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This manual documents version 4.1 of the GNU file utilities.
1. Introduction Caveats, overview, and authors. 2. Common options 3. File permissions Access modes. 4. Date input formats Specifying date strings. 5. Directory listing ls dir vdir d v dircolors 6. Basic operations cp dd install mv rm shred 7. Special file types ln mkdir rmdir mkfifo mknod 8. Changing file attributes chgrp chmod chown touch 9. Disk usage df du sync Index General index.
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This manual is incomplete: No attempt is made to explain basic file concepts in a way suitable for novices. Thus, if you are interested, please get involved in improving this manual. The entire GNU community will benefit.
The GNU file utilities are mostly compatible with the POSIX.2 standard.
Please report bugs to [email protected]. Remember to include the version number, machine architecture, input files, and any other information needed to reproduce the bug: your input, what you expected, what you got, and why it is wrong. Diffs are welcome, but please include a description of the problem as well, since this is sometimes difficult to infer. See section `Bugs' in GNU CC.
This manual was originally derived from the Unix man pages in the distribution, which were written by David MacKenzie and updated by Jim Meyering. What you are reading now is the authoritative documentation for these utilities; the man pages are no longer being maintained. François Pinard did the initial conversion to Texinfo format. Karl Berry did the indexing, some reorganization, and editing of the results. Richard Stallman contributed his usual invaluable insights to the overall process.
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Certain options are available in all of these programs (in fact, every GNU program should accept them). Rather than writing identical descriptions for each of the programs, they are described here.
2.1 Backup options -b -S -V, in some programs. 2.2 Block size BLOCK_SIZE and --block-size, in some programs. 2.3 Target directory --target-directory, in some programs. 2.4 Trailing slashes --strip-trailing-slashes, in some programs.
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Some GNU programs (at least cp, install, ln, and
mv) optionally make backups of files before writing new versions.
These options control the details of these backups. The options are also
briefly mentioned in the descriptions of the particular programs.
VERSION_CONTROL
environment variable is used. And if VERSION_CONTROL is not set,
the default backup type is `existing'.
Note that the short form of this option, `-b' does not accept any argument. Using `-b' is equivalent to using `--backup=existing'.
This option corresponds to the Emacs variable `version-control'; the values for method are the same as those used in Emacs. This option also accepts more descriptive names. The valid methods are (unique abbreviations are accepted):
SIMPLE_BACKUP_SUFFIX
environment variable is used. And if SIMPLE_BACKUP_SUFFIX is not
set, the default is `~', just as in Emacs.
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Some GNU programs (at least df, du, and ls) display
file sizes in "blocks". You can adjust the block size to make file
sizes easier to read. The block size used for display is independent of
any filesystem block size.
Normally, disk usage sizes are rounded up, disk free space sizes are rounded down, and other sizes are rounded to the nearest value with ties rounding to an even value.
The default block size is chosen by examining the following environment variables in turn; the first one that is set determines the block size.
DF_BLOCK_SIZE
df command.
Similarly, DU_BLOCK_SIZE specifies the default for du and
LS_BLOCK_SIZE for ls.
BLOCK_SIZE
POSIXLY_CORRECT
command_BLOCK_SIZE nor the BLOCK_SIZE
variables are set, but this variable is set, the block size defaults to 512.
If none of the above environment variables are set, the block size currently defaults to 1024 bytes, but this number may change in the future.
A block size specification can be a positive integer specifying the number
of bytes per block, or it can be human-readable or si to
select a human-readable format.
With human-readable formats, output sizes are followed by a size letter
such as `M' for megabytes. BLOCK_SIZE=human-readable uses
powers of 1024; `M' stands for 1,048,576 bytes.
BLOCK_SIZE=si is similar, but uses powers of 1000; `M' stands
for 1,000,000 bytes. (SI, the International System of Units, defines
these power-of-1000 prefixes.)
An integer block size can be followed by a size letter to specify a
multiple of that size. When this notation is used, the size letters
normally stand for powers of 1024, and can be followed by an optional
`B' for "byte"; but if followed by `D' (for "decimal
byte"), they stand for powers of 1000. For example,
BLOCK_SIZE=4MB is equivalent to BLOCK_SIZE=4194304, and
BLOCK_SIZE=4MD is equivalent to BLOCK_SIZE=4000000.
The following size letters are defined. Large sizes like 1Y
may be rejected by your computer due to limitations of its arithmetic.
human-readable,
or 10^3 = 1000 for si.
Block size defaults can be overridden by an explicit
`--block-size=size' option. The `-k' or
`--kilobytes' option is equivalent to `--block-size=1k', which
is the default unless the POSIXLY_CORRECT environment variable is
set. The `-h' or `--human-readable' option is equivalent to
`--block-size=human-readable'. The `--si' option
is equivalent to `--block-size=si'.
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Some GNU programs (at least cp, install, ln, and
mv) allow you to specify the target directory via this option:
The interface for most programs is that after processing options and a
finite (possibly zero) number of fixed-position arguments, the remaining
argument list is either expected to be empty, or is a list of items
(usually files) that will all be handled identically. The xargs
program is designed to work well with this convention.
The commands in the mv-family are unusual in that they take
a variable number of arguments with a special case at the end
(namely, the target directory). This makes it nontrivial to perform some
operations, e.g., "move all files from here to ../d/", because
mv * ../d/ might exhaust the argument space, and ls | xargs ...
doesn't have a clean way to specify an extra final argument for each
invocation of the subject command. (It can be done by going through a
shell command, but that requires more human labor and brain power than
it should.)
The --target-directory option allows the cp,
install, ln, and mv programs to be used conveniently
with xargs. For example, you can move the files from the
current directory to a sibling directory, d like this:
(However, this doesn't move files whose names begin with `.'.)
ls |xargs mv --target-directory=../d |
If you use the GNU find program, you can move all
files with this command:
find . -mindepth 1 -maxdepth 1 \ | xargs mv --target-directory=../d |
But that will fail if there are no files in the current directory
or if any file has a name containing a newline character.
The following example removes those limitations and requires both
GNU find and GNU xargs:
find . -mindepth 1 -maxdepth 1 -print0 \
| xargs --null --no-run-if-empty \
mv --target-directory=../d
|
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Some GNU programs (at least cp and mv)
allow you to remove any trailing slashes from each source
argument before operating on it. The --strip-trailing-slashes
option enables this behavior.
This is useful when a source argument may have a trailing slash and
specify a symbolic link to a directory. This scenario is in fact rather
common because some shells can automatically append a trailing slash when
performing file name completion on such symbolic links. Without this
option, mv, for example, (via the system's rename function) must
interpret a trailing slash as a request to dereference the symbolic link
and so must rename the indirectly referenced directory and not
the symbolic link. Although it may seem surprising that such behavior
be the default, it is required by POSIX.2 and is consistent with
other parts of that standard.
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Each file has a set of permissions that control the kinds of access that users have to that file. The permissions for a file are also called its access mode. They can be represented either in symbolic form or as an octal number.
3.1 Structure of File Permissions Structure of file permissions. 3.2 Symbolic Modes Mnemonic permissions representation. 3.3 Numeric Modes Permissions as octal numbers.
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There are three kinds of permissions that a user can have for a file:
There are three categories of users who may have different permissions to perform any of the above operations on a file:
Files are given an owner and group when they are created. Usually the
owner is the current user and the group is the group of the directory
the file is in, but this varies with the operating system, the
filesystem the file is created on, and the way the file is created. You
can change the owner and group of a file by using the chown and
chgrp commands.
In addition to the three sets of three permissions listed above, a file's permissions have three special components, which affect only executable files (programs) and, on some systems, directories:
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Symbolic modes represent changes to files' permissions as
operations on single-character symbols. They allow you to modify either
all or selected parts of files' permissions, optionally based on
their previous values, and perhaps on the current umask as well
(see section 3.2.6 The Umask and Protection).
The format of symbolic modes is:
[ugoa...][[+-=][rwxXstugo...]...][,...] |
The following sections describe the operators and other details of symbolic modes.
3.2.1 Setting Permissions Basic operations on permissions. 3.2.2 Copying Existing Permissions Copying existing permissions. 3.2.3 Changing Special Permissions Special permissions. 3.2.4 Conditional Executability Conditionally affecting executability. 3.2.5 Making Multiple Changes Making multiple changes. 3.2.6 The Umask and Protection The effect of the umask.
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The basic symbolic operations on a file's permissions are adding, removing, and setting the permission that certain users have to read, write, and execute the file. These operations have the following format:
users operation permissions |
The spaces between the three parts above are shown for readability only; symbolic modes cannot contain spaces.
The users part tells which users' access to the file is changed. It consists of one or more of the following letters (or it can be empty; see section 3.2.6 The Umask and Protection, for a description of what happens then). When more than one of these letters is given, the order that they are in does not matter.
u
g
o
a
The operation part tells how to change the affected users' access to the file, and is one of the following symbols:
+
-
=
The permissions part tells what kind of access to the file should be changed; it is zero or more of the following letters. As with the users part, the order does not matter when more than one letter is given. Omitting the permissions part is useful only with the `=' operation, where it gives the specified users no access at all to the file.
r
w
x
For example, to give everyone permission to read and write a file, but not to execute it, use:
a=rw |
To remove write permission for from all users other than the file's owner, use:
go-w |
The above command does not affect the access that the owner of the file has to it, nor does it affect whether other users can read or execute the file.
To give everyone except a file's owner no permission to do anything with that file, use the mode below. Other users could still remove the file, if they have write permission on the directory it is in.
go= |
Another way to specify the same thing is:
og-rxw |
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You can base a file's permissions on its existing permissions. To do this, instead of using `r', `w', or `x' after the operator, you use the letter `u', `g', or `o'. For example, the mode
o+g |
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In addition to changing a file's read, write, and execute permissions, you can change its special permissions. See section 3.1 Structure of File Permissions, for a summary of these permissions.
To change a file's permission to set the user ID on execution, use `u' in the users part of the symbolic mode and `s' in the permissions part.
To change a file's permission to set the group ID on execution, use `g' in the users part of the symbolic mode and `s' in the permissions part.
To change a file's permission to stay permanently on the swap device, use `o' in the users part of the symbolic mode and `t' in the permissions part.
For example, to add set user ID permission to a program, you can use the mode:
u+s |
To remove both set user ID and set group ID permission from it, you can use the mode:
ug-s |
To cause a program to be saved on the swap device, you can use the mode:
o+t |
Remember that the special permissions only affect files that are executable, plus, on some systems, directories (on which they have different meanings; see section 3.1 Structure of File Permissions). Also, the combinations `u+t', `g+t', and `o+s' have no effect.
The `=' operator is not very useful with special permissions; for example, the mode:
o=t |
does cause the file to be saved on the swap device, but it also removes all read, write, and execute permissions that users not in the file's group might have had for it.
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There is one more special type of symbolic permission: if you use `X' instead of `x', execute permission is affected only if the file already had execute permission or is a directory. It affects directories' execute permission even if they did not initially have any execute permissions set.
For example, this mode:
a+X |
gives all users permission to execute files (or search directories) if anyone could before.
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The format of symbolic modes is actually more complex than described above (see section 3.2.1 Setting Permissions). It provides two ways to make multiple changes to files' permissions.
The first way is to specify multiple operation and permissions parts after a users part in the symbolic mode.
For example, the mode:
og+rX-w |
gives users other than the owner of the file read permission and, if it is a directory or if someone already had execute permission to it, gives them execute permission; and it also denies them write permission to the file. It does not affect the permission that the owner of the file has for it. The above mode is equivalent to the two modes:
og+rX og-w |
The second way to make multiple changes is to specify more than one simple symbolic mode, separated by commas. For example, the mode:
a+r,go-w |
gives everyone permission to read the file and removes write permission on it for all users except its owner. Another example:
u=rwx,g=rx,o= |
sets all of the non-special permissions for the file explicitly. (It gives users who are not in the file's group no permission at all for it.)
The two methods can be combined. The mode:
a+r,g+x-w |
gives all users permission to read the file, and gives users who are in the file's group permission to execute it, as well, but not permission to write to it. The above mode could be written in several different ways; another is:
u+r,g+rx,o+r,g-w |
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If the users part of a symbolic mode is omitted, it defaults to
`a' (affect all users), except that any permissions that are
set in the system variable umask are not affected.
The value of umask can be set using the
umask command. Its default value varies from system to system.
Omitting the users part of a symbolic mode is generally not useful
with operations other than `+'. It is useful with `+' because
it allows you to use umask as an easily customizable protection
against giving away more permission to files than you intended to.
As an example, if umask has the value 2, which removes write
permission for users who are not in the file's group, then the mode:
+w |
adds permission to write to the file to its owner and to other users who are in the file's group, but not to other users. In contrast, the mode:
a+w |
ignores umask, and does give write permission for
the file to all users.
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File permissions are stored internally as integers. As an alternative to giving a symbolic mode, you can give an octal (base 8) number that corresponds to the internal representation of the new mode. This number is always interpreted in octal; you do not have to add a leading 0, as you do in C. Mode 0055 is the same as mode 55.
A numeric mode is usually shorter than the corresponding symbolic mode, but it is limited in that it cannot take into account a file's previous permissions; it can only set them absolutely.
On most systems, the permissions granted to the user, to other users in the file's group, and to other users not in the file's group are each stored as three bits, which are represented as one octal digit. The three special permissions are also each stored as one bit, and they are as a group represented as another octal digit. Here is how the bits are arranged, starting with the lowest valued bit:
Value in Corresponding
Mode Permission
Other users not in the file's group:
1 Execute
2 Write
4 Read
Other users in the file's group:
10 Execute
20 Write
40 Read
The file's owner:
100 Execute
200 Write
400 Read
Special permissions:
1000 Save text image on swap device
2000 Set group ID on execution
4000 Set user ID on execution
|
For example, numeric mode 4755 corresponds to symbolic mode `u=rwxs,go=rx', and numeric mode 664 corresponds to symbolic mode `ug=rw,o=r'. Numeric mode 0 corresponds to symbolic mode `ugo='.
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First, a quote:
Our units of temporal measurement, from seconds on up to months, are so complicated, asymmetrical and disjunctive so as to make coherent mental reckoning in time all but impossible. Indeed, had some tyrannical god contrived to enslave our minds to time, to make it all but impossible for us to escape subjection to sodden routines and unpleasant surprises, he could hardly have done better than handing down our present system. It is like a set of trapezoidal building blocks, with no vertical or horizontal surfaces, like a language in which the simplest thought demands ornate constructions, useless particles and lengthy circumlocutions. Unlike the more successful patterns of language and science, which enable us to face experience boldly or at least level-headedly, our system of temporal calculation silently and persistently encourages our terror of time.... It is as though architects had to measure length in feet, width in meters and height in ells; as though basic instruction manuals demanded a knowledge of five different languages. It is no wonder then that we often look into our own immediate past or future, last Tuesday or a week from Sunday, with feelings of helpless confusion. ...
--- Robert Grudin, Time and the Art of Living.
This section describes the textual date representations that GNU
programs accept. These are the strings you, as a user, can supply as
arguments to the various programs. The C interface (via the
getdate function) is not described here.
Although the date syntax here can represent any possible time since the
year zero, computer integers often cannot represent such a wide range of
time. On POSIX systems, the clock starts at 1970-01-01 00:00:00
UTC: POSIX does not require support for times before the
POSIX Epoch and times far in the future. Traditional Unix systems
have 32-bit signed time_t and can represent times from 1901-12-13
20:45:52 through 2038-01-19 03:14:07 UTC. Systems with 64-bit
signed time_t can represent all the times in the known
lifetime of the universe.
4.1 General date syntax Common rules. 4.2 Calendar date items 19 Dec 1994. 4.3 Time of day items 9:20pm. 4.4 Time zone items EST, PDT, GMT, ... 4.5 Day of week items Monday and others. 4.6 Relative items in date strings next tuesday, 2 years ago. 4.7 Pure numbers in date strings 19931219, 1440. 4.8 Authors of getdateBellovin, Eggert, Salz, Berets, et al.
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A date is a string, possibly empty, containing many items separated by whitespace. The whitespace may be omitted when no ambiguity arises. The empty string means the beginning of today (i.e., midnight). Order of the items is immaterial. A date string may contain many flavors of items:
We describe each of these item types in turn, below.
A few numbers may be written out in words in most contexts. This is most useful for specifying day of the week items or relative items (see below). Here is the list: `first' for 1, `next' for 2, `third' for 3, `fourth' for 4, `fifth' for 5, `sixth' for 6, `seventh' for 7, `eighth' for 8, `ninth' for 9, `tenth' for 10, `eleventh' for 11 and `twelfth' for 12. Also, `last' means exactly -1.
When a month is written this way, it is still considered to be written numerically, instead of being "spelled in full"; this changes the allowed strings.
In the current implementation, only English is supported for words and abbreviations like `AM', `DST', `EST', `first', `January', `Sunday', `tomorrow', and `year'.
The output of date is not always acceptable as a date string,
not only because of the language problem, but also because there is no
standard meaning for time zone items like `IST'. When using
date to generate a date string intended to be parsed later,
specify a date format that is independent of language and that does not
use time zone items other than `UTC' and `Z'. Here are some
ways to do this:
$ LC_ALL=C TZ=UTC0 date Fri Dec 15 19:48:05 UTC 2000 $ TZ=UTC0 date +"%Y-%m-%d %H:%M:%SZ" 2000-12-15 19:48:05Z $ date --iso-8601=seconds # a GNU extension 2000-12-15T11:48:05-0800 $ date --rfc-822 # a GNU extension Fri, 15 Dec 2000 11:48:05 -0800 $ date +"%Y-%m-%d %H:%M:%S %z" # %z is a GNU extension. 2000-12-15 11:48:05 -0800 |
Alphabetic case is completely ignored in dates. Comments may be introduced between round parentheses, as long as included parentheses are properly nested. Hyphens not followed by a digit are currently ignored. Leading zeros on numbers are ignored.
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A calendar date item specifies a day of the year. It is specified differently, depending on whether the month is specified numerically or literally. All these strings specify the same calendar date:
1972-09-24 # ISO 8601.
72-9-24 # Assume 19xx for 69 through 99,
# 20xx for 00 through 68.
72-09-24 # Leading zeros are ignored.
9/24/72 # Common U.S. writing.
24 September 1972
24 Sept 72 # September has a special abbreviation.
24 Sep 72 # Three-letter abbreviations always allowed.
Sep 24, 1972
24-sep-72
24sep72
|
The year can also be omitted. In this case, the last specified year is used, or the current year if none. For example:
9/24 sep 24 |
Here are the rules.
For numeric months, the ISO 8601 format `year-month-day' is allowed, where year is any positive number, month is a number between 01 and 12, and day is a number between 01 and 31. A leading zero must be present if a number is less than ten. If year is 68 or smaller, then 2000 is added to it; otherwise, if year is less than 100, then 1900 is added to it. The construct `month/day/year', popular in the United States, is accepted. Also `month/day', omitting the year.
Literal months may be spelled out in full: `January', `February', `March', `April', `May', `June', `July', `August', `September', `October', `November' or `December'. Literal months may be abbreviated to their first three letters, possibly followed by an abbreviating dot. It is also permitted to write `Sept' instead of `September'.
When months are written literally, the calendar date may be given as any of the following:
day month year day month month day year day-month-year |
Or, omitting the year:
month day |
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A time of day item in date strings specifies the time on a given day. Here are some examples, all of which represent the same time:
20:02:0 20:02 8:02pm 20:02-0500 # In EST (U.S. Eastern Standard Time). |
More generally, the time of the day may be given as `hour:minute:second', where hour is a number between 0 and 23, minute is a number between 0 and 59, and second is a number between 0 and 59. Alternatively, `:second' can be omitted, in which case it is taken to be zero.
If the time is followed by `am' or `pm' (or `a.m.' or `p.m.'), hour is restricted to run from 1 to 12, and `:minute' may be omitted (taken to be zero). `am' indicates the first half of the day, `pm' indicates the second half of the day. In this notation, 12 is the predecessor of 1: midnight is `12am' while noon is `12pm'. (This is the zero-oriented interpretation of `12am' and `12pm', as opposed to the old tradition derived from Latin which uses `12m' for noon and `12pm' for midnight.)
The time may alternatively be followed by a time zone correction, expressed as `shhmm', where s is `+' or `-', hh is a number of zone hours and mm is a number of zone minutes. When a time zone correction is given this way, it forces interpretation of the time relative to Coordinated Universal Time (UTC), overriding any previous specification for the time zone or the local time zone. The minute part of the time of the day may not be elided when a time zone correction is used. This is the best way to specify a time zone correction by fractional parts of an hour.
Either `am'/`pm' or a time zone correction may be specified, but not both.
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A time zone item specifies an international time zone, indicated by a small set of letters, e.g., `UTC' or `Z' for Coordinated Universal Time. Any included periods are ignored. By following a non-daylight-saving time zone by the string `DST' in a separate word (that is, separated by some white space), the corresponding daylight saving time zone may be specified.
Time zone items other than `UTC' and `Z' are obsolescent and are not recommended, because they are ambiguous; for example, `EST' has a different meaning in Australia than in the United States. Instead, it's better to use unambiguous numeric time zone corrections like `-0500', as described in the previous section.
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The explicit mention of a day of the week will forward the date (only if necessary) to reach that day of the week in the future.
Days of the week may be spelled out in full: `Sunday', `Monday', `Tuesday', `Wednesday', `Thursday', `Friday' or `Saturday'. Days may be abbreviated to their first three letters, optionally followed by a period. The special abbreviations `Tues' for `Tuesday', `Wednes' for `Wednesday' and `Thur' or `Thurs' for `Thursday' are also allowed.
A number may precede a day of the week item to move forward supplementary weeks. It is best used in expression like `third monday'. In this context, `last day' or `next day' is also acceptable; they move one week before or after the day that day by itself would represent.
A comma following a day of the week item is ignored.
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Relative items adjust a date (or the current date if none) forward or backward. The effects of relative items accumulate. Here are some examples:
1 year 1 year ago 3 years 2 days |
The unit of time displacement may be selected by the string `year' or `month' for moving by whole years or months. These are fuzzy units, as years and months are not all of equal duration. More precise units are `fortnight' which is worth 14 days, `week' worth 7 days, `day' worth 24 hours, `hour' worth 60 minutes, `minute' or `min' worth 60 seconds, and `second' or `sec' worth one second. An `s' suffix on these units is accepted and ignored.
The unit of time may be preceded by a multiplier, given as an optionally signed number. Unsigned numbers are taken as positively signed. No number at all implies 1 for a multiplier. Following a relative item by the string `ago' is equivalent to preceding the unit by a multiplier with value -1.
The string `tomorrow' is worth one day in the future (equivalent to `day'), the string `yesterday' is worth one day in the past (equivalent to `day ago').
The strings `now' or `today' are relative items corresponding to zero-valued time displacement, these strings come from the fact a zero-valued time displacement represents the current time when not otherwise changed by previous items. They may be used to stress other items, like in `12:00 today'. The string `this' also has the meaning of a zero-valued time displacement, but is preferred in date strings like `this thursday'.
When a relative item causes the resulting date to cross a boundary where the clocks were adjusted, typically for daylight-saving time, the resulting date and time are adjusted accordingly.
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The precise interpretation of a pure decimal number depends on the context in the date string.
If the decimal number is of the form yyyymmdd and no other calendar date item (see section 4.2 Calendar date items) appears before it in the date string, then yyyy is read as the year, mm as the month number and dd as the day of the month, for the specified calendar date.
If the decimal number is of the form hhmm and no other time of day item appears before it in the date string, then hh is read as the hour of the day and mm as the minute of the hour, for the specified time of the day. mm can also be omitted.
If both a calendar date and a time of day appear to the left of a number in the date string, but no relative item, then the number overrides the year.
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getdate
getdate was originally implemented by Steven M. Bellovin
([email protected]) while at the University of North Carolina
at Chapel Hill. The code was later tweaked by a couple of people on
Usenet, then completely overhauled by Rich $alz ([email protected])
and Jim Berets ([email protected]) in August, 1990. Various
revisions for the GNU system were made by David MacKenzie, Jim Meyering,
Paul Eggert and others.
This chapter was originally produced by François Pinard ([email protected]) from the `getdate.y' source code, and then edited by K. Berry ([email protected]).
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This chapter describes the ls command and its variants dir
and vdir, which list information about files.
5.1 ls: List directory contentsList directory contents. 5.2 dir: Briefly list directory contentsBriefly ls. 5.3 vdir: Verbosely list directory contentsVerbosely ls. 5.4 dircolors: Color setup forlsColor setup for ls, etc.
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ls: List directory contents
The ls program lists information about files (of any type,
including directories). Options and file arguments can be intermixed
arbitrarily, as usual.
For non-option command-line arguments that are directories, by default
ls lists the contents of directories, not recursively, and
omitting files with names beginning with `.'. For other non-option
arguments, by default ls lists just the file name. If no
non-option arguments are specified, ls lists the contents of the
current directory.
By default, the output is sorted alphabetically. If standard output is a terminal, the output is in columns (sorted vertically) and control characters are output as question marks; otherwise, the output is listed one per line and control characters are output as-is.
Because ls is such a fundamental program, it has accumulated many
options over the years. They are described in the subsections below;
within each section, options are listed alphabetically (ignoring case).
The division of options into the subsections is not absolute, since some
options affect more than one aspect of ls's operation.
The `-g' option is accepted but ignored, for compatibility with Unix. Also see 2. Common options.
5.1.1 Which files are listed 5.1.2 What information is listed 5.1.3 Sorting the output 5.1.4 More details about version sort 5.1.5 General output formatting 5.1.6 Formatting the file names
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These options determine which files ls lists information for.
By default, any files and the contents of any directories on the command
line are shown.
$ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*' |
The first option ignores names of length 3 or more that start with `.', the second ignores all two-character names that start with `.' except `..', and the third ignores names that start with `#'.
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These options affect the information that ls displays. By
default, only file names are shown.
//DIRED// beg1 end1 beg2 end2 ... |
The begN and endN are unsigned integers that record the byte position of the beginning and end of each file name in the output. This makes it easy for Emacs to find the names, even when they contain unusual characters such as space or newline, without fancy searching.
If directories are being listed recursively (-R), output a similar
line after each subdirectory:
//SUBDIRED// format beg1 end1 ... |
Finally, output a line of the form:
//DIRED-OPTIONS// --quoting-style=word |
ls, so we
provide this option for compatibility.)
Warning: the meaning of `-H' will change in the future to conform to POSIX. Use `--si' for the old meaning.
make that
rely on file times.
For each directory that is listed, preface the files with a line `total blocks', where blocks is the total disk allocation for all files in that directory. The block size currently defaults to 1024 bytes, but this can be overridden (see section 2.2 Block size). The blocks computed counts each hard link separately; this is arguably a deficiency.
The permissions listed are similar to symbolic mode specifications
(see section 3.2 Symbolic Modes). But ls combines multiple bits into the
third character of each set of permissions as follows:
Following the permission bits is a single character that specifies whether an alternate access method applies to the file. When that character is a space, there is no alternate access method. When it is a printing character (e.g., `+'), then there is such a method.
ls.
Normally the disk allocation is printed in units of 1024 bytes, but this can be overridden (see section 2.2 Block size).
For files that are NFS-mounted from an HP-UX system to a BSD system,
this option reports sizes that are half the correct values. On HP-UX
systems, it reports sizes that are twice the correct values for files
that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
it also affects the HP-UX ls program.
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These options change the order in which ls sorts the information
it outputs. By default, sorting is done by character code (e.g., ASCII
order).
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The version sort takes into account the fact that file names frequently include indices or version numbers. Standard sorting functions usually do not produce the ordering that people expect because comparisons are made on a character-by-character basis. The version sort addresses this problem, and is especially useful when browsing directories that contain many files with indices/version numbers in their names:
> ls -1 > ls -1v
foo.zml-1.gz foo.zml-1.gz
foo.zml-100.gz foo.zml-2.gz
foo.zml-12.gz foo.zml-6.gz
foo.zml-13.gz foo.zml-12.gz
foo.zml-2.gz foo.zml-13.gz
foo.zml-25.gz foo.zml-25.gz
foo.zml-6.gz foo.zml-100.gz
|
Note also that numeric parts with leading zeroes are considered as fractional one:
> ls -1 > ls -1v
abc-1.007.tgz abc-1.007.tgz
abc-1.012b.tgz abc-1.01a.tgz
abc-1.01a.tgz abc-1.012b.tgz
|
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These options affect the appearance of the overall output.
ls when standard
output is not a terminal.
ls if standard output is a terminal. It is always the default
for the dir and d programs.
GNU ls uses variable width columns to display as many files as
possible in the fewest lines.
more or
less usually produces unreadable results. However, using
more -f does seem to work.
date,
but this is planned to change in a future release, partly because modern
file time stamps have more precision. It's not
possible to change the format, but you can extract out the date string with
cut and then pass the result to date -d. See section `date invocation' in Shell utilities.
This is most useful because the time output includes the seconds. (Unix filesystems store file timestamps only to the nearest second, so this option shows all the information there is.) For example, this can help when you have a Makefile that is not regenerating files properly.
ls uses tabs where possible in the output, for efficiency. If
cols is zero, do not use tabs at all.
COLUMNS is used if it is set; otherwise the default
is 80.
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These options change how file names themselves are printed.
ls.
You can specify the default value of the `--quoting-style' option
with the environment variable QUOTING_STYLE. If that environment
variable is not set, the default value is `literal', but this
default may change to `shell' in a future version of this package.
ls.
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dir: Briefly list directory contents
dir (also installed as d) is equivalent to ls -C
-b; that is, by default files are listed in columns, sorted vertically,
and special characters are represented by backslash escape sequences.
See section ls.
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vdir: Verbosely list directory contents
vdir (also installed as v) is equivalent to ls -l
-b; that is, by default files are listed in long format and special
characters are represented by backslash escape sequences.
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dircolors: Color setup for ls
dircolors outputs a sequence of shell commands to set up the
terminal for color output from ls (and dir, etc.).
Typical usage:
eval `dircolors [option]... [file]` |
If file is specified, dircolors reads it to determine which
colors to use for which file types and extensions. Otherwise, a
precompiled database is used. For details on the format of these files,
run `dircolors --print-database'.
The output is a shell command to set the LS_COLORS environment
variable. You can specify the shell syntax to use on the command line,
or dircolors will guess it from the value of the SHELL
environment variable.
The program accepts the following options. Also see 2. Common options.
SHELL
environment variable is set and does not end with `csh' or
`tcsh'.
SHELL ends with
csh or tcsh.
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This chapter describes the commands for basic file manipulation: copying, moving (renaming), and deleting (removing).
6.1 cp: Copy files and directoriesCopy files. 6.2 dd: Convert and copy a fileConvert and copy a file. 6.3 install: Copy files and set attributesCopy files and set attributes. 6.4 mv: Move (rename) filesMove (rename) files. 6.5 rm: Remove files or directoriesRemove files or directories. 6.6 shred: Remove files more securelyRemove files more securely.
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cp: Copy files and directories
cp copies files (or, optionally, directories). The copy is
completely independent of the original. You can either copy one file to
another, or copy arbitrarily many files to a destination directory.
Synopsis:
cp [option]... source dest cp [option]... source... directory |
If the last argument names an existing directory, cp copies each
source file into that directory (retaining the same name).
Otherwise, if only two files are given, it copies the first onto the
second. It is an error if the last argument is not a directory and more
than two non-option arguments are given.
Generally, files are written just as they are read. For exceptions, see the `--sparse' option below.
By default, cp does not copy directories. However, the
`-R', `-a', and `-r' options cause cp to
copy recursively by descending into source directories and copying files
to corresponding destination directories.
By default, cp follows symbolic links only when not copying
recursively. This default can be overridden with the
`--no-dereference' (`-d'), `--dereference'
(`-L'), and `-H' options. If more than one of these
options is specified, the last one silently overrides the others.
cp generally refuses to copy a file onto itself, with the
following exception: if `--force --backup' is specified with
source and dest identical, and referring to a regular file,
cp will make a backup file, either regular or numbered, as
specified in the usual ways (see section 2.1 Backup options). This is useful when
you simply want to make a backup of an existing file before changing it.
The program accepts the following options. Also see 2. Common options.
cp makes a backup of source when the force
and backup options are given and source and dest are the same
name for an existing, regular file. One useful application of this
combination of options is this tiny Bourne shell script:
#!/bin/sh # Usage: backup FILE... # Create a GNU-style backup of each listed FILE. for i in "$@"; do cp --backup --force "$i" "$i" done |
cp then unlinks it and
tries to open it again. Contrast this behavior with that enabled by
`--link' and `--symbolic-link', whereby the destination file
is never opened but rather is unlinked unconditionally. Also see the
description of `--remove-destination'.
cp must be the name of an existing directory.
For example, the command:
cp --parents a/b/c existing_dir |
copies the file `a/b/c' to `existing_dir/a/b/c', creating any missing intermediate directories.
Warning: the meaning of `-P' will change in the future to conform to POSIX. Use `--parents' for the old meaning, and `--no-dereference' for the new.
cp -r to special files like FIFOs and the ones typically
found in the `/dev' directory. In most cases, cp -r
will hang indefinitely trying to read from FIFOs and special files
like `/dev/console', and it will fill up your destination disk
if you use it to copy `/dev/zero'.
Use the `--recursive' (`-R') option instead if you want
to copy special files, preserving their special nature
rather than reading from them to copy their contents.
cp detects holes in input source files via a crude
heuristic and makes the corresponding output file sparse as well.
The when value can be one of the following:
mkswap command,
since such a file must not have any holes.
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dd: Convert and copy a file
dd copies a file (from standard input to standard output, by
default) with a changeable I/O block size, while optionally performing
conversions on it. Synopsis:
dd [option]... |
The program accepts the following options. Also see 2. Common options.
The numeric-valued options below (bytes and blocks) can be followed by a multiplier: `b'=512, `c'=1, `w'=2, `xm'=m, or any of the standard block size suffixes like `k'=1024 (see section 2.2 Block size).
Use different dd invocations to use different block sizes for
skipping and I/O. For example, the following shell commands copy data
in 512 kB blocks between a disk and a tape, but do not save or restore a
4 kB label at the start of the disk:
disk=/dev/rdsk/c0t1d0s2 tape=/dev/rmt/0 # Copy all but the label from disk to tape. (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape # Copy from tape back to disk, but leave the disk label alone. (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk |
dd truncates file to zero
bytes (or the size specified with `seek=').
Conversions:
dd, unlike others, works
when an odd number of bytes are read--the last byte is simply copied
(since there is nothing to swap it with).
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install: Copy files and set attributes
install copies files while setting their permission modes and, if
possible, their owner and group. Synopses:
install [option]... source dest install [option]... source... directory install -d [option]... directory... |
In the first of these, the source file is copied to the dest target file. In the second, each of the source files are copied to the destination directory. In the last, each directory (and any missing parent directories) is created.
install is similar to cp, but allows you to control the
attributes of destination files. It is typically used in Makefiles to
copy programs into their destination directories. It refuses to copy
files onto themselves.
The program accepts the following options. Also see 2. Common options.
install.
install, which
gives directories that it creates the default attributes.)
chmod, with 0 as the point of departure (see section 3. File permissions). The default mode is `u=rwx,go=rx'---read, write,
and execute for the owner, and read and execute for group and other.
install has appropriate privileges (is run as root), set the
ownership of installed files or directories to owner. The default
is root. owner may be either a user name or a numeric user
ID.
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mv: Move (rename) files
mv moves or renames files (or directories). Synopsis:
mv [option]... source dest mv [option]... source... directory |
If the last argument names an existing directory, mv moves each
other given file into a file with the same name in that directory.
Otherwise, if only two files are given, it renames the first as
the second. It is an error if the last argument is not a directory
and more than two files are given.
mv can move any type of file from one filesystem to another.
Prior to version 4.0 of the fileutils,
mv could move only regular files between filesystems.
For example, now mv can move an entire directory hierarchy
including special device files from one partition to another. It first
uses some of the same code that's used by cp -a to copy the
requested directories and files, then (assuming the copy succeeded)
it removes the originals. If the copy fails, then the part that was
copied to the destination partition is removed. If you were to copy
three directories from one partition to another and the copy of the first
directory succeeded, but the second didn't, the first would be left on
the destination partion and the second and third would be left on the
original partition.
If a destination file exists but is normally unwritable, standard input
is a terminal, and the `-f' or `--force' option is not given,
mv prompts the user for whether to replace the file. (You might
own the file, or have write permission on its directory.) If the
response does not begin with `y' or `Y', the file is skipped.
Warning: If you try to move a symlink that points to a directory,
and you specify the symlink with a trailing slash, then mv
doesn't move the symlink but instead moves the directory referenced
by the symlink. See section 2.4 Trailing slashes.
The program accepts the following options. Also see 2. Common options.
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rm: Remove files or directories
rm removes each given file. By default, it does not remove
directories. Synopsis:
rm [option]... [file]... |
If a file is unwritable, standard input is a terminal, and the `-f'
or `--force' option is not given, or the `-i' or
`--interactive' option is given, rm prompts the user
for whether to remove the file. If the response does not begin with
`y' or `Y', the file is skipped.
The program accepts the following options. Also see 2. Common options.
unlink instead of rmdir, and
don't require a directory to be empty before trying to unlink it. This works
only if you have appropriate privileges and if your operating system supports
unlink for directories. Because unlinking a directory causes any files
in the deleted directory to become unreferenced, it is wise to fsck the
filesystem after doing this.
One common question is how to remove files whose names begin with a
`-'. GNU rm, like every program that uses the getopt
function to parse its arguments, lets you use the `--' option to
indicate that all following arguments are non-options. To remove a file
called `-f' in the current directory, you could type either:
rm -- -f |
or:
rm ./-f |
The Unix rm program's use of a single `-' for this purpose
predates the development of the getopt standard syntax.
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shred: Remove files more securely
shred overwrites devices or files, to help prevent even
very expensive hardware from recovering the data.
Ordinarily when you remove a file (see section 6.5 rm: Remove files or directories), the data is
not actually destroyed. Only the index listing where the file is
stored is destroyed, and the storage is made available for reuse.
There are undelete utilities that will attempt to reconstruct the index
and can bring the file back if the parts were not reused.
On a busy system with a nearly-full drive, space can get reused in a few seconds. But there is no way to know for sure. If you have sensitive data, you may want to be sure that recovery is not possible by actually overwriting the file with non-sensitive data.
However, even after doing that, it is possible to take the disk back to a laboratory and use a lot of sensitive (and expensive) equipment to look for the faint "echoes" of the original data underneath the overwritten data. If the data has only been overwritten once, it's not even that hard.
The best way to remove something irretrievably is to destroy the media
it's on with acid, melt it down, or the like. For cheap removable media
like floppy disks, this is the preferred method. However, hard drives
are expensive and hard to melt, so the shred utility tries
to achieve a similar effect non-destructively.
This uses many overwrite passes, with the data patterns chosen to maximize the damage they do to the old data. While this will work on floppies, the patterns are designed for best effect on hard drives. For more details, see the source code and Peter Gutmann's paper Secure Deletion of Data from Magnetic and Solid-State Memory, from the proceedings of the Sixth USENIX Security Symposium (San Jose, California, 22--25 July, 1996). The paper is also available online http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html.
Please note that shred relies on a very important assumption:
that the filesystem overwrites data in place. This is the traditional
way to do things, but many modern filesystem designs do not satisfy this
assumption. Exceptions include:
If you are not sure how your filesystem operates, then you should assume that it does not overwrite data in place, which means that shred cannot reliably operate on regular files in your filesystem.
Generally speaking, it is more reliable to shred a device than a file,
since this bypasses the problem of filesystem design mentioned above.
However, even shredding devices is not always completely reliable. For
example, most disks map out bad sectors invisibly to the application; if
the bad sectors contain sensitive data, shred won't be able to
destroy it.
shred makes no attempt to detect or report these problem, just as
it makes no attempt to do anything about backups. However, since it is
more reliable to shred devices than files, shred by default does
not truncate or remove the output file. This default is more suitable
for devices, which typically cannot be truncated and should not be
removed.
shred [option]... file[...] |
The program accepts the following options. Also see 2. Common options.
shred uses 25 passes of overwrite. This is enough
for all of the useful overwrite patterns to be used at least once.
You can reduce this to save time, or increase it if you have a lot of
time to waste.
shred writes is made up of
random data. If this would be conspicuous on your hard drive (for
example, because it looks like encrypted data), or you just think
it's tidier, the `--zero' option adds an additional overwrite pass with
all zero bits. This is in addition to the number of passes specified
by the `--iterations' option.
This argument is considered an option. If the common `--' option has been used to indicate the end of options on the command line, then `-' will be interpreted as an ordinary file name.
The intended use of this is to shred a removed temporary file. For example
i=`tempfile -m 0600` exec 3<>"$i" rm -- "$i" echo "Hello, world" >&3 shred - >&3 exec 3>- |
Note that the shell command `shred - >file' does not shred the
contents of file, since it truncates file before invoking
shred. Use the command `shred file' or (if using a
Bourne-compatible shell) the command `shred - 1<>file' instead.
You might use the following command to erase all trace of the file system you'd created on the floppy disk in your first drive. That command takes about 20 minutes to erase a 1.44MB floppy.
shred --verbose /dev/fd0 |
Similarly, to erase all data on a selected partition of your hard disk, you could give a command like this:
shred --verbose /dev/sda5 |
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This chapter describes commands which create special types of files (and
rmdir, which removes directories, one special file type).
Although Unix-like operating systems have markedly fewer special file types than others, not everything can be treated only as the undifferentiated byte stream of normal files. For example, when a file is created or removed, the system must record this information, which it does in a directory---a special type of file. Although you can read directories as normal files, if you're curious, in order for the system to do its job it must impose a structure, a certain order, on the bytes of the file. Thus it is a "special" type of file.
Besides directories, other special file types include named pipes (FIFOs), symbolic links, sockets, and so-called special files.
7.1 ln: Make links between filesMake links between files. 7.2 mkdir: Make directoriesMake directories. 7.3 mkfifo: Make FIFOs (named pipes)Make FIFOs (named pipes). 7.4 mknod: Make block or character special filesMake block or character special files. 7.5 rmdir: Remove empty directoriesRemove empty directories.
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ln: Make links between files
ln makes links between files. By default, it makes hard links;
with the `-s' option, it makes symbolic (or soft) links.
Synopses:
ln [option]... target [linkname] ln [option]... target... directory |
ln creates a
link to each target file in that directory, using the
targets' names. (But see the description of the
`--no-dereference' option below.)
ln creates a link from the
second to the first.
ln creates a link to that
file in the current directory.
ln will not remove an existing file. Use the `--backup'
option to make ln rename existing files.
A hard link is another name for an existing file; the link and the original are indistinguishable. Technically speaking, they share the same inode, and the inode contains all the information about a file--indeed, it is not incorrect to say that the inode is the file. On all existing implementations, you cannot make a hard link to a directory, and hard links cannot cross filesystem boundaries. (These restrictions are not mandated by POSIX, however.)
Symbolic links (symlinks for short), on the other hand, are a special file type (which not all kernels support: System V release 3 (and older) systems lack symlinks) in which the link file actually refers to a different file, by name. When most operations (opening, reading, writing, and so on) are passed the symbolic link file, the kernel automatically dereferences the link and operates on the target of the link. But some operations (e.g., removing) work on the link file itself, rather than on its target. See section `Symbolic Links' in The GNU C Library Reference Manual.
The program accepts the following options. Also see 2. Common options.
When the destination is an actual directory (not a symlink to one),
there is no ambiguity. The link is created in that directory.
But when the specified destination is a symlink to a directory,
there are two ways to treat the user's request. ln can
treat the destination just as it would a normal directory and create
the link in it. On the other hand, the destination can be viewed as a
non-directory--as the symlink itself. In that case, ln
must delete or backup that symlink before creating the new link.
The default is to treat a destination that is a symlink to a directory
just like a directory.
Examples:
ln -s /some/name # creates link ./name pointing to /some/name ln -s /some/name myname # creates link ./myname pointing to /some/name ln -s a b .. # creates links ../a and ../b pointing to ./a and ./b |
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mkdir: Make directories
mkdir creates directories with the specified names. Synopsis:
mkdir [option]... name... |
If a name is an existing file but not a directory, mkdir prints a
warning message on stderr and will exit with a status of 1 after
processing any remaining names. The same is done when a name is an
existing directory and the -p option is not given. If a name is an
existing directory and the -p option is given, mkdir will ignore it.
That is, mkdir will not print a warning, raise an error, or change
the mode of the directory (even if the -m option is given), and will
move on to processing any remaining names.
The program accepts the following options. Also see 2. Common options.
chmod and uses `a=rwx' (read, write and execute allowed for
everyone) minus the bits set in the umask for the point of the
departure. See section 3. File permissions.
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mkfifo: Make FIFOs (named pipes)
mkfifo creates FIFOs (also called named pipes) with the
specified names. Synopsis:
mkfifo [option] name... |
A FIFO is a special file type that permits independent processes to communicate. One process opens the FIFO file for writing, and another for reading, after which data can flow as with the usual anonymous pipe in shells or elsewhere.
The program accepts the following option. Also see 2. Common options.
chmod and uses `a=rw' (read and write allowed for everyone) minus
the bits set in the umask for the point of departure. See section 3. File permissions.
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mknod: Make block or character special files
mknod creates a FIFO, character special file, or block special
file with the specified name. Synopsis:
mknod [option]... name type [major minor] |
Unlike the phrase "special file type" above, the term special
file has a technical meaning on Unix: something that can generate or
receive data. Usually this corresponds to a physical piece of hardware,
e.g., a printer or a disk. (These files are typically created at
system-configuration time.) The mknod command is what creates
files of this type. Such devices can be read either a character at a
time or a "block" (many characters) at a time, hence we say there are
block special files and character special files.
The arguments after name specify the type of file to make:
When making a block or character special file, the major and minor device numbers must be given after the file type.
The program accepts the following option. Also see 2. Common options.
chmod and uses `a=rw' minus the bits set in the umask as the point
of departure. See section 3. File permissions.
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rmdir: Remove empty directories
rmdir removes empty directories. Synopsis:
rmdir [option]... directory... |
If any directory argument does not refer to an existing empty directory, it is an error.
The program accepts the following option. Also see 2. Common options.
rmdir to
exit unsuccessfully.
See section 6.5 rm: Remove files or directories, for how to remove non-empty directories (recursively).
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A file is not merely its contents, a name, and a file type (see section 7. Special file types). A file also has an owner (a userid), a group (a group id), permissions (what the owner can do with the file, what people in the group can do, and what everyone else can do), various timestamps, and other information. Collectively, we call these a file's attributes.
These commands change file attributes.
8.1 chown: Change file owner and groupChange file owners and groups. 8.2 chgrp: Change group ownershipChange file groups. 8.3 chmod: Change access permissionsChange access permissions. 8.4 touch: Change file timestampsChange file timestamps.
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chown: Change file owner and group
chown changes the user and/or group ownership of each given file
to new-owner or to the user and group of an existing reference file.
Synopsis:
chown [option]... {new-owner | --reference=ref_file} file...
|
If used, new-owner specifies the new owner and/or group as follows (with no embedded white space):
[owner] [ [:] [group] ] |
Specifically:
chown performs the same function as chgrp.
You may use `.' in place of the `:' separator. This is a
GNU extension for compatibility with older scripts.
New scripts should avoid the use of `.' because GNU chown
may fail if owner contains `.' characters.
The program accepts the following options. Also see 2. Common options.
root might run
find / -owner OLDUSER -print0 | xargs -0 chown NEWUSER |
But that is dangerous because the interval between when the find
tests the existing file's owner and when the chown is actually run
may be quite large.
One way to narrow the gap would be to invoke chown for each file
as it is found:
find / -owner OLDUSER -exec chown NEWUSER {} \;
|
But that is very slow if there are many affected files. With this option, it is safer (the gap is narrower still) though still not perfect:
chown -R --from=OLDUSER NEWUSER / |
lchown system call.
On systems that do not provide the lchown system call,
chown fails when a file specified on the command line
is a symbolic link.
By default, no diagnostic is issued for symbolic links encountered
during a recursive traversal, but see `--verbose'.
lchown system call, and `--no-dereference'
is in effect, then issue a diagnostic saying neither the symbolic link nor
its referent is being changed.
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chgrp: Change group ownership
chgrp changes the group ownership of each given file
to group (which can be either a group name or a numeric group id)
or to the group of an existing reference file. Synopsis:
chgrp [option]... {group | --reference=ref_file} file...
|
The program accepts the following options. Also see 2. Common options.
lchown system call.
On systems that do not provide the lchown system call,
chgrp fails when a file specified on the command line
is a symbolic link.
By default, no diagnostic is issued for symbolic links encountered
during a recursive traversal, but see `--verbose'.
lchown system call, and `--no-dereference'
is in effect, then issue a diagnostic saying neither the symbolic link nor
its referent is being changed.
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chmod: Change access permissions
chmod changes the access permissions of the named files. Synopsis:
chmod [option]... {mode | --reference=ref_file} file...
|
chmod never changes the permissions of symbolic links, since
the chmod system call cannot change their permissions.
This is not a problem since the permissions of symbolic links are
never used. However, for each symbolic link listed on the command
line, chmod changes the permissions of the pointed-to file.
In contrast, chmod ignores symbolic links encountered during
recursive directory traversals.
If used, mode specifies the new permissions. For details, see the section on 3. File permissions.
The program accepts the following options. Also see 2. Common options.
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touch: Change file timestamps
touch changes the access and/or modification times of the
specified files. Synopsis:
touch [option]... file... |
If the first file would be a valid argument to the `-t' option and no timestamp is given with any of the `-d', `-r', or `-t' options and the `--' argument is not given, that argument is interpreted as the time for the other files instead of as a file name. Warning: this usage is obsolescent, and future versions of POSIX will require that support for it be withdrawn. Use `-t' instead.
Any file that does not exist is created empty.
If changing both the access and modification times to the current
time, touch can change the timestamps for files that the user
running it does not own but has write permission for. Otherwise, the
user must own the files.
Although touch provides options for changing two of the times --
the times of last access and modification -- of a file, there is actually
a third one as well: the inode change time. This is often referred to
as a file's ctime.
The inode change time represents the time when the file's meta-information
last changed. One common example of this is when the permissions of a
file change. Changing the permissions doesn't access the file, so
the atime doesn't change, nor does it modify the file, so the mtime
doesn't change. Yet, something about the file itself has changed,
and this must be noted somewhere. This is the job of the ctime field.
This is necessary, so that, for example, a backup program can make a
fresh copy of the file, including the new permissions value.
Another operation that modifies a file's ctime without affecting
the others is renaming. In any case, it is not possible, in normal
operations, for a user to change the ctime field to a user-specified value.
The program accepts the following options. Also see 2. Common options.
touch.
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No disk can hold an infinite amount of data. These commands report on
how much disk storage is in use or available. (This has nothing much to
do with how much main memory, i.e., RAM, a program is using when
it runs; for that, you want ps or pstat or swap
or some such command.)
9.1 df: Report filesystem disk space usageReport filesystem disk space usage. 9.2 du: Estimate file space usageEstimate file space usage. 9.3 sync: Synchronize data on disk with memorySynchronize memory and disk.
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df: Report filesystem disk space usage
df reports the amount of disk space used and available on
filesystems. Synopsis:
df [option]... [file]... |
With no arguments, df reports the space used and available on all
currently mounted filesystems (of all types). Otherwise, df
reports on the filesystem containing each argument file.
Normally the disk space is printed in units of 1024 bytes, but this can be overridden (see section 2.2 Block size).
If an argument file is a disk device file containing a mounted
filesystem, df shows the space available on that filesystem
rather than on the filesystem containing the device node (i.e., the root
filesystem). GNU df does not attempt to determine the disk usage
on unmounted filesystems, because on most kinds of systems doing so
requires extremely nonportable intimate knowledge of filesystem
structures.
The program accepts the following options. Also see 2. Common options.
sync system call before getting any usage data.
This may make df run significantly faster on systems with many
disks, but on some systems (notably SunOS) the results may be slightly
out of date. This is the default.
sync system call before getting any usage data. On
some systems (notably SunOS), doing this yields more up to date results,
but in general this option makes df much slower, especially when
there are many or very busy filesystems.
df.
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du: Estimate file space usage
du reports the amount of disk space used by the specified files
and for each subdirectory (of directory arguments). Synopsis:
du [option]... [file]... |
With no arguments, du reports the disk space for the current
directory. Normally the disk space is printed in units of
1024 bytes, but this can be overridden (see section 2.2 Block size).
The program accepts the following options. Also see 2. Common options.
du --max-depth=0 is equivalent to du -s.
du --exclude='*.o' excludes files whose names
end in `.o'.
On BSD systems, du reports sizes that are half the correct
values for files that are NFS-mounted from HP-UX systems. On HP-UX
systems, it reports sizes that are twice the correct values for
files that are NFS-mounted from BSD systems. This is due to a flaw
in HP-UX; it also affects the HP-UX du program.
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sync: Synchronize data on disk with memory
sync writes any data buffered in memory out to disk. This can
include (but is not limited to) modified superblocks, modified inodes,
and delayed reads and writes. This must be implemented by the kernel;
The sync program does nothing but exercise the sync system
call.
The kernel keeps data in memory to avoid doing (relatively slow) disk
reads and writes. This improves performance, but if the computer
crashes, data may be lost or the filesystem corrupted as a
result. sync ensures everything in memory is written to disk.
Any arguments are ignored, except for a lone `--help' or `--version' (see section 2. Common options).
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| Jump to: | -
4
A B C D E F G H I K L M N O P Q R S T U V W Y |
|---|
| Jump to: | -
4
A B C D E F G H I K L M N O P Q R S T U V W Y |
|---|
| [Top] | [Contents] | [Index] | [ ? ] |
getdate
ls: List directory contents
dir: Briefly list directory contents
vdir: Verbosely list directory contents
dircolors: Color setup for ls
cp: Copy files and directories
dd: Convert and copy a file
install: Copy files and set attributes
mv: Move (rename) files
rm: Remove files or directories
shred: Remove files more securely
ln: Make links between files
mkdir: Make directories
mkfifo: Make FIFOs (named pipes)
mknod: Make block or character special files
rmdir: Remove empty directories
chown: Change file owner and group
chgrp: Change group ownership
chmod: Change access permissions
touch: Change file timestamps
df: Report filesystem disk space usage
du: Estimate file space usage
sync: Synchronize data on disk with memory
| [Top] | [Contents] | [Index] | [ ? ] |
1. Introduction
2. Common options
3. File permissions
4. Date input formats
5. Directory listing
6. Basic operations
7. Special file types
8. Changing file attributes
9. Disk usage
Index
| [Top] | [Contents] | [Index] | [ ? ] |
| Button | Name | Go to | From 1.2.3 go to |
|---|---|---|---|
| [ < ] | Back | previous section in reading order | 1.2.2 |
| [ > ] | Forward | next section in reading order | 1.2.4 |
| [ << ] | FastBack | previous or up-and-previous section | 1.1 |
| [ Up ] | Up | up section | 1.2 |
| [ >> ] | FastForward | next or up-and-next section | 1.3 |
| [Top] | Top | cover (top) of document | |
| [Contents] | Contents | table of contents | |
| [Index] | Index | concept index | |
| [ ? ] | About | this page |