8. Changing file attributes

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 group		Change file owners and groups.
8.2 chgrp: Change group ownership		Change file groups.
8.3 chmod: Change access permissions		Change access permissions.
8.4 touch: Change file timestamps		Change file timestamps.

8.1 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:

owner

If only an owner (a user name or numeric user id) is given, that user is made the owner of each given file, and the files' group is not changed.

owner`:'group

If the owner is followed by a colon and a group (a group name or numeric group id), with no spaces between them, the group ownership of the files is changed as well (to group).

owner`:'

If a colon but no group name follows owner, that user is made the owner of the files and the group of the files is changed to owner's login group.

`:'group

If the colon and following group are given, but the owner is omitted, only the group of the files is changed; in this case, 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.

`-c'

`--changes'

Verbosely describe the action for each file whose ownership actually changes.

`-f'

`--silent'

`--quiet'

Do not print error messages about files whose ownership cannot be changed.

`--from=old-owner'

Change a file's ownership only if it has current attributes specified by old-owner. old-owner has the same form as new-owner described above. This option is useful primarily from a security standpoint in that it narrows considerably the window of potential abuse. For example, to reflect a UID numbering change for one user's files without an option like this, 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 /

`--dereference'

Do not act on symbolic links themselves but rather on what they point to.

`-h'

`--no-dereference'

Act on symbolic links themselves instead of what they point to. This is the default. This mode relies on the 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'.

`--reference=ref_file'

Change the user and group of each file to be the same as those of ref_file. If ref_file is a symbolic link, do not use the user and group of the symbolic link, but rather those of the file it refers to.

`-v'

`--verbose'

Output a diagnostic for every file processed. If a symbolic link is encountered during a recursive traversal on a system without the lchown system call, and `--no-dereference' is in effect, then issue a diagnostic saying neither the symbolic link nor its referent is being changed.

`-R'

`--recursive'

Recursively change ownership of directories and their contents.

8.2 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.

`-c'

`--changes'

Verbosely describe the action for each file whose group actually changes.

`-f'

`--silent'

`--quiet'

Do not print error messages about files whose group cannot be changed.

`--dereference'

Do not act on symbolic links themselves but rather on what they point to.

`-h'

`--no-dereference'

Act on symbolic links themselves instead of what they point to. This is the default. This mode relies on the 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'.

`--reference=ref_file'

Change the group of each file to be the same as that of ref_file. If ref_file is a symbolic link, do not use the group of the symbolic link, but rather that of the file it refers to.

`-v'

`--verbose'

Output a diagnostic for every file processed. If a symbolic link is encountered during a recursive traversal on a system without the lchown system call, and `--no-dereference' is in effect, then issue a diagnostic saying neither the symbolic link nor its referent is being changed.

`-R'

`--recursive'

Recursively change the group ownership of directories and their contents.

8.3 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.

`-c'

`--changes'

Verbosely describe the action for each file whose permissions actually changes.

`-f'

`--silent'

`--quiet'

Do not print error messages about files whose permissions cannot be changed.

`-v'

`--verbose'

Verbosely describe the action or non-action taken for every file.

`--reference=ref_file'

Change the mode of each file to be the same as that of ref_file. See section 3. File permissions. If ref_file is a symbolic link, do not use the mode of the symbolic link, but rather that of the file it refers to.

`-R'

`--recursive'

Recursively change permissions of directories and their contents.

8.4 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.

`-a'

`--time=atime'

`--time=access'

`--time=use'

Change the access time only.

`-c'

`--no-create'

Do not create files that do not exist.

`-d'

`--date=time'

Use time instead of the current time. It can contain month names, timezones, `am' and `pm', etc. See section 4. Date input formats.

`-f'

Ignored; for compatibility with BSD versions of touch.

`-m'

`--time=mtime'

`--time=modify'

Change the modification time only.

`-r file'

`--reference=file'

Use the times of the reference file instead of the current time.

`-t [[CC]YY]MMDDhhmm[.ss]'

Use the argument (optional four-digit or two-digit years, months, days, hours, minutes, optional seconds) instead of the current time. If the year is specified with only two digits, then CC is 20 for years in the range 0 ... 68, and 19 for years in 69 ... 99. If no digits of the year are specified, the argument is interpreted as a date in the current year.