Linux and Unix chmod command

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About chmod
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About chmod

Changes the permissions of a file.

Syntax

chmod [OPTION]... MODE[,MODE]... FILE...
chmod [OPTION]... OCTAL-MODE FILE...
chmod [OPTION]... --reference=RFILE FILE...

Permissions
u - User who owns the file.
g - Group that owns the file.
o - Other.
a - All.
r - Read the file.
w - Write or edit the file.
x - Execute or run the file as a program.

Numeric Permissions:
CHMOD can also to attributed by using Numeric Permissions:

400 read by owner
040 read by group
004 read by anybody (other)
200 write by owner
020 write by group
002 write by anybody
100 execute by owner
010 execute by group
001 execute by anybody

Description

chmod changes the file mode bits of each given file according to mode, which can be either a symbolic representation of changes to make, or an octal number representing the bit pattern for the new mode bits.

The format of a symbolic mode is

[ugoa...][[+-=][perms...]...]

where perms is either zero or more letters from the set rwxXst, or a single letter from the set ugo. Multiple symbolic modes can be given, separated by commas.

A combination of the letters ugoa controls which users' access to the file will be changed: the user who owns it (u), other users in the file's group (g), other users not in the file's group (o), or all users (a). If none of these are given, the effect is as if a were given, but bits that are set in the umask are not affected.

The operator + causes the selected file mode bits to be added to the existing file mode bits of each file; - causes them to be removed; and = causes them to be added and causes unmentioned bits to be removed except that a directory's unmentioned set user and group ID bits are not affected.

The letters rwxXst select file mode bits for the affected users: read (r), write (w), execute (or search for directories) (x), execute/search only if the file is a directory or already has execute permission for some user (X), set user or group ID on execution (s), restricted deletion flag or sticky bit (t). Instead of one or more of these letters, you can specify exactly one of the letters ugo: the permissions granted to the user who owns the file (u), the permissions granted to other users who are members of the file's group (g), and the permissions granted to users that are in neither of the two preceding categories (o).

A numeric mode is from one to four octal digits (0-7), derived by adding up the bits with values 4, 2, and 1. Omitted digits are assumed to be leading zeros. The first digit selects the set user ID (4) and set group ID (2) and restricted deletion or sticky (1) attributes. The second digit selects permissions for the user who owns the file: read (4), write (2), and execute (1); the third selects permissions for other users in the file's group, with the same values; and the fourth for other users not in the file's group, with the same values.

chmod never changes the permissions of symbolic links; 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.

This manual page documents the GNU version of chmod.

Options

chmod changes the mode of each FILE to MODE. With --reference, change the mode of each FILE to that of RFILE. MODE is of the form "[ugoa]*([-+=]([rwxXst]*|[ugo]))+|[-+=][0-7]+".

-c, --changes like verbose, but report only when a change is made.
-f, --silent, --quiet suppress most error messages.
-v, --verbose output a diagnostic for every file processed.
--no-preserve-root do not treat '/' (the root directory) in any special way. This is the default.
--preserve-root Do not operate recursively on '/'.
--reference=RFILE use RFILE's mode instead of MODE values.
-R, --recursive change files and directories recursively.
--help display a help message and exit.
--version output version information and exit.

Examples

chmod 644 file.htm

Set the permission of file.htm to "read/write by owner" and "read only by the group and everyone else" (-rw-r--r--).

Note: Files such as scripts that need to be executed will need execute permissions.

chmod u=w example.jpg

Clear the user permission bit of example.jpg and set it to write-only.

chmod u+s comphope.txt

Set the User ID bit of comphope.txt, so that anyone who attempts to access that file does so as if they are the owner of the file.

chmod u-s comphope.txt

Same as above, but unset the bit.

chmod 755 file.cgi

Set the permission of file.cgi to "read, write, and execute by owner" and "read and execute by the group and everyone else" (-rwxr-xr-x). This would be the following 400+040+004+200+100+010+001 = 755 where you are giving all the rights except the capability for anyone to write to the file.cgi file.

chmod 666 file.txt

Set the permission of file.txt to "read and write by everyone" (-rw-rw-rw-).

 

Tip: The above commands are all done through the command line. However, if you upload a file using FTP these permissions can also be adjusted through many FTP clients by right-clicking the file and choosing permissions.

Additional information

Below is an example of how a file may be listed when typing ( ls -l ) at the prompt as well as information on how to interpret the permissions.

-rw-rw-r-- 1   hope 123   Feb 03 15:36   file.txt

- rw rw- r-- 1 hope 123 Feb 03 15:36 file.txt
File owner group everyone else links owner size mod date file name

Related commands

access
chown
getfacl
ls