chmod

change file mode bits

Syntax

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

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.

Options

--no-preserve-root
       do not treat ‘/’ specially (the default)

-v, --verbose
       output a diagnostic for every file processed

--reference=RFILE
       use RFILE’s mode instead of MODE values

-R, --recursive
              change files and directories recursively

Symbolic Mode

       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.

Octal Mode

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.

Example 1

Remove all permissions from a file for group and others.

[eleiss@eleiss ~]$ ls -l
total 4
-rw-rw-r--. 1 eleiss eleiss 22 Sep  4 19:22 cricket
[eleiss@eleiss ~]$ chmod g-rwx,o-rwx cricket 
[eleiss@eleiss ~]$ ls -l
total 4
-rw-------. 1 eleiss eleiss 22 Sep  4 19:22 cricket
[eleiss@eleiss ~]$

Here permissions are given in symbolic representation.

Example 2

Give all the permissions for all to a file.

[eleiss@eleiss ~]$ ls -l
total 4
-rw-------. 1 eleiss eleiss 22 Sep  4 19:22 cricket
[eleiss@eleiss ~]$ chmod 777 cricket 
[eleiss@eleiss ~]$ ls -l
total 4
-rwxrwxrwx. 1 eleiss eleiss 22 Sep  4 19:22 cricket
[eleiss@eleiss ~]$

Think twice before you do this. This will let everyone read, write and execute your file.


You may write to resources@eleiss.com for any Linux related queries.

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