The chmod command in Unix and Linux is used to change the file mode (permissions) of a file or directory. File permissions determine who can read, write, or execute a file. Understanding and using chmod is essential for maintaining proper security and access control on a system.
Basic Usage
The basic syntax for chmod is:
chmod [options] mode file...
mode: The permissions to set, specified either symbolically or numerically.
file: The file or directory whose permissions you want to change. Multiple files or directories can be specified.
Understanding File Permissions
File permissions are represented as a set of three groups:
User (owner): Permissions for the file's owner.
Group: Permissions for the group associated with the file.
Others: Permissions for all other users.
Each group has three types of permissions:
Read (r): Permission to read the file.
Write (w): Permission to write to the file.
Execute (x): Permission to execute the file (for scripts and binaries).
Symbolic Mode
Symbolic mode uses characters to specify changes to permissions. The format is:
chmod [ugoa][+-=][rwx] file...
u: User (owner)
g: Group
o: Others
a: All (user, group, and others)
+: Add permission
-: Remove permission
=: Set exact permission
Examples
Add Execute Permission for User:
chmod u+x script.sh
Remove Write Permission for Group:
chmod g-w document.txt
Set Read and Write Permissions for All:
chmod a=rw file.txt
Numeric Mode
Numeric mode uses octal numbers to represent permissions. Each permission type is represented by a number:
Read (r): 4
Write (w): 2
Execute (x): 1
The permissions for user, group, and others are combined into a three-digit number:
User: First digit
Group: Second digit
Others: Third digit
Examples
Set Read, Write, and Execute for User; Read and Execute for Group and Others:
chmod 755 file.txt
Set Read and Write for User and Group; Read for Others:
chmod 664 document.txt
Options
-c: Report only when a change is made.
-f: Suppress most error messages.
-v: Output a diagnostic for every file processed.
-R: Operate recursively, changing the permissions of all files and directories within the specified directory.
Examples with Explanations
Verbose Mode:
To change the permissions of file.txt to read, write, and execute for the user, and read and execute for group and others, with verbose output:
chmod -v 755 file.txt
Recursive Change:
To recursively set read and write permissions for user, and read for group and others on all files in mydir:
chmod -R 644 mydir
Suppress Errors:
To suppress error messages while changing permissions of logs/ to read, write, and execute for all users:
chmod -f 777 logs/
Practical Use Cases
Security: Ensuring sensitive files have restricted permissions.
Script Execution: Granting execute permissions to scripts.
Collaboration: Adjusting file permissions for shared projects to ensure proper access.
Summary
The chmod command is crucial for managing file permissions in Unix and Linux systems. By using symbolic and numeric modes, you can precisely control who can read, write, or execute a file, thus maintaining security and proper access control. Understanding chmod enhances your ability to effectively manage a multi-user environment.
help
Usage: chmod [OPTION]... MODE[,MODE]... FILE...
or: chmod [OPTION]... OCTAL-MODE FILE...
or: chmod [OPTION]... --reference=RFILE FILE...
Change the mode of each FILE to MODE.
With --reference, change the mode of each FILE to that of RFILE.
-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 '/' specially (the default)
--preserve-root fail to operate recursively on '/'
--reference=RFILE use RFILE's mode instead of MODE values
-R, --recursive change files and directories recursively
--help display this help and exit
--version output version information and exit
Each MODE is of the form '[ugoa]*([-+=]([rwxXst]*|[ugo]))+|[-+=][0-7]+'.
man
NAME
chmod - change file mode bits
SYNOPSIS
chmod [OPTION]... MODE[,MODE]... FILE...
chmod [OPTION]... OCTAL-MODE FILE...
chmod [OPTION]... --reference=RFILE FILE...
DESCRIPTION
This manual page documents the GNU version of chmod. 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 ef‐
fect 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 exe‐
cution (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 mem‐
bers 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.
SETUID AND SETGID BITS
chmod clears the set-group-ID bit of a regular file if the file's group ID does not match the user's effective group ID or one
of the user's supplementary group IDs, unless the user has appropriate privileges. Additional restrictions may cause the set-
user-ID and set-group-ID bits of MODE or RFILE to be ignored. This behavior depends on the policy and functionality of the un‐
derlying chmod system call. When in doubt, check the underlying system behavior.
For directories chmod preserves set-user-ID and set-group-ID bits unless you explicitly specify otherwise. You can set or clear
the bits with symbolic modes like u+s and g-s. To clear these bits for directories with a numeric mode requires an additional
leading zero, or leading = like 00755 , or =755
SETUID AND SETGID BITS
chmod clears the set-group-ID bit of a regular file if the file's group ID does not match the user's effective group ID or one
of the user's supplementary group IDs, unless the user has appropriate privileges. Additional restrictions may cause the set-
user-ID and set-group-ID bits of MODE or RFILE to be ignored. This behavior depends on the policy and functionality of the un‐
derlying chmod system call. When in doubt, check the underlying system behavior.
For directories chmod preserves set-user-ID and set-group-ID bits unless you explicitly specify otherwise. You can set or clear
the bits with symbolic modes like u+s and g-s. To clear these bits for directories with a numeric mode requires an additional
leading zero, or leading = like 00755 , or =755
RESTRICTED DELETION FLAG OR STICKY BIT
The restricted deletion flag or sticky bit is a single bit, whose interpretation depends on the file type. For directories, it
prevents unprivileged users from removing or renaming a file in the directory unless they own the file or the directory; this is
called the restricted deletion flag for the directory, and is commonly found on world-writable directories like /tmp. For regu‐
lar files on some older systems, the bit saves the program's text image on the swap device so it will load more quickly when
run; this is called the sticky bit.
OPTIONS
Change the mode of each FILE to MODE. With --reference, change the mode of each FILE to that of RFILE.
-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 '/' specially (the default)
--preserve-root
fail to operate recursively on '/'
--reference=RFILE
use RFILE's mode instead of MODE values
-R, --recursive
change files and directories recursively
--help display this help and exit
--version
output version information and exit
Each MODE is of the form '[ugoa]*([-+=]([rwxXst]*|[ugo]))+|[-+=][0-7]+'.