chmod

chmod

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

  1. Add Execute Permission for User:

    chmod u+x script.sh

  2. Remove Write Permission for Group:

    chmod g-w document.txt

  3. 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

  1. Set Read, Write, and Execute for User; Read and Execute for Group and Others:

    chmod 755 file.txt

  2. 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

  1. 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

  2. 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

  3. 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]+'.
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