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:

  • r: read
  • w: write (w)
  • x: execute (or search for directories)
  • X: execute/search only if the file is a directory or already has execute permission for some user
  • s: set user or group ID on execution
  • t: restricted deletion flag or sticky bit.

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.


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 underlying chmod system call. When in doubt, check the underlying system behavior.

chmod preserves a directory's 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, and you can set (but not clear) the bits with a numeric mode.


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 regular 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.


On older Unix systems, the sticky bit caused executable files to be hoarded in swap space. This feature is not useful on modern VM systems, and the Linux kernel ignores the sticky bit on files. Other kernels may use the sticky bit on files for system-defined purposes. On some systems, only the superuser can set the sticky bit on files.


When the sticky bit is set on a directory, files in that directory may be unlinked or renamed only by root or their owner. Without the sticky bit, anyone able to write to the directory can delete or rename files. The sticky bit is commonly found on directories, such as /tmp, that are world-writable.

Marking a directory setgid (g+s) will make new files inherit the group ownership of the directory, but the -g option of rsync will attempt to override this.

chmod a+r file
chmod a=r file
umask 023 // specifies default file to 754

chmod u+s file    // set UID access
chmod g+s file    // set GID access

The t access mode tell Unix to keep an executable image in memory even after the process that was using it has exited.

The SUID and SGID access modes: When set on an executable file, processes which run it are granted access to system resource based on the file's owner or group owner, rather than based on the user who created the process.

Save text on directories: If the sticky bit is set on a directory, a user may only delete files that she owns or for which she has explicit write permission.

Setgid on directory: when this mode is set on a directory, it means that files created in that directory will have the same group ownership as the directory itself.

The write access mode on a directory override the file access mode of individual file.

chmod u+t /tmp        // Turns on sticky bit on tmp to avoid users deleting files from each other in common area
chmod g+s /pub/chemz
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