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Commands Reference, Volume 1

chmod Command


Changes file modes.


To Change File Modes Symbolically

chmod [ -R ] [ -h ] [ -f ] [ [ u ] [ g ] [ o ] | [ a ] ] { { - | + | = } [ r ] [ w ] [ x ] [ X ] [ s ] [ t ] } { File ... | Directory ... }

To Change File Modes Numerically

chmod [ -R ] [ -h ] [ -f ] PermissionCode { File ... | Directory ... }


The chmod command modifies the mode bits and the extended access control lists (ACLs) of the specified files or directories. The mode can be defined symbolically or numerically (absolute mode).

When a symbolic link is encountered and you have not specified the -h flag, the chmod command changes the mode of the file or directory pointed to by the link and not the mode of the link itself. If you specify the -h flag, the chmod command prevents this mode change.

If you specify both the -h flag and the -R flag, the chmod command descends the specified directories recursively, and when a symbolic link is encountered, the mode of the file or directory pointed to by the link is not changed.


-f Suppresses all error reporting except invalid permissions and usage statements.
-h Suppresses a mode change for the file or directory pointed to by the encountered symbolic link.

Note: This behavior is slightly different from the behavior of the -h flag on the chgrp and chown commands because mode bits cannot be set on symbolic links.
-R Descends only directories recursively, as specified by the pattern File...|Directory.... The -R flag changes the file mode bits of each directory and of all files matching the specified pattern. See Example 6.

When a symbolic link is encountered and the link points to a directory, the file mode bits of that directory are changed but the directory is not further traversed.

Symbolic Mode

To specify a mode in symbolic form, you must specify three sets of flags.

Note: Do not separate flags with spaces.

The first set of flags specifies who is granted or denied the specified permissions, as follows:

u File owner.
g Group and extended ACL entries pertaining to the file's group.
o All others.
a User, group, and all others. The a flag has the same effect as specifying the ugo flags together. If none of these flags are specified, the default is the a flag and the file creation mask (umask) is applied.

The second set of flags specifies whether the permissions are to be removed, applied, or set:

- Removes specified permissions.
+ Applies specified permissions.
= Clears the selected permission field and sets it to the permission specified. If you do not specify a permission following =, the chmod command removes all permissions from the selected field.

The third set of flags specifies the permissions that are to be removed, applied, or set:

r Read permission.
w Write permission.
x Execute permission for files; search permission for directories.
X Execute permission for files if the current (unmodified) mode bits have at least one of the user, group, or other execute bits set. The X flag is ignored if the File parameter is specified and none of the execute bits are set in the current mode bits.

Search permission for directories.

s Set-user-ID-on-execution permission if the u flag is specified or implied. Set-group-ID-on-execution permission if the g flag is specified or implied.
t For directories, indicates that only file owners can link or unlink files in the specified directory. For files, sets the save-text attribute.

Numeric or Absolute Mode

The chmod command also permits you to use octal notation for the mode. The numeric mode is the sum of one or more of the following values:

4000 Sets user ID on execution.
2000 Sets group ID on execution.
1000 Sets the link permission to directories or sets the save-text attribute for files.
0400 Permits read by owner.
0200 Permits write by owner.
0100 Permits execute or search by owner.
0040 Permits read by group.
0020 Permits write by group.
0010 Permits execute or search by group.
0004 Permits read by others.
0002 Permits write by others.
0001 Permits execute or search by others.

  1. Specifying the mode numerically disables any extended ACLs. Refer to "Access Control Lists" in AIX 5L Version 5.2 System User's Guide: Operating System and Devices for more information.
  2. Changing group access permissions symbolically also affects the extended ACL entries. The group entries in the ACL that are equal to the owning group of the file are denied any permission that is removed from the mode. Refer to "Access Control Lists" for more information.
  3. You can specify multiple symbolic modes separated with commas. Operations are performed in the order they appear from left to right.
  4. You must specify the mode symbolically when removing the set-group-ID-on-execution permission from directories.


Access Control: This program should be installed as a normal user program in the Trusted Computing Base.

Only the owner of the file or the root user can change the mode of a file.

Exit Status

This command returns the following exit values:

0 The command executed successfully and all requested changes were made.
>0 An error occurred.


  1. To add a type of permission to several files:

    chmod  g+w  chap1  chap2

    This adds write permission for group members to the files chap1 and chap2.

  2. To make several permission changes at once:

    chmod  go-w+x  mydir

    This denies group members and others the permission to create or delete files in mydir (go-w) and allows group members and others to search mydir or use it in a path name (go+x). This is equivalent to the command sequence:

    chmod  g-w  mydir
    chmod  o-w  mydir
    chmod  g+x  mydir
    chmod  o+x  mydir
  3. To permit only the owner to use a shell procedure as a command:

    chmod  u=rwx,go= cmd

    This gives read, write, and execute permission to the user who owns the file (u=rwx). It also denies the group and others the permission to access cmd in any way (go=).

    If you have permission to execute the cmd shell command file, then you can run it by entering:


    Note: Depending on the PATH shell variable, you may need to specify the full path to the cmd file.
  4. To use Set-ID Modes:

    chmod  ug+s  cmd

    When the cmd command is executed, the effective user and group IDs are set to those that own the cmd file. Only the effective IDs associated with the child process that runs the cmd command are changed. The effective IDs of the shell session remain unchanged.

    This feature allows you to permit access to restricted files. Suppose that the cmd program has the Set-User-ID Mode enabled and is owned by a user called dbms. The user dbms is not actually a person, but might be associated with a database management system. The user betty does not have permission to access any of dbms's data files. However, she does have permission to execute the cmd command. When she does so, her effective user ID is temporarily changed to dbms, so that the cmd program can access the data files owned by the user dbms.

    This way the user betty can use the cmd command to access the data files, but she cannot accidentally damage them with the standard shell commands.

  5. To use the absolute mode form of the chmod command:

    chmod  644  text

    This sets read and write permission for the owner, and it sets read-only mode for the group and others. This also removes all extended ACLs that might be associated with the file.

  6. To recursively descend directories and change file and directory permissions given the tree structure:




    enter this command sequence:

    chmod -R 777 f*

    which will change permissions on ./dir1/file1.

    But given the tree structure of:




    the command sequence:

    chmod -R 777 f*

    will change permissions on:






/usr/bin/chmod Contains the chmod command .

Related Information

The acledit command, aclget command, aclput command, chown command, chgrp command, ls command.

The chmod subroutine, fchmod subroutine.

File Ownership and User Groups in AIX 5L Version 5.2 System User's Guide: Operating System and Devices introduces file ownership and permissions to access files and directories.

Security Administration in AIX 5L Version 5.2 System Management Concepts: Operating System and Devices describes system security.

Installing and Configuring the Trusted Computing Base in AIX 5L Version 5.2 Security Guide.

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