Section (1) chmod

Linux manual pages Section 1  


chmod — change file mode bits


chmod [OPTION...] MODE[,MODE]... FILE...


chmod [OPTION...] −−reference=RFILE FILE...


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_zsingle_quotesz_ access to the file will be changed: the user who owns it (u), other users in the file_zsingle_quotesz_s group (g), other users not in the file_zsingle_quotesz_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_zsingle_quotesz_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 execution (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 members of the file_zsingle_quotesz_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_zsingle_quotesz_s group, with the same values; and the fourth for other users not in the file_zsingle_quotesz_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_zsingle_quotesz_s group ID does not match the user_zsingle_quotesz_s effective group ID or one of the user_zsingle_quotesz_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.

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


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_zsingle_quotesz_s text image on the swap device so it will load more quickly when run; this is called the sticky bit.


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


do not treat _zsingle_quotesz_/_zsingle_quotesz_ specially (the default)


fail to operate recursively on _zsingle_quotesz_/_zsingle_quotesz_


use RFILE_zsingle_quotesz_s mode instead of MODE values

−R, −−recursive

change files and directories recursively


display this help and exit


output version information and exit

Each MODE is of the form _zsingle_quotesz_[ugoa]*([−+=]([rwxXst]*|[ugo]))+|[−+=][0−7]+_zsingle_quotesz_.


Written by David MacKenzie and Jim Meyering.


GNU coreutils online help: <>

Report any translation bugs to <>



Full documentation <>

or available locally via: info _zsingle_quotesz_(coreutils) chmod invocation_zsingle_quotesz_


Copyright © 2019 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <>.

This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.

Section (2) chmod

Linux manual pages Section 2  


chmod, fchmod, fchmodat — change permissions of a file


#include <sys/stat.h>
int chmod( const char *pathname,
  mode_t mode);
int fchmod( int fd,
  mode_t mode);
#include <fcntl.h>            /* Definition of AT_* constants */
#include <sys/stat.h>
int fchmodat( int dirfd,
  const char *pathname,
  mode_t mode,
  int flags);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
Since glibc 2.24:
_POSIX_C_SOURCE >= 199309L
Glibc 2.19 to 2.23 _POSIX_C_SOURCE
Glibc 2.16 to 2.19:
Glibc 2.12 to 2.16:
Glibc 2.11 and earlier:
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:


The chmod() and fchmod() system calls change a files mode bits. (The file mode consists of the file permission bits plus the set-user-ID, set-group-ID, and sticky bits.) These system calls differ only in how the file is specified:

  • chmod() changes the mode of the file specified whose pathname is given in pathname, which is dereferenced if it is a symbolic link.

  • fchmod() changes the mode of the file referred to by the open file descriptor fd.

The new file mode is specified in mode, which is a bit mask created by ORing together zero or more of the following:

S_ISUID (04000)

set-user-ID (set process effective user ID on execve(2))

S_ISGID (02000)

set-group-ID (set process effective group ID on execve(2); mandatory locking, as described in fcntl(2); take a new file_zsingle_quotesz_s group from parent directory, as described in chown(2) and mkdir(2))

S_ISVTX (01000)

sticky bit (restricted deletion flag, as described in unlink(2))

S_IRUSR (00400)

read by owner

S_IWUSR (00200)

write by owner

S_IXUSR (00100)

execute/search by owner (search applies for directories, and means that entries within the directory can be accessed)

S_IRGRP (00040)

read by group

S_IWGRP (00020)

write by group

S_IXGRP (00010)

execute/search by group

S_IROTH (00004)

read by others

S_IWOTH (00002)

write by others

S_IXOTH (00001)

execute/search by others

The effective UID of the calling process must match the owner of the file, or the process must be privileged (Linux: it must have the CAP_FOWNER capability).

If the calling process is not privileged (Linux: does not have the CAP_FSETID capability), and the group of the file does not match the effective group ID of the process or one of its supplementary group IDs, the S_ISGID bit will be turned off, but this will not cause an error to be returned.

As a security measure, depending on the filesystem, the set-user-ID and set-group-ID execution bits may be turned off if a file is written. (On Linux, this occurs if the writing process does not have the CAP_FSETID capability.) On some filesystems, only the superuser can set the sticky bit, which may have a special meaning. For the sticky bit, and for set-user-ID and set-group-ID bits on directories, see inode(7).

On NFS filesystems, restricting the permissions will immediately influence already open files, because the access control is done on the server, but open files are maintained by the client. Widening the permissions may be delayed for other clients if attribute caching is enabled on them.


The fchmodat() system call operates in exactly the same way as chmod(), except for the differences described here.

If the pathname given in pathname is relative, then it is interpreted relative to the directory referred to by the file descriptor dirfd (rather than relative to the current working directory of the calling process, as is done by chmod() for a relative pathname).

If pathname is relative and dirfd is the special value AT_FDCWD, then pathname is interpreted relative to the current working directory of the calling process (like chmod()).

If pathname is absolute, then dirfd is ignored.

flags can either be 0, or include the following flag:


If pathname is a symbolic link, do not dereference it: instead operate on the link itself. This flag is not currently implemented.

See openat(2) for an explanation of the need for fchmodat().


On success, zero is returned. On error, −1 is returned, and errno is set appropriately.


Depending on the filesystem, errors other than those listed below can be returned.

The more general errors for chmod() are listed below:


Search permission is denied on a component of the path prefix. (See also path_resolution(7).)


pathname points outside your accessible address space.


An I/O error occurred.


Too many symbolic links were encountered in resolving pathname.


pathname is too long.


The file does not exist.


Insufficient kernel memory was available.


A component of the path prefix is not a directory.


The effective UID does not match the owner of the file, and the process is not privileged (Linux: it does not have the CAP_FOWNER capability).


The file is marked immutable or append-only. (See ioctl_iflags(2).)


The named file resides on a read-only filesystem.

The general errors for fchmod() are listed below:


The file descriptor fd is not valid.


See above.


See above.


See above.

The same errors that occur for chmod() can also occur for fchmodat(). The following additional errors can occur for fchmodat():


dirfd is not a valid file descriptor.


Invalid flag specified in flags.


pathname is relative and dirfd is a file descriptor referring to a file other than a directory.


flags specified AT_SYMLINK_NOFOLLOW, which is not supported.


fchmodat() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.


chmod(), fchmod(): 4.4BSD, SVr4, POSIX.1-2001i, POSIX.1-2008.

fchmodat(): POSIX.1-2008.


C library/kernel differences

The GNU C library fchmodat() wrapper function implements the POSIX-specified interface described in this page. This interface differs from the underlying Linux system call, which does not have a flags argument.

Glibc notes

On older kernels where fchmodat() is unavailable, the glibc wrapper function falls back to the use of chmod(). When pathname is a relative pathname, glibc constructs a pathname based on the symbolic link in /proc/self/fd that corresponds to the dirfd argument.


chmod(1), chown(2), execve(2), open(2), stat(2), inode(7), path_resolution(7), symlink(7)


This page is part of release 4.16 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at−pages/.

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