Content-type: text/html; charset=UTF-8 Man page of CHMOD

CHMOD

Section: User Commands (1)
Updated: January 2018
Index Return to Main Contents

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 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: 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'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 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.

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 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.

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]+'.

AUTHOR

Written by David MacKenzie and Jim Meyering.

REPORTING BUGS

GNU coreutils online help: <http://www.gnu.org/software/coreutils/>
Report chmod translation bugs to <http://translationproject.org/team/>

COPYRIGHT

Copyright © 2017 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.

Index

NAME
SYNOPSIS
DESCRIPTION
SETUID AND SETGID BITS
RESTRICTED DELETION FLAG OR STICKY BIT
OPTIONS
AUTHOR
REPORTING BUGS
COPYRIGHT
SEE ALSO

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Time: 04:45:20 GMT, September 16, 2022 Content-type: text/html; charset=UTF-8 Man page of CHMOD

CHMOD

Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
Index Return to Main Contents

NAME

chmod, fchmod, fchmodat - change permissions of a file

SYNOPSIS

#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);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

fchmod():

Since glibc 2.24:
_POSIX_C_SOURCE >= 199309L

Glibc 2.19 to 2.23
_POSIX_C_SOURCE

Glibc 2.16 to 2.19:
_BSD_SOURCE || _POSIX_C_SOURCE

Glibc 2.12 to 2.16:
_BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
_POSIX_C_SOURCE >= 200809L

Glibc 2.11 and earlier:
_BSD_SOURCE || _XOPEN_SOURCE >= 500

fchmodat():

Since glibc 2.10:: _POSIX_C_SOURCE >= 200809L
Before glibc 2.10:: _ATFILE_SOURCE

DESCRIPTION

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'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.

fchmodat()

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:

AT_SYMLINK_NOFOLLOW: 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().

RETURN VALUE

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

ERRORS

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

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

EACCES: Search permission is denied on a component of the path prefix. (See also path_resolution(7).)
EFAULT: pathname points outside your accessible address space.
EIO: An I/O error occurred.
ELOOP: Too many symbolic links were encountered in resolving pathname.
ENAMETOOLONG: pathname is too long.
ENOENT: The file does not exist.
ENOMEM: Insufficient kernel memory was available.
ENOTDIR: A component of the path prefix is not a directory.
EPERM: 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).
EPERM: The file is marked immutable or append-only. (See ioctl_iflags(2).)
EROFS: The named file resides on a read-only filesystem.

The general errors for fchmod() are listed below:

EBADF: The file descriptor fd is not valid.
EIO: See above.
EPERM: See above.
EROFS: See above.

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

EBADF: dirfd is not a valid file descriptor.
EINVAL: Invalid flag specified in flags.
ENOTDIR: pathname is relative and dirfd is a file descriptor referring to a file other than a directory.
ENOTSUP: flags specified AT_SYMLINK_NOFOLLOW, which is not supported.

VERSIONS

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

CONFORMING TO

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

fchmodat(): POSIX.1-2008.

NOTES

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.

COLOPHON

This page is part of release 4.15 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 https://www.kernel.org/doc/man-pages/.