chmod
change file mode bits
1. chmod.1.man
Manpage of CHMOD
CHMOD
Section: User Commands (1)Updated: January 2008
Index Return to Main Contents
NAME
chmod - change file mode bitsSYNOPSIS
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.
- -c, --changes
- like verbose but report only when a change is made
- --no-preserve-root
- do not treat `/' specially (the default)
- --preserve-root
- fail to operate recursively on `/'
- -f, --silent, --quiet
- suppress most error messages
- -v, --verbose
- output a diagnostic for every file processed
- --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]))+'.
AUTHOR
Written by David MacKenzie and Jim Meyering.REPORTING BUGS
Report bugs to <bug-coreutils@gnu.org>.COPYRIGHT
Copyright © 2008 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl>This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.
SEE ALSO
chmod(2)The full documentation for chmod is maintained as a Texinfo manual. If the info and chmod programs are properly installed at your site, the command
- info chmod
should give you access to the complete manual.
Index
- NAME
- SYNOPSIS
- DESCRIPTION
- SETUID AND SETGID BITS
- RESTRICTED DELETION FLAG OR STICKY BIT
- OPTIONS
- AUTHOR
- REPORTING BUGS
- COPYRIGHT
- SEE ALSO
This document was created by man2html using the manual pages.
Time: 23:20:44 GMT, July 09, 2008
2. chmod.2.man
Manpage of CHMOD
CHMOD
Section: Linux Programmer's Manual (2)Updated: 2007-07-26
Index Return to Main Contents
NAME
chmod, fchmod - change permissions of a fileSYNOPSIS
#include <sys/stat.h>
int chmod(const char *path, mode_t mode);
int fchmod(int fd, mode_t mode);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fchmod(): _BSD_SOURCE || _XOPEN_SOURCE >= 500
DESCRIPTION
The mode of the file given by path or referenced by fd is changed.Modes are specified by or'ing the following:
-
- S_ISUID
- 04000 set user ID on execution
- S_ISGID
- 02000 set group ID on execution
- S_ISVTX
- 01000 sticky bit
- S_IRUSR
- 00400 read by owner
- S_IWUSR
- 00200 write by owner
- S_IXUSR
- 00100 execute/search by owner
- 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 file system, 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 file systems, 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 stat(2).
On NFS file systems, 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.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.ERRORS
Depending on the file system, other errors 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
- path points outside your accessible address space.
- EIO
- An I/O error occurred.
- ELOOP
- Too many symbolic links were encountered in resolving path.
- ENAMETOOLONG
- path 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).
- EROFS
- The named file resides on a read-only file system.
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.
CONFORMING TO
4.4BSD, SVr4, POSIX.1-2001.SEE ALSO
chown(2), execve(2), fchmodat(2), open(2), stat(2), path_resolution(7)COLOPHON
This page is part of release 2.78 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
Index
This document was created by man2html using the manual pages.
Time: 23:20:44 GMT, July 09, 2008
