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There have been various requests to add -h to avoid following symlinks for security reasons. This wasn't provided previously as chmod(1) already ignored symlinks unless specified on the command line. Note chmod defaults to -H mode rather than the chown default of -P, as usually chown can work directly on symlinks and so defaults to not traversing those specified on the command line. Note FreeBSD chmod does default to -P mode, but we retain the -H mode default also for compatibility with existing chmod behavior. Adding -HLP will allow chmod to disable traversing CLI symlinks to dirs. Adding -h will allow to disable following CLI symlinks to files/dirs, also operating on all symlinks on systems that support that. Adding --dereference will be significant with -H (the default). I.e. symlinks to dirs not recursed, but symlinks are dereferenced. Adding these options will also be consistent with chown(1), chgrp(1), and chmod(1) on other systems. Note since chmod(1) currently ignores symlinks by default, and -h is primarily a mechanism to avoid following symlinks, rather than for operating on the symlink itself, we make -h try to chmod a symlink, but ignore ENOTSUP. In that way we're consistent with chown(1) where it also ignores ENOTSUP for symlinks, and we don't fail when trying to be extra secure with command line params. * doc/coreutils.texi (chmod invocation): Reference the -H,-L,-P descriptions, and adjust the corresponding macros to say the default is -H or -P as appropriate. Add --dereference and -h,--no-dereference descriptions. * man/chmod.x: Adjust discussion of symlink handling. * src/chmod.c (main): Accept new options and set fts flags appropriately. (process_file): Process / dereference symlinks as necessary. * src/system.h (emit_symlink_recurse_options): A new function refactored from chown.c and chmod.c usage(). * tests/chmod/symlinks.sh: New test for the new options. * tests/local.mk: Reference the new test. * NEWS: Mention the new feature.
121 lines
4.7 KiB
Plaintext
121 lines
4.7 KiB
Plaintext
'\" Copyright (C) 1998-2024 Free Software Foundation, Inc.
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'\"
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'\" This is free software. You may redistribute copies of it under the terms
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'\" of the GNU General Public License <https://www.gnu.org/licenses/gpl.html>.
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'\" There is NO WARRANTY, to the extent permitted by law.
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[NAME]
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chmod \- change file mode bits
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[DESCRIPTION]
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This manual page
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documents the GNU version of
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.BR chmod .
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.B chmod
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changes the file mode bits of each given file according to
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.IR mode ,
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which can be either a symbolic representation of changes to make, or
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an octal number representing the bit pattern for the new mode bits.
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.PP
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The format of a symbolic mode is [\c
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\fBugoa\fP.\|.\|.][[\fB-+=\fP][\fIperms\fP.\|.\|.].\|.\|.],
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where
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.I "perms"
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is either zero or more letters from the set
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\fBrwxXst\fP, or a single letter from the set \fBugo\fP.
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Multiple symbolic
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modes can be given, separated by commas.
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.PP
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A combination of the letters \fBugoa\fP controls which users' access
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to the file will be changed: the user who owns it (\fBu\fP), other
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users in the file's group (\fBg\fP), other users not in the file's
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group (\fBo\fP), or all users (\fBa\fP). If none of these are given,
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the effect is as if (\fBa\fP) were
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given, but bits that are set in the umask are not affected.
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.PP
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The operator \fB+\fP causes the selected file mode bits to be added to
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the existing file mode bits of each file; \fB-\fP causes them to be
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removed; and \fB=\fP causes them to be added and causes unmentioned
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bits to be removed except that a directory's unmentioned set user and
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group ID bits are not affected.
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.PP
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The letters \fBrwxXst\fP select file mode bits for the affected users:
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read (\fBr\fP), write (\fBw\fP), execute (or search for directories)
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(\fBx\fP), execute/search only if the file is a directory or already
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has execute permission for some user (\fBX\fP), set user or group ID
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on execution (\fBs\fP), restricted deletion flag or sticky bit
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(\fBt\fP). Instead of one or more of these letters, you can specify
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exactly one of the letters \fBugo\fP: the permissions granted to the
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user who owns the file (\fBu\fP), the permissions granted to other
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users who are members of the file's group (\fBg\fP),
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and the permissions granted to users that are in neither of the two preceding
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categories (\fBo\fP).
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.PP
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A numeric mode is from one to four octal digits (0\-7), derived by
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adding up the bits with values 4, 2, and 1. Omitted digits are
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assumed to be leading zeros.
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The first digit selects the set user ID (4) and set group ID (2) and
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restricted deletion or sticky (1) attributes. The second digit
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selects permissions for the user who owns the file: read (4), write (2),
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and execute (1); the third selects permissions for other users in the
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file's group, with the same values; and the fourth for other users not
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in the file's group, with the same values.
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.PP
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.B chmod
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doesn't change the permissions of symbolic links; the
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.B chmod
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system call cannot change their permissions on most systems,
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and most systems ignore permissions of symbolic links.
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However, for each symbolic link listed on the command line,
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.B chmod
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changes the permissions of the pointed-to file.
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In contrast,
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.B chmod
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ignores symbolic links encountered during recursive directory
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traversals. Options that modify this behavior are described in the
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.B OPTIONS
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section.
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.SH "SETUID AND SETGID BITS"
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.B chmod
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clears the set-group-ID bit of a
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regular file if the file's group ID does not match the user's
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effective group ID or one of the user's supplementary group IDs,
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unless the user has appropriate privileges. Additional restrictions
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may cause the set-user-ID and set-group-ID bits of
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.I MODE
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or
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.I RFILE
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to be ignored. This behavior depends on the policy and
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functionality of the underlying
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.B chmod
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system call. When in
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doubt, check the underlying system behavior.
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.PP
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For directories
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.B chmod
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preserves set-user-ID and set-group-ID bits unless you
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explicitly specify otherwise. You can set or clear the bits with
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symbolic modes like
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.B u+s
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and
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.BR g\-s .
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To clear these bits for directories with a numeric mode requires
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an additional leading zero like
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.BR 00755 ,
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leading minus like
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.BR \-6000 ,
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or leading equals like
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.BR =755 .
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.SH "RESTRICTED DELETION FLAG OR STICKY BIT"
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The restricted deletion flag or sticky bit is a single bit, whose
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interpretation depends on the file type. For directories, it prevents
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unprivileged users from removing or renaming a file in the directory
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unless they own the file or the directory; this is called the
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.I "restricted deletion flag"
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for the directory, and is commonly found on world-writable directories
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like \fB/tmp\fP. For regular files on some older systems, the bit
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saves the program's text image on the swap device so it will load more
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quickly when run; this is called the
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.IR "sticky bit" .
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.SH OPTIONS
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[SEE ALSO]
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chmod(2)
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