Linux Command Directory
This directory of Linux commands is from Linux in a Nutshell, 5th Edition.
Click on any of the 687 commands below to get a description and list of available options. All links in the command summaries point to the online version of the book on Safari Bookshelf.
Buy it now, or read it online on Safari Bookshelf.
chmod
chmod [options] mode files chmod [options] --reference=filename files
Change the access mode (permissions) of one or more files. Only the owner of a file or a privileged user may change the mode. mode can be numeric or an expression in the form of who opcode permission. who is optional (if omitted, default is a); choose only one opcode. Multiple modes are separated by commas.
Options
Print information about files that are changed.
Do not notify user of files that chmod cannot change.
Print help message and then exit.
Traverse subdirectories recursively, applying changes.
Change permissions to match those associated with filename.
Print information about each file, whether changed or not.
Print version information and then exit.
Who
User.
Group.
Other.
All (default).
Opcode
Add permission.
Remove permission.
Assign permission (and remove permission of the unspecified fields).
Permissions
Read.
Write.
Execute.
Set user (or group) ID.
Sticky bit; used on directories to prevent removal of files by non-owners.
User's present permission.
Group's present permission.
Other's present permission.
Alternatively, specify permissions by a three-digit octal number. The first digit designates owner permission; the second, group permission; and the third, other's permission. Permissions are calculated by adding the following octal values:
Read.
Write.
Execute.
Note that a fourth digit may precede this sequence. This digit assigns the following modes:
Set user ID on execution to grant permissions to process based on the file's owner, not on permissions of the user who created the process.
Set group ID on execution to grant permissions to process based on the file's group, not on permissions of the user who created the process.
Set sticky bit.
Examples
Add execute-by-user permission to file:
chmod u+x file
Either of the following will assign read/write/execute permission by owner (7), read/execute permission by group (5), and execute-only permission by others (1) to file:
chmod 751 file chmod u=rwx,g=rx,o=x file
Any one of the following will assign read-only permission to file for everyone:
chmod =r file chmod 444 file chmod a-wx,a+r file
The following makes the executable setuid, assigns read/write/execute permission by owner, and assigns read/execute permission by group and others:
chmod 4755 file
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