Chmod

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Revision as of 15:02, 19 December 2018 by Nina-loseke-fd7a@rwth-aachen.de (talk | contribs) (Numerical Permissions)
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General

Chmod is the system call used to change the access permissions for files and directories. It is also capable of changing additional permissions or special modes. The current status can be checked with "ls -l".

Command Syntax

The general syntax of the Chmod call is

$ chmod [options] mode[,mode] file1 [file2 ...]
Options
-r Recursive, i.e. include objects in subdirectories
-f force processing to continue if errors occur
-v verbose, show objects changed (unchanged objects are not shown)

Octal modes

The main part of the chmod calls are the octal modes.

Structure:

with

Each triad
first character r: readable
second character w: writable
third character x: executable

s or t: setuid/setgid or sticky (also executable)

S or T: setuid/setgid or sticky (not executable)

"Set user id" (setuid for short) allows users to execute the program with the same rights as its owner. Similarly, "set group id" (setgid) lets group members execute the program with the same permissions as the group owner. The so-called "sticky bit" is deprecated regarding executable files. It used to ensure that the program instructions were kept inside the swap space after the execution had ended. For shared directories, however, the sticky bit guarantees that only the owner of a file is allowed to delete and rename said file. The permissions to read, write or execute a file are not affected by the sticky bit.

The very first character (in front of the first triad) shows the type of the file and cannot be changed with chmod.

Common file types
d a directory
- a file (e.g. executable, document, picture, etc.)
l a link

Numerical Permissions

The file permissions can also be represented in decimal numbers in the chmod call. Up to 4 digits can be set where the leading digit is optional and used to specify the special setuid, setgid, and sticky flags. The remaining 3 digits represent the read, write and execute permissions.

Decimal Permission rwx Binary
7 read, write and execute rwx 111
6 read and write rw- 110
5 read and execute r-x 101
4 read only r-- 100
3 write and execute -wx 011
2 write only -w- 010
1 execute only --x 001
0 none --- 000

For example, the call

$ chmod 777 file1

enables reading, writing and executing for the owner, the group and all other users (use with care!).

Commonly used calls

A few example calls that are commonly used:

Command Explanation
chmod 664 file1 sets read and write permissions for owner and group, and provides read to others.
chmod 744 file1 sets read, write and execute for the owner and read only for the group and all others.
chmod 777 file1 sets read, write and execute for everyone.