Linux - The chmod command

3.4.2.1. The chmod command

A normal consequence of applying strict file permissions, and sometimes a nuisance, is that access rights will need to be changed for all kinds of reasons. We use the chmod command to do this, and eventually to chmod has become an almost acceptable English verb, meaning the changing of the access mode of a file. The chmod command can be used with alphanumeric or numeric options, whatever you like best.

The example below uses alphanumeric options in order to solve a problem that commonly occurs with new users:

asim:~> ./hello
bash: ./hello: bad interpreter: Permission denied

asim:~> cat hello
#!/bin/bash
echo "Hello, World"

asim:~> ls -l hello
-rw-rw-r--    1 asim    asim    32 Jan 15 16:29 hello

asim:~> chmod u+x hello

asim:~> ./hello
Hello, World

asim:~> ls -l hello
-rwxrw-r--   1 asim    asim    32 Jan 15 16:29 hello*

The + and - operators are used to grant or deny a given right to a given group. Combinations separated by commas are allowed. The Info and man pages contain useful examples. Here's another one, which makes the file from the previous example a private file to user asim:

asim:~> chmod u+rwx,go-rwx hello

asim:~> ls -l hello
-rwx------    1 asim    asim    32 Jan 15 16:29 hello*

The kind of problem resulting in an error message saying that permission is denied somewhere is usually a problem with access rights in most cases. Also, comments like, "It worked yesterday," and "When I run this as root it works," are most likely caused by the wrong file permissions.

When using chmod with numeric arguments, the values for each granted access right have to be counted together per group. Thus we get a 3-digit number, which is the symbolic value for the settings chmod has to make. The following table lists the most common combinations:

Table 3-9. File protection with chmod

Command	Meaning
chmod `400` `file`	To protect a file against accidental overwriting.
chmod `500` `directory`	To protect yourself from accidentally removing, renaming or moving files from this directory.
chmod `600` `file`	A private file only changeable by the user who entered this command.
chmod `644` `file`	A publicly readable file that can only be changed by the issuing user.
chmod `660` `file`	Users belonging to your group can change this file, others don't have any access to it at all.
chmod `700` `file`	Protects a file against any access from other users, while the issuing user still has full access.
chmod `755` `directory`	For files that should be readable and executable by others, but only changeable by the issuing user.
chmod `775` `file`	Standard file sharing mode for a group.
chmod `777` `file`	Everybody can do everything to this file.

If you enter a number with less than three digits as an argument to chmod, omitted characters are replaced with zeros starting from the left. There is actually a fourth digit on Linux systems, that precedes the first three and sets special access modes. Everything about these and many more are located in the Info pages.