File Permissions
Each file and directory has a set of permissions associated with it. Traditionally, Unix has assigned permissions to the owner (the user), the group (a set of users), others (users not the owner nor a member of the group that may have permissions on the file), and all (all users on the system). These are generally denoted by ugoa.
Within each category, a user can have permission to read, write, or execute the file. These are denoted by rwx.
The command
ls -l
shows the permissions associated with each file. Since in Unix a directory is a special type of file, the permissions are the same.
$ls -l
total 8
drwxr-xr-x. 2 mst3k mst3k 4096 Dec 18 10:39 data
drwxr-xr-x. 2 mst3k mst3k 4096 Dec 18 10:22 shakespeare
The output of ls -l is
| Permissions | Number of “Hard Links” | Owner | Group | Size | Date and Time Last Modified | Name |
|---|---|---|---|---|---|---|
| drwxr-xr-x | 2 | mst3k | mst3k | 4096 | Dec 18 10:39 | data |
Permissions not granted to a user are indicated with a hyphen -.
The permissions layout in the first columns should be read as
| Type | Owner | Group Owner | Others |
|---|---|---|---|
| d | rwx | r-x | r-x |
In the above example, the d indicates the listing is a directory. A directory must have “execute” x permission in order for a user to enter it.
Listing the files in a directory will result in output such as
$ls -l shakespeare
-rwxr-xr-x. 1 mst3k mst3k 91 Dec 18 10:22 2col.txt
-rwxr-xr-x. 1 mst3k mst3k 173940 Dec 18 10:22 Hamlet.txt
-rwxr-xr-x. 1 mst3k mst3k 162563 Dec 18 10:22 HamletWords.txt
-rwxr-xr-x. 1 mst3k mst3k 34 Dec 18 10:22 k2sort.txt
-rwxr-xr-x. 1 mst3k mst3k 180857 Dec 18 10:22 Lear.txt
-rwxr-xr-x. 1 mst3k mst3k 154520 Dec 18 10:22 Othello.txt
-rwxr-xr-x. 1 mst3k mst3k 25628 Dec 18 10:22 uniques
-rwxr-xr-x. 1 mst3k mst3k 25628 Dec 18 10:22 uniques.txt
-rwxr-xr-x. 1 mst3k mst3k 72 Dec 18 10:22 wcdemo.txt
-rwxr-xr-x. 1 mst3k mst3k 26 Dec 18 10:22 words_and_num.txt
The hyphen as Type indicates an ordinary file.
Changing Permissions
The owner of a file or directory may change the permissions with the chmod command. Two formats are supported: a three- or four-digit code (in octal, i.e. base 8) indicating permissions, or the add/remove symbolic format. The digit format is advanced so we will not discuss it; a reference is available
here. The symbolic format is more intuitive and mnemonic.
Examples
Add execute permission to a file for its owner. This is frequently used with shell scripts.
$chmod u+x ascript.sh
Add execute permissions for the owner and group members
$chmod ug+x ascript.sh
Allow others to read and write a file
$chmod o+wr myfile.txt
Please note that on multiuser, central systems such as an HPC cluster, the administrators may not allow individual users to change the permissions on certain file sets such as home and scratch directories, for reasons of data security and privacy.