Unix File Permissions

Overview

This document provides an overview of the Unix file permission model, which governs access to files and directories on REPACSS systems. Each object in the file system is associated with an owner and permission flags that define read, write, and execute privileges for different user classes.

Permissions are typically reviewed using the ls -l command:

ls -l

Example output:

drwx------ 2 bsencer bsencer  2048 Jun 12 2012 private
-rw------- 1 bsencer bsencer  1327 Apr  9 2012 try.f90
-rwx------ 1 bsencer bsencer 12040 Apr  9 2012 a.out
drwxr-xr-x 3 bsencer bsencer  2048 Nov 13 2011 public

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File Permission Structure

Each line of output in ls -l contains a 10-character string representing the file type and permission bits:

Position	Description
1	File type: d (directory) or - (regular file)
2–4	Permissions for the user (owner)
5–7	Permissions for the group (not evaluated in current REPACSS scope)
8–10	Permissions for others (i.e., all users)

Permission flags are defined as follows:

• r: Read permission
• w: Write permission
• x: Execute permission
• -: Permission not granted

Special flags such as s (setgid) are not used in standard user scenarios on REPACSS.

Interpreting Permission Strings

• drwx------: A directory accessible only by the owner
• -rw-------: A file readable and writable only by the owner
• -rwx------: An executable file restricted to the owner
• drwxr-xr-x: A directory accessible to all users for reading and execution, but writable only by the owner

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Managing Default Permissions: umask

The umask command controls the default permission settings for newly created files and directories. The following table outlines common umask values and their corresponding default permissions:

umask	File Permissions	Directory Permissions
002	rw-rw-r--	rwxrwxr-x
007	rw-rw----	rwxrwx---
022	rw-r--r--	rwxr-xr-x
027	rw-r-----	rwxr-x---
077	rw-------	rwx------

Users may configure their preferred umask value by adding the command to their .bash_profile.

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Modifying Permissions: chmod

The chmod utility is used to alter file and directory permissions. This can be done using either octal or symbolic notation.

Octal Notation

Octal Value	Permission Bits	Description
0	---	No permissions
1	--x	Execute only
2	-w-	Write only
3	-wx	Write and execute
4	r--	Read only
5	r-x	Read and execute
6	rw-	Read and write
7	rwx	All permissions

Example:

chmod 755 file

This sets the file to be fully accessible by the owner, and readable and executable by others (rwxr-xr-x).

Symbolic Notation

Users may also modify permissions with symbolic notation:

chmod u+x,go+rx file

This command grants execute permission to the user, and read/execute permissions to group and others.

Class	Definition
u	User (owner)
o	Other (non-owners)
a	All users

Operator	Meaning
+	Add permission
-	Remove permission
=	Set exact permission

Mode	Action
r	Read
w	Write
x	Execute
X	Conditional execute (directories or if already executable)

Recursive operations may be executed as follows:

chmod -R o+rX directory/

This command grants read and execute access to others for all applicable files and directories within the specified directory.

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Summary

• Use ls -l to audit file and directory permissions.
• Apply chmod to update access control.
• Configure umask to enforce default file security.
• Sensitive files should be explicitly protected using:

chmod 600 sensitive_file

This ensures exclusive read/write access to the owner and denies access to all other users.

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