File Permissions

Understanding and managing file permissions on the cluster.

Understanding Permissions

Linux files have three types of permissions:

Permission	Symbol	Octal	Description
Read	r	4	View file contents
Write	w	2	Modify file contents
Execute	x	1	Run as program/script

Files also have three permission levels:

Level	Description
Owner	User who owns the file
Group	Users in the file’s group
Others	All other users

Important

On ARC systems the ‘other’ bit should most always be set to 0

chmod - Change Permissions

Modify permissions using octal values or their shorthand:

Usage

chmod [OPTION]... MODE FILE...

Common Permission Values

Octal	Symbol	Use Case
750	rwxr-xr-x	Scripts, executables
640	rw-r--r--	Regular files (default)
600	rw-------	Private files
700	rwx------	Private directories

Warning

NEVER use permission mode 777 on files or directories. This grants anyone permissions to change or delete it.

Examples

chmod 755 script.sh    # rwx for owner, rx for group/others
chmod 644 file.txt     # rw for owner, r for group/others
chmod +x script.sh     # Add execute permission
chmod -R 755 dir/      # Recursive permission change
chmod go=u-w           # set permissions for group and other to be the same as user but without write

touch - Create or Update Files

Create empty files or update timestamps:

touch newfile.txt           # Create empty file
touch file1.txt file2.txt   # Create multiple files
touch existing.txt          # Update timestamp

Viewing Permissions

ls -l file.txt
# Output: -rwxr-xr-x 1 username group 1234 Jan 1 12:00 file.txt
#          ^^^^^^^^^
#          Permission string

Tips

• Regular files default to 644 (rw-r–r–)

• Scripts need execute permission (755)

• Use chmod +x to add execute, chmod -x to remove

• Directories need execute permission to be accessible