File permissions

Who can do what

• Every file has permissions that say who may read, write, or execute it.
• ls -l shows them on the left of each line:

ls -l script.sh

• You see something like -rwxr-xr-x. Read it in groups of three.

Reading the flags

• The ten characters are: type, then owner, group, others:

-  rwx  r-x  r-x
file owner group others

• r = read, w = write, x = execute, - = not allowed. So the owner here can do everything; others can read and run.

Changing permissions

• chmod changes the mode. Numbers are a quick way: r=4, w=2, x=1, added per group:

chmod 755 script.sh

• 7 = 4+2+1 (rwx) for the owner; 5 = 4+1 (r-x) for group and others. 755 is the classic "runnable program".

Common mistakes

• chmod sets read, write and execute for user, group and others.
• On a real system, direct execution such as ./script.sh needs execute permission and a suitable interpreter. sh script.sh instead asks the shell to read the file, so the script itself needs read permission, not its execute bit. Our simulator displays modes but does not execute scripts.