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.
• You need execute permission to run a script.