Permissions and Getting Help

Introduction

On Linux and macOS, every file says who may read it, change it or run it. This protects your system, and it explains many "permission denied" errors.

Reading permissions

Run ls -l and look at the start of each line:

-rwxr-xr-- 1 sam staff 120 Nov 6 10:00 script.sh
drwxr-xr-x 2 sam staff  64 Nov 6 09:00 projects

The first character shows the type: - for a file and d for a directory. The next nine characters are three groups of three:

• Owner (user), group, and others
• In each group: r read, w write, x execute (run)

So rwxr-xr-- means the owner can read, write and run, the group can read and run, and everyone else can only read.

Changing permissions with chmod

$ chmod +x script.sh      # make it runnable
$ chmod -w notes.txt      # remove write permission
$ chmod 644 notes.txt     # owner read and write, others read

The numbers add up as read 4, write 2, execute 1. So 6 is read plus write, and 7 is all three. Use the least permission needed. Avoid chmod 777, which lets everyone do everything.

Running as administrator

Some tasks need extra rights. sudo runs one command as the administrator:

$ sudo apt update

It asks for your password. Only use sudo with commands you understand, because it can change or delete important system files.

Getting help

You do not need to memorise everything. Ask the computer:

$ ls --help          # short summary of options
$ man ls             # the full manual page; press q to quit

Searching the web for the exact error message also works well. Read answers carefully before pasting commands.

Try it yourself

Create a file, run ls -l and read its permissions. Remove write permission with chmod -w, then try to add a line with echo "hi" >> file. What happens? Restore it with chmod +w.

Summary

• Permissions control who can read, write and execute each file.
• chmod changes permissions, and sudo runs a command as administrator.
• Use --help and man to learn what a command can do.