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Bash Tutorial · Linux System Skills · Lesson 24 of 30

Bash File Permissions

Read and change file permissions with ls -l, chmod and umask, and understand owners and groups.

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Reading permissions

ls -l shows something like -rwxr-x---: the type (- file, d directory), then read/write/execute for the owner, the group and others.

chmod

  • Symbolic: chmod u+x script.sh, chmod go-w file, chmod -R a+r public/
  • Numeric: r=4, w=2, x=1 per class: chmod 755 (rwxr-xr-x), chmod 644 (rw-r--r--), chmod 600 (private)
chmod and numeric modes
touch script.sh secret.key public.txt
chmod 755 script.sh
chmod 600 secret.key
chmod u=rw,go=r public.txt
ls -l script.sh secret.key public.txt | awk '{ print $1, $NF }'
stat -c '%a %n' script.sh secret.key public.txt
umask

Output

-rw-r--r-- public.txt
-rwxr-xr-x script.sh
-rw------- secret.key
755 script.sh
600 secret.key
644 public.txt
0002

Run this on your computer — see “Get Started” for setup.

chown user:group file changes the owner (usually needs sudo). umask sets the default permissions for new files.

Never fix a permissions problem with chmod 777: it lets every user on the machine change the file.

Test yourself

What does chmod 600 key.pem allow?

  1. Everyone can read it
  2. Only the owner can read and write it
  3. Only the owner can execute it
  4. Nobody can access it
Show answer

Answer: Only the owner can read and write it

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