File Permissions: rwx, chmod, chown

The 9 bits that decide everything

Every Unix file has three sets of three permission bits: read, write, execute — for owner, group, others. ls -l shows them as rwxr-xr--: owner has all three, group has read+execute, others only read.

Symbolic chmod

chmod u+x file — add execute for owner
chmod g-w file — remove write from group
chmod o= file — remove all permissions for others
chmod a+r file — read for all (a = u+g+o)

Numeric chmod

Each rwx is one octal digit. r=4, w=2, x=1. Add them: rwx=7, rw-=6, r-x=5, r--=4. Three digits = owner/group/others.

chmod 755 script.sh — rwxr-xr-x. Standard for executables.
chmod 644 file.txt — rw-r--r--. Standard for regular files.
chmod 600 ~/.ssh/id_ed25519 — rw-------. Required for SSH private keys.
chmod 700 ~/.ssh — rwx------. SSH dir.

Directories

On a directory, x means "can enter" (cd into), r means "can list contents," w means "can add/remove entries." That's why scripts often need chmod 755 on a dir, not 644.

chown — change owner

chown alice file changes owner to alice. chown alice:staff file sets owner and group. -R for recursive. Often requires sudo because you usually can't give your files away to other users.

umask — default permissions

When you create a file, the kernel uses 0666 & ~umask; for directories 0777 & ~umask. The default 022 umask gives 644 files and 755 dirs. umask 077 makes new files readable only by you — useful in dotfiles where private data lives.

Code

Common chmod patterns·bash

chmod +x script.sh                # add x for everyone
chmod 755 script.sh               # rwxr-xr-x
chmod 644 README.md               # rw-r--r--
chmod 600 ~/.ssh/id_ed25519       # SSH key — must be 600
chmod 700 ~/.ssh                  # SSH dir — must be 700
chmod -R u+rwX,go-w project/      # recursive, smart X for dirs

Decode rwx to octal·text

rwx = 4+2+1 = 7
rw- = 4+2   = 6
r-x = 4+1   = 5
r-- = 4     = 4
rwxr-xr-x = 755
rw-r--r-- = 644