$ ls -l
-rwxr-xr-- 1 alice staff 4096 Mar 14 10:00 script.sh
│├──┤├──┤├──┤
│ │   │   └── Others: read only
│ │   └────── Group: read + execute
│ └────────── Owner: read + write + execute
└──────────── File type (- = file, d = directory, l = symlink)
Permission letters:
r = read    (4)
w = write   (2)
x = execute (1)
- = none    (0)

# Each digit = owner, group, others
# r=4, w=2, x=1, add them up
chmod 755 file    # rwxr-xr-x  (owner: all, group: read+exec, others: read+exec)
chmod 644 file    # rw-r—r—  (owner: read+write, group: read, others: read)
chmod 700 file    # rwx------  (owner: all, nobody else)
chmod 600 file    # rw-------  (owner: read+write, nobody else)
chmod 777 file    # rwxrwxrwx  (everyone: all — usually bad!)
chmod 000 file    # ---------- (nobody can do anything)
How to calculate:
rwx = 4+2+1 = 7
rw- = 4+2+0 = 6
r-x = 4+0+1 = 5
r— = 4+0+0 = 4
--- = 0+0+0 = 0

# Who: u=owner, g=group, o=others, a=all
# Action: +=add, -=remove, ==set exactly
# Permission: r=read, w=write, x=execute
chmod u+x file        # Add execute for owner
chmod g+rw file       # Add read+write for group
chmod o-w file        # Remove write for others
chmod a+r file        # Add read for everyone
chmod u=rwx,g=rx,o=r file  # Set exactly
Common patterns
chmod +x script.sh    # Make executable (for everyone)
chmod -x script.sh    # Remove executable

chmod 644 file.txt     # rw-r--r--  Normal file (owner writes, everyone reads)
chmod 600 secret.key   # rw-------  Private file (owner only)
chmod 755 script.sh    # rwxr-xr-x  Executable script
chmod 700 script.sh    # rwx------  Private executable
chmod 666 shared.txt   # rw-rw-rw-  Everyone can read/write (rare)

chmod 755 mydir/       # rwxr-xr-x  Normal directory
chmod 700 private/     # rwx------  Private directory
chmod 750 team/        # rwxr-x---  Owner + group can access
For directories:
r = list contents (ls)
w = create/delete files inside
x = enter the directory (cd)

SSH is strict about permissions. Wrong permissions = silent failure.

chmod 700 ~/.ssh
chmod 600 ~/.ssh/id_ed25519        # Private key
chmod 644 ~/.ssh/id_ed25519.pub    # Public key
chmod 600 ~/.ssh/config
chmod 600 ~/.ssh/authorized_keys

See also: Git permission denied fix

# Recursive (apply to all files and subdirectories)
chmod -R 755 mydir/
Better: different permissions for files vs directories
find mydir/ -type f -exec chmod 644 {} +    # Files: 644
find mydir/ -type d -exec chmod 755 {} +    # Dirs: 755
Change ownership
chown alice file.txt              # Change owner
chown alice:staff file.txt        # Change owner and group
chown -R alice:staff mydir/       # Recursive
Change group only
chgrp staff file.txt