How do you use chmod to change file permissions?

Two styles: symbolic like chmod u+x file makes a RELATIVE tweak (add/remove one bit, leave the rest alone), while numeric like chmod 755 file sets ALL nine bits at once to an absolute value.

Pick by intent. Symbolic (u+x, g-w, o=r) is surgical — "add execute for the owner, touch nothing else" — ideal when you don't know or care about the current mode. Numeric is declarative — "the mode IS 755" — ideal when you want a known end state and don't mind overwriting whatever was there. The = operator in symbolic mode is the bridge: chmod u=rwx,g=rx,o= sets exact bits like numeric does.

Symbolic mode: chmod [who][operator][permission] file

Symbolic examples:

chmod u+x file        # Add execute for owner
chmod g-w file        # Remove write from group
chmod o=r file        # Set other to read-only
chmod a+r file        # Add read for everyone
chmod ug+rw file      # Add read+write for user and group

Numeric mode: chmod ### file

Each digit = user, group, other (r=4, w=2, x=1)

# rwxr-xr-x
chmod 755 file
# rw-r--r--
chmod 644 file
# rw-------
chmod 600 file
# rwx------
chmod 700 dir

Common permissions:

Go deeper:

• chmod(1) — symbolic and numeric modes — the [ugoa][+-=][rwx] symbolic syntax vs absolute octal modes.