What chmod does

chmod changes permissions (read r, write w, execute x) for three audiences:

• u (owner), g (group), o (others), and a (all = ugo)

Permissions mean different things for files vs directories:

• Files: r read, w modify, x run as a program.

• Directories: r list names, w create/delete/rename entries, x enter/traverse.

Check current perms:

ls -l file_or_dir

stat -c '%A %a %n' file_or_dir   # shows symbolic & octal, e.g., rwxr-xr-- 754

Two ways to use chmod

1) Symbolic mode (add/remove/assign bits)

chmod [ugoa][+-=][rwxXst] file

Examples:

chmod u+x script.sh        # owner gets execute

chmod g-w notes.txt        # remove write from group

chmod a+r report.pdf       # everyone gets read

chmod u=rw,go=r file.txt   # owner rw, group/others r

chmod -R a+rX project/     # recursive: +r for all; +X gives x to dirs (and exec files)

Common letters

• r,w,x = read, write, execute

• X = execute only if item is a directory or any execute bit is already set (GNU extension; great for recursive fixes)

• s = setuid/setgid (advanced; see below)

• t = sticky bit (for shared dirs)

2) Octal (numeric) mode (set exact bits)

Digits are sums of r=4, w=2, x=1 for u/g/o:

chmod 755 script.sh   # u=rwx (7), g=rx (5), o=rx (5)

chmod 640 file.txt    # u=rw (6), g=r (4), o=--- (0)

chmod 750 dir/        # typical for private project dirs

Directories vs files (watch the x)

• To enter a directory, you need x on that directory.

• Typical patterns:

chmod 644 *.txt           # files readable by all, editable by owner

chmod 755 bin/ -R         # dirs & (maybe) files executable—often too broad

# Safer recursive pattern:

find myproj -type d -exec chmod 755 {} +    # dirs: 755

find myproj -type f -exec chmod 644 {} +    # files: 644

Recursive changes

chmod -R u+rwX,go-rwx project/   # owner rwX; remove all perms for group/others

chmod -R a+rX shared_docs/       # everyone can read; x only on dirs/execs

Special permission bits (exam-relevant)

Examples:

chmod 4755 /usr/local/bin/prog   # setuid + 755

chmod 2775 /shared/team          # setgid on shared dir

chmod 1777 /some/shared/tmp      # sticky bit on shared tmp dir

Note: setuid on scripts is usually ignored for security. Use with care.

Handy options & tricks

chmod -R … path           # recursive

chmod -v 644 file         # verbose (show changes)

chmod -c 644 file         # report only if changes were made

chmod --reference=REF F   # copy perms from REF to F

The magic X (capital X)

chmod -R a+rX project/    # add execute only to directories (and already-exec files)

This is perfect for fixing a whole tree without making every file executable.

Common patterns you’ll use a lot

# Make a script runnable

chmod u+x run.sh

# Lock down a private dir

chmod 700 ~/.ssh

# Share a project within a group

chgrp -R devs /srv/app

chmod -R 2750 /srv/app        # setgid on dirs + u=rwx,g=rx

# Web content (readable by all, owner edits)

chmod -R u=rwX,go=rX /var/www/html

Typical errors & fixes

• “Operation not permitted” → you’re not the owner (or FS is RO). Use sudo if appropriate, or chown by an admin.

• “Read-only file system” → the mount is read-only (e.g., recovery). Remount RW or choose another location.

• “No such file or directory” → path/glob typo; tab-complete to verify.

• Changed files but can’t enter directory → missing x on the directory; add +x for the audience that needs to traverse.

chmod vs umask

• umask controls default perms at creation time.

• chmod edits perms after creation.
  Common umask for teamwork: 002 (files become 664, dirs 775).

Student-safe guidelines

• Typical defaults: files 644, dirs 755; private work: files 640, dirs 750/700.

• Avoid chmod -R 777 (security & accidental edits). Use groups instead.

• For big trees, treat files and dirs separately (see find example above).

• Prefer a+rX over a+rx when going recursive.

Mini-lab (15–20 min)

mkdir -p ~/lab/chmod/{bin,docs,shared}

echo -e '#!/usr/bin/env bash\necho Hi' > ~/lab/chmod/bin/hello.sh

echo "Notes" > ~/lab/chmod/docs/notes.txt

# 1) scripts and files

chmod 644 ~/lab/chmod/bin/hello.sh

./lab/chmod/bin/hello.sh      # should fail (no x)

chmod u+x ~/lab/chmod/bin/hello.sh && ~/lab/chmod/bin/hello.sh

# 2) fix a tree safely with X

chmod -R a+rX ~/lab/chmod/

find ~/lab/chmod -maxdepth 1 -type d -exec ls -ld {} \;

# 3) shared dir with setgid

sudo groupadd devs 2>/dev/null || true

sudo chgrp devs ~/lab/chmod/shared

chmod 2775 ~/lab/chmod/shared

ls -ld ~/lab/chmod/shared     # look for the 's' in group perms

# 4) copy permissions from a reference file

touch ~/lab/chmod/template && chmod 640 ~/lab/chmod/template

chmod --reference=~/lab/chmod/template ~/lab/chmod/docs/notes.txt

stat -c '%A %a %n' ~/lab/chmod/docs/notes.txt

Exam-ready bullets

• chmod changes perms for u/g/o/a using symbolic (u+x, go-r) or octal (755, 640) modes.

• X adds execute only to dirs (and already-exec files) — great for recursion.

• Recursive: -R; treat dirs and files differently via find.

• Special bits: setuid(4xxx), setgid(2xxx), sticky(1xxx); setgid on dirs → new files inherit group.

• Use ls -l / stat -c '%A %a %n' to verify. Avoid 777; use groups and least privilege.