Changing Permissions: chmod
chmod sets file permissions using either symbolic letters or numeric codes.
Syntax
chmod MODE file
# e.g. chmod 755 file or chmod u+x filechmod changes who can do what with a file. There are two ways to express the new permissions.
Symbolic mode
Combine who (user, group, other, all) with + / - and a permission letter.
chmod u+x file— give the owner execute.chmod go-w file— remove write from group and other.
Numeric (octal) mode
Each permission has a value: read = 4, write = 2, execute = 1. Add them per group to get one digit each for owner, group and other.
755= rwx r-x r-x (common for programs and folders).644= rw- r-- r-- (common for regular files).
Example
# Make a script executable
chmod +x deploy.sh
# Numeric: owner rwx, group & other r-x
chmod 755 deploy.sh
# A private file only the owner can read/write
chmod 600 secret.key
# Apply to a folder and everything inside it
chmod -R 644 website/When to use it
- A developer runs 'chmod +x deploy.sh' to make a shell script executable before running it.
- A sysadmin sets 'chmod 600 ~/.ssh/id_rsa' to restrict a private key to owner-only access, satisfying SSH security requirements.
- A web server admin uses 'chmod -R 755 /var/www/html' to ensure the web root is readable by the web server process.
More examples
Add execute permission symbolically
Uses symbolic mode (u+x, +x) to add execute permission to a script without affecting other bits.
# Add execute for owner only
chmod u+x script.sh
# Add execute for everyone
chmod +x script.sh
ls -l script.sh
# -rwxr-xr-x 1 alice alice 256 Jul 16 script.shSet permissions with octal
Sets permissions using octal notation where each digit sums r(4)+w(2)+x(1) for owner, group, and others.
# 600: owner rw, group none, others none
chmod 600 ~/.ssh/id_rsa
# 755: owner rwx, group r-x, others r-x
chmod 755 /usr/local/bin/mytool
# 644: owner rw, group r, others r
chmod 644 /etc/app/config.iniRecursive permission change
Uses find+chmod for fine-grained recursive changes, applying different modes to files vs directories.
# Set all directories to 755 and files to 644
find /var/www/html -type d -exec chmod 755 {} \;
find /var/www/html -type f -exec chmod 644 {} \;
# Or use chmod -R (applies same mode to everything)
chmod -R 750 /opt/myapp/
Discussion