Linux file permissions and chmod explained

Every File Has Three Permission Sets

Run ls -l and you see something like -rwxr-xr--. The first character is the file type (- for file, d for directory). The next nine characters are three sets of three: permissions for the owner, the group, and everyone else.

Reading Permission Bits

Each set has three positions: r (read=4), w (write=2), x (execute=1). A dash means the permission is absent. rwx = 4+2+1 = 7. r-x = 4+0+1 = 5. r-- = 4+0+0 = 4.

# Make a script executable
chmod +x deploy.sh

# Set exact permissions using octal
# 755 = owner:rwx group:r-x other:r-x
chmod 755 deploy.sh

# 600 = owner:rw- only (good for private keys)
chmod 600 ~/.ssh/id_rsa

# Change ownership to another user
chown www-data:www-data /var/www/html

# Check current permissions
ls -l deploy.sh

Why Permission Denied Happens

You get Permission denied when you try to read a file without r, write without w, or execute without x. Use sudo to run a command as root — but only when necessary. Avoid running your whole application as root.