How to Use chmod in Linux — File Permission Commands Explained

What chmod does¶

chmod stands for change mode. It modifies the read, write, and execute permissions on a file or directory for three groups: the owning user (u), the owning group (g), and everyone else (o).

Linux represents permissions as a 9-bit string — three bits per group:

-rwxr-xr--
 ^^^    owner: rwx (read + write + execute)
    ^^^ group: r-x (read + execute)
       ^^^ others: r-- (read only)

Symbolic mode¶

The symbolic form uses letters and operators:

# Add execute permission for the owner
chmod u+x script.sh

# Remove write permission from group
chmod g-w report.txt

# Set exact permissions (= replaces, does not add)
chmod u=rw,g=r,o= private.conf

# Apply to everyone
chmod a+r public.html

Operators: + adds, - removes, = sets exactly.

Octal mode¶

The octal form uses three digits, one per group. Each digit is the sum of: 4 (read) + 2 (write) + 1 (execute).

# Common patterns
chmod 755 script.sh      # rwxr-xr-x  — executable, others can read/execute
chmod 644 config.txt     # rw-r--r--  — owner edits, others read
chmod 600 id_rsa         # rw-------  — SSH private key, no group/other access
chmod 700 ~/.ssh         # rwx------  — SSH directory, owner only
chmod 777 /tmp/shared    # rwxrwxrwx  — everyone full access (use with care)

Recursive chmod¶

# Apply recursively to a directory tree
chmod -R 755 /var/www/html

# Fix a common mistake: executable bit set on all files
find /var/www/html -type f -exec chmod 644 {} \;
find /var/www/html -type d -exec chmod 755 {} \;

Tip: Use find with -type f and -type d to apply different permissions to files and directories — recursive chmod -R applies the same mode to both.

Special permission bits¶

Beyond the standard 9 bits, three special bits exist:

chmod u+s /usr/bin/sudo    # setuid — 4755
chmod g+s /shared          # setgid on directory — 2755
chmod +t /tmp              # sticky bit — 1777

Check current permissions¶

ls -l file.txt
# -rw-r--r-- 1 alice devs 1234 May 28 2026 file.txt

stat file.txt
# Access: (0644/-rw-r--r--)

Key takeaways¶

• Symbolic mode (u+x) is easier to read; octal mode (755) is faster to type.
• 644 for files, 755 for directories is the safe default for web content.
• 600 for private keys and sensitive config — no group, no others.
• Prefer find over -R when files and directories need different modes.