The chmod Command in Linux: Symbolic and Numeric Format, SUID, SGID, and Sticky Bit

Copied a script to the server — it does not run. Uploaded a file via FTP — cannot read it. Created a directory — another user cannot write to it. Nine times out of ten the cause is the same: wrong file permissions. chmod fixes that.

How Permissions Work in Linux

Every file has three permission groups — for the owner (u), the group (g), and everyone else (o). Each group has three bits:

• r — read
• w — write
• x — execute

Check current permissions:

ls -l filename

The first 9 characters after - are the permissions: first three for the owner (rwx), next three for the group (r-x), last three for others (r--).

Symbolic Format: Readable and Precise

To add or remove a specific permission — use symbolic format. + adds, - removes, = sets exactly.

Give the owner execute permission:

chmod u+x script.sh

Remove write permission from group and others:

chmod go-w file.txt

Give the group read and execute:

chmod g+rx file.txt

Set permissions for all three at once (ugo or the shorthand a):

chmod a+r file.txt

Set exact permissions for the group — read only, remove everything else:

chmod g=r file.txt

Change permissions for multiple groups in one command:

chmod u+x,go-x script.sh

Numeric Format: Fast and Universal

Each permission has a numeric value: r=4, w=2, x=1. Permissions are summed:

Three digits — for owner, group, others. Most common masks:

Owner full access (7), group and others read and execute (5). Standard for executables and web server directories:

chmod 755 script.sh

Owner read and write (6), others read only (4). Standard for configuration files:

chmod 644 config.conf

Owner only reads and writes. Required minimum for SSH keys — without this ssh refuses to use them:

chmod 600 private_key

Full access for everyone. Use only where genuinely needed:

chmod 777 file.txt

Recursive Change: the -R Flag

Change permissions for an entire directory and all its contents:

chmod -R 755 /var/www/mysite

The -v flag shows what changed:

chmod -Rv 755 /var/www/mysite

The -c flag — only files whose permissions actually changed (not those already at 755):

chmod -Rc 755 /var/www/mysite

A common server task — set 755 for directories and 644 for files separately. Via find:

find /var/www/mysite -type d -exec chmod 755 {} +
find /var/www/mysite -type f -exec chmod 644 {} +

Special Bits: SUID, SGID, Sticky

SUID (Set User ID) — the file runs with the owner's permissions rather than the permissions of the user who launched it. Used for system utilities like passwd — a regular user runs it under their own account, but it operates with root privileges.

Set SUID:

chmod u+s /usr/bin/program

In ls -l output, s appears instead of x for the owner: -rwsr-xr-x.

SGID (Set Group ID) — on a directory, new files inside inherit the directory's group rather than the creating user's group. Useful for shared team folders:

chmod g+s /shared/team

Sticky bit — on a directory, prevents users from deleting each other's files. Classic example — /tmp: anyone can create a file, but only the owner can delete it.

chmod +t /shared/uploads

In ls -l output, t appears at the end: drwxrwxrwt.

Numeric values for special bits — a fourth leading digit:

chmod 4755 /usr/bin/program   # SUID
chmod 2775 /shared/team       # SGID
chmod 1777 /tmp               # Sticky bit

Quick Reference