chmod

change file access permissions

TLDR

Make file executable

$ chmod +x [script.sh]

Set specific permissions

$ chmod 755 [file]

Remove write permission

$ chmod -w [file]

Recursive change

$ chmod -R 755 [directory/]

Set permissions symbolically

$ chmod u=rwx,g=rx,o=r [file]

chmod (change mode) modifies file access permissions. It controls read, write, and execute permissions for three classes of users: the file owner (user), the file's group, and all other users.
Permissions can be specified in two ways: numeric (octal) mode using digits 0-7, or symbolic mode using letters (u/g/o/a, +/-/=, r/w/x). Numeric mode sets all permissions at once, while symbolic mode allows adding or removing individual permissions. The command is fundamental to Unix file security and access control.

PARAMETERS

-R, --recursive

Change files and directories recursively

-v, --verbose

Verbose output

-c, --changes

Report only changes

--reference=file

Use permissions from reference file

NUMERIC MODE

Octal digits (0-7):
- 4 - Read (r)
- 2 - Write (w)
- 1 - Execute (x)
Common modes:
- 755 - rwxr-xr-x (owner full, others read/execute)
- 644 - rw-r--r-- (owner read/write, others read)
- 600 - rw------- (owner only)
- 777 - rwxrwxrwx (all permissions, usually bad idea)

SYMBOLIC MODE

Format: [ugoa][+-=][rwxXst]
Who:
- u - User (owner)
- g - Group
- o - Others
- a - All
Operation:
- + - Add permission
- - - Remove permission
- = - Set exact permission
Permissions:
- r - Read
- w - Write
- x - Execute
- X - Execute only if directory
- s - Setuid/setgid
- t - Sticky bit

WORKFLOW

$ # Make script executable
chmod +x script.sh

# Set standard file permissions
chmod 644 file.txt

# Set directory permissions
chmod 755 directory/

# Recursive change
chmod -R 755 public_html/

# Remove write for group and others
chmod go-w file.txt

# Add execute for all
chmod a+x program

# Set exact permissions
chmod u=rwx,g=rx,o= private_script

SPECIAL MODES

Setuid (4000):

Run as file owner

Setgid (2000):

Run as file group or inherit directory group

Sticky (1000):

Only owner can delete (common for /tmp)

Example: `chmod 4755 program` (setuid + rwxr-xr-x)

CAVEATS

Requires ownership or root privileges. 777 permissions are security risk. Recursive changes can break systems. Symbolic links not affected (use chmod -h on some systems). Execute on files vs directories different meaning.

HISTORY

chmod has been part of Unix since the early 1970s, implementing the Unix permission model designed by Dennis Ritchie and Ken Thompson.