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.