chmod — Linux basic command

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A command that changes the access permissions of files and directories.

Basic Information

What is this command?

chmod is an abbreviation for change mode, a command that changes access permissions for files and directories. In Linux, permissions for reading (r), writing (w), and executing (x) are granted individually for the owner, group, and others.

Permissions can be specified in two ways: numeric (octal) mode and symbolic mode, and it is most frequently used for granting execute permissions to scripts.

What problem does it solve?

• Grants execute permission to script files.
• Strengthens file access permissions.
• Sets shared directory permissions.
• Corrects files with incorrectly granted permissions.

Key Features

Numeric (octal) Mode

Permissions are specified numerically using the combination r=4, w=2, x=1.

• Usage scenario: When setting exact permissions at once.
• Result: Example: 755 = owner (rwx), group (r-x), others (r-x)
• Limitations: If numbers are specified incorrectly, permissions may become too open.

Symbolic Mode

Uses +, -, = operators for u (owner), g (group), o (others), a (all).

• Usage scenario: When changing permissions for a specific target.
• Result: Example: chmod u+x script.sh (grant execute permission to owner)
• Limitations: Symbolic mode may be harder to read than numeric.

Recursive Change (-R)

Applies permissions to the directory and all files beneath it.

• Usage scenario: When changing permissions for an entire directory.
• Result: All sub-files are changed as well.
• Limitations: Excessive recursive changes may cause system issues.

Reference File Copy (--reference)

Copies permissions from another file as-is.

• Usage scenario: When consistent permission settings are needed.
• Result: The same permissions as the reference file are applied.
• Limitations: May not be supported on some file systems.

Installation and Execution

Check Pre-installation

Included by default in all Linux distributions.

which chmod
chmod --version

Command to Check Permissions (ls -l)

To check current permissions, use ls -l.

ls -l file.txt
# -rw-r--r-- 1 user group 1024 Oct 18 12:00 file.txt
#  ^^^ Owner permissions
#      ^^^ Group permissions
#          ^^^ Others' permissions

Real-world Usage Examples

Change Permissions in Numeric Mode

Set permissions in octal.

# Grant execute permission to script (755)
chmod 755 script.sh

# Read/write only (600)
chmod 600 secret.key

# Read for all (644)
chmod 644 public-file.txt

# Directory traverse permission (755)
chmod 755 /var/www/html

Change Permissions in Symbolic Mode

Change permissions for a specific target only.

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

# Remove execute permission from others
chmod o-x script.sh

# Add read permission to group
cat /etc/hosts | sudo tee /var/log/shared.txt
group_name="developers"
chmod g+r /var/log/shared.txt
chgrp $group_name /var/log/shared.txt

# Set read permission for all
chmod a=r file.txt

Recursive Permission Change

Apply permissions to the entire directory.

# Grant execute permission to directory and sub-files
chmod -R a+x /opt/scripts/

# Change only directory permissions (keep files)
chmod 755 /var/www/html/
find /var/www/html -type f -exec chmod 644 {} \;
find /var/www/html -type d -exec chmod 755 {} \;