“chmod” command in Linux
In Unix operating systems, the chmod command is used to change the access mode of a file, controlling the permissions such as read, write, and execute for different categories of users (owner, group, others). The name “chmod” is an abbreviation of “change mode.” Let’s explore the syntax, options, and modes available in the chmod command.
Syntax of chmod command
chmod [options] [mode] [file_name]
Options:
- -R: Apply the permission change recursively to all files and directories within the specified directory.
- -v: Display a message for each file that is processed, indicating the permission change.
- -c: Similar to -v, but only displays messages for files whose permission is changed.
- -f: Avoid displaying error messages.
- -h: Change the permissions of symbolic links instead of the files they point to.
Mode:
The permissions to be set, are represented by a three-digit octal number or symbolic notation (e.g., u=rw,go=rx).
Modes in chmod command in Linux
Symbolic mode
In symbolic mode, a combination of letters and operators is used to specify permissions.
Operators: + (add permissions), – (remove permissions), = (set permissions to the specified values).
Letters: r (read), w (write), x (execute).
References: u (Owner), g (Group), o (Others), a (All: owner, groups, others).
Examples
Grant read, write, and execute permissions to the file owner:
chmod u+rwx [file_name]
Remove write permission for the group and others:
chmod go-w [file_name]
Set read and write for owner, and read-only for the group and others:
chmod u+rw,go+r [file_name]
Octal mode
Octal mode specifies permissions using a three-digit number where each digit represents the permission for Owner, Group, and Others. The digits are calculated by adding the values of the individual permissions (4 for read, 2 for write, 1 for execute).
Example
Give read and write permission to the file owner, read, write, and execute permission to the group, and read-only permission to others:
chmod 674 [file_name]
Practical Implementation – Making a script executable in Linux:
To make a script executable, you can use the following steps:
Navigate to the script’s directory
cd /path/to/your/script
Check current permissions:
ls -l
Make the script executable:
chmod +x example.sh
Frequently Asked Questions (FAQs):
Q: How do I view the current permissions of a file or directory?
ls -l
Q: What are the different types of permissions in Linux, and what do they mean?
r: Read Permission
w: Write Permission
x: Execute Permission
These permissions are applied to:
u: Owner
g: Group
o: Others
Q: What is “chmod 777,” “chmod 755,” and “chmod +x” or “chmod a+x”?
chmod 777 [file_name]: Gives all three permissions to everyone (owner, group, others).
chmod a+x [file_name]: Makes a file executable for everyone.
chmod 755 [file_name]: Gives read, write, and execute permissions to the owner, and read-only permissions to the group and others.
Q: How can we revert changes made by “chmod” command in Linux?
To undo changes, use the chmod command again with the correct permission. For example:
chmod 644 [file_or_directory_name]
These steps involve determining the correct permission and applying it using chmod.
In conclusion, the chmod command in Linux is a crucial tool for managing file and directory permissions, providing flexibility through symbolic and octal modes. Understanding and using chmod effectively is essential for maintaining security and control over file access.