chmod command

chmod [options] permissions filename

Step 1: Decide who you want to permit:

• u → user (the file owner)
• g → group
• o → others (everyone else)
• a → all (user + group + others)

Step 2: Decide what permission you want to give:

• r → read (ability to read the file)
• w → write (ability to edit or delete the file)
• x → execute (ability to run the file/program)

Step 3: Choose how to set the permissions:

1. Symbolic method: Use letters (u, g, o) with +, -, =

• u → user/owner
• g → group
• o → others
• + → adds permission
• - → removes permission
• = → sets exact permission

Example:

chmod u+x file.txt
chmod g-w file.txt
chmod o=r file.txt

Here, when 7 is set for the owner, it gives full rights to read, write, and execute the file, whereas 5 for the group and others means they are limited to just reading and executing it.

Explanation:

• u+x → Owner can now execute the file.
• g-w → Group cannot write to the file anymore.
• o=r → it lets others read the file only, removing all other permissions.

Note: This method is useful when you want to change specific permissions without affecting others.

2. Numeric method: Assign numbers to permissions (r=4, w=2, x=1) and combine them to set access.

Examples:

chmod 755 file.txt  # owner=rwx, group=r-x, others=r-x
chmod 644 file.txt  # owner=rw-, group=r--, others=r--

Explanation:

755 means:

• 7 = 4+2+1 → rwx
• 5 = 4+1 → r-x
• 5 = 4+1 → r-x

When the number 7 is given to the owner, they can read, write, and execute the file, while 5 is given to the group and others, which means they can only read and execute it.

Step 4: Apply the chmod command on the file or folder:

chmod [options] permissions filename

Step 5: Check the permissions to verify:

ls -l file.txt

Output will show something like:

-rwxr-xr-x 1 user group 0 Apr 05 12:00 file.txt