Change Selected Bits Symbolically

The teacher's report already has useful permissions, and she wants to add just group read without clearing other bits. Symbolic mode gives you a way to express that limited change. You select a class, an operation and one or more permissions.

Read g+r as 'for the group class, add read.' The symbols +, - and = have different meanings: add, remove and assign. Choosing between them is the main reasoning step, not simply learning another spelling for a numeric mode.

Prepare the example

Use a new folder and an owned empty report. We first set ordinary mode 600 so the group has no bits. The next change adds one group bit, giving you a clear before-and-after result to inspect.

Bash · Linux terminal

touch marks.txt
chmod 600 marks.txt
chmod g+r marks.txt
ls -l marks.txt

Expected behavior; machine-dependent details

The mode begins -rw-r-----.

chmod 600 marks.txt prepares owner read/write and no group or others bits. chmod g+r marks.txt then adds group read. The resulting ordinary mode is 640, which ls -l displays as the familiar triplets.

Now compare g=r. That assigns the group triplet to read rather than merely adding read, so it removes other ordinary group bits that were present. The current example starts with none, making the result look similar, but the distinction matters on a differently configured file. Select the operator based on what should be preserved.

Symbolic chmod describes a permission change with letters. It is useful when you want to modify one part of a mode without replacing every permission bit.

What is chmod?

The chmod command is used to change the permissions of files and directories in Linux. Permissions decide who can read, write, or execute a file.

Understanding Symbolic Mode

In symbolic mode, we use letters and symbols to describe permissions:

• u = user (owner)
• g = group
• o = others
• a = all (u+g+o)

Then we use operators:

• + = add permission
• - = remove permission
• = = set exact permission

• r = read
• w = write
• x = execute

Let's Practice!

Step 1: Create a sample file

touch demo.txt

Step 2: Check the current permissions

ls -l demo.txt

-rw-r--r-- 1 user user 0 Jul  2 10:00 demo.txt

This means the owner can read and write, and group/others can only read.

Add execute permission for the user

chmod u+x demo.txt

Check again:

ls -l demo.txt

-rwxr--r-- 1 user user 0 Jul  2 10:00 demo.txt

Now the user has execute permission too.

Remove read permission from others

chmod o-r demo.txt

ls -l demo.txt

-rwxr----- 1 user user 0 Jul  2 10:00 demo.txt

Others can no longer read the file.

Set group permissions to exactly read and execute

chmod g=rx demo.txt

ls -l demo.txt

-rwxr-x--- 1 user user 0 Jul  2 10:00 demo.txt

The group now has only read and execute permissions.

Tip: Use a to affect all

chmod a-w demo.txt

ls -l demo.txt

-r-xr-xr-- 1 user user 0 Jul  2 10:00 demo.txt

This removes write permission from everyone.

Target user, group and other permissions symbolically

• chmod in symbolic mode is readable and intuitive
• Use u, g, o, a to choose who you're modifying
• Use +, -, = to set permissions
• Use r, w, x to define permission types

Practice with different combinations and try it out on both files and directories!

Your task: Add group read without requesting group write.

Use a new folder and an owned empty report. We first set ordinary mode 600 so the group has no bits. The next change adds one group bit, giving you a clear before-and-after result to inspect.

Bash · complete example for this question

touch marks.txt
chmod 600 marks.txt
chmod g+r marks.txt
ls -l marks.txt

Expected behavior; machine-dependent details

The mode begins -rw-r-----.

Worked answer

chmod g+r marks.txt

Use chmod g+r marks.txt for the add operation. g+w asks for write instead. g=r requests assignment, which may clear pre-existing group bits. The requirement describes a limited change, so the preservation behavior is part of the answer.

Say the symbolic expression in words before running it: whose bits, which operation, which permission. Adding, removing and replacing are different tasks. Inspect the updated mode afterward, especially when the starting file had other permissions you intended to keep.