Linux File Permissions : chmod, chown, and Fixing Issues

Last week, I was working on a small project on my Ubuntu laptop. I set up a web server (Apache) to host a webpage and wrote a script log.sh to save visitor data into data.txt. When I ran the script in the terminal, it worked fine. But when the web server tried to run it, I got “Permission denied.” I was stuck! After some digging, I learned it was a permissions issue, the web server user (www-data) couldn’t access my files. Here’s what I learned and how I fixed it.

What Are Linux Permissions?

Every file in Linux has rules about who can read, write, or run it. There are three types of users:

• Owner: The person who made the file.

• Group: A group of users.

• Others: Everyone else.

And three types of permissions:

• Read (r): Can view the file.

• Write (w): Can edit the file.

• Execute (x): Can run the file (like a script).

Run ls -l to see permissions. Example:

-rwxr-xr--  1 zasem mygroup  1024 Apr 28 2025 log.sh

• rwxr-xr--: Owner has rwx (read, write, execute), group has r-x (read, execute), others have r-- (read).

• zasem: Owner (shoutout to my buddy who inspired me to start this journey!).

• mygroup: Group.

Using chmod to Change Permissions

chmod changes permissions. I use the number method because it’s easier:

• Read = 4

• Write = 2

• Execute = 1

Add them up for each user. So rwx = 7, rw- = 6, r-x = 5, etc. You use three numbers for owner, group, and others.

For my script, I needed it to be executable:

chmod 755 log.sh

• 7: Owner can do everything (rwx).

• 5: Group can read and execute (r-x).

• 5: Others can read and execute (r-x).

Now the web server could run it!

Using chown to Change Ownership

My script was owned by my user (zasem), but the web server runs as www-data. So, I used chown to change the owner:

sudo chown www-data:www-data log.sh data.txt

This made www-data the owner and group for both files. Now the web server could access them.

Common Permission Problems and Fixes

Here are issues I faced and how I fixed them:

1. “Permission Denied” When Running a Script
   Check with ls -l. If there’s no x, add it:

    chmod +x log.sh
   

   Also, make sure the right user owns the file (use chown if needed).

2. Can’t Write to a File
   My web server couldn’t write to data.txt. I added write permission:

    chmod 664 data.txt
   

   • 6: Owner can read and write.

   • 6: Group can read and write.

   • 4: Others can read.

3. Too Much Access
   I once did chmod 777 (everyone can do everything)big mistake! It’s not safe. Use 755 for scripts or 644 for files instead.

How I Fixed My Issue

Here’s what I did to solve my web server problem:

1. Checked permissions: ls -l log.sh showed no execute permission.

2. Added execute: chmod 755 log.sh.

3. Changed owner: sudo chown www-data:www-data log.sh data.txt.

4. Fixed the log file: chmod 664 data.txt so the web server could write to it.

It worked! My script now logs visitor data without errors.

What I Learned

This taught me to always check permissions with ls -l first. Use chmod to set the right access and chown to fix ownership. Don’t give too much access, stick to what’s needed. I’m getting more confident with Linux now, and I hope this helps you too! If you’ve had permission issues, share them in the comments.