Permissions and Users

Permission problems are a lot more common than you expect.

Core commands

ls -l
id                            # show your user ID and group membership
whoami
groups
chmod 640 <file>              # set read/write for owner and read for group
chmod 755 <dir-or-script>     # set common execute permissions for a dir or script
chown <user>:<group> <path>   # change file owner and group
sudo -u <user> <command>      # run a command as another user

What the bits mean

r is read, w is write, x is execute.

The three groups are:

1. Owner
2. Group
3. Everyone else

Example:

-rw-r----- 1 app app config.yaml

That means owner can read and write, group can read, others get nothing.

High-value checks

Check who owns a file:

ls -l /path/to/file

Check the user a service runs as:

systemctl cat <service-name>

Test a command as that user:

sudo -u <user> <command>

Common failure patterns

• Service user cannot read its config
• Service user cannot write to its log or data directory
• Script is present but not executable
• Directory permissions block traversal even when file permissions look fine

Practical rule

When debugging access problems, test as the same user the service runs as. Root succeeding does not prove the app can do the same thing.