Computer Science › Operating Systems
File Permissions
Read, write and execute rights for user, group and others.
Also known as: chmod, Unix permissions, rwx, file mode
On Linux and macOS, every file has permissions saying who can read it, write it and execute it. They’re set separately for three classes: the owner (user), the owner’s group, and everyone else (others).
ls -l
# -rwxr-xr-- 1 ana devs 1204 Oct 10 09:00 deploy.sh
# │└┬┘└┬┘└┬┘
# │ │ │ └─ others: r-- (read only)
# │ │ └──── group: r-x (read and execute)
# │ └─────── owner: rwx (read, write, execute)
# └───────── file type: - is a regular file, d is a directory| Letter | On a file | On a directory |
|---|---|---|
r | read the contents | list the files in it |
w | change the contents | create and delete files in it |
x | run it as a program | enter it (cd) |
Changing them
chmod +x script.sh # make it executable
chmod u+w,g-w file # owner can write, group can't
chmod 644 notes.txt # numeric form
chmod 755 deploy.sh
chown ana:devs file # change owner and group (usually needs sudo)The numbers add up per class: read = 4, write = 2, execute = 1.
| Mode | Meaning |
|---|---|
644 | owner read/write, everyone else read |
755 | owner all, others read/execute (scripts, directories) |
600 | owner read/write only (private files) |
700 | owner only (private directories) |
Common problems
- “Permission denied” when running a script: it needs
x(chmod +x), or you aren’t the right user. - SSH keys must be private (
chmod 600 ~/.ssh/id_ed25519), or SSH refuses them (SSH keys). - A web server can’t read your files: its user lacks read permission or directory
x. chmod 777“fixes” everything by letting everybody do anything. It’s insecure. Find the right owner and mode instead (least privilege).- Don’t run things as root to get around permissions (sudo and root).
- Containers and mounted volumes can map users differently, causing mismatches.