student@ubuntu:~$
shell 3/5 35 XP

File Permissions

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Quick Reference

Permission values:

Symbol Meaning Octal
r Read 4
w Write 2
x Execute 1
- No permission 0

Reading a permission string:

-    rwx    r-x    r-x
│    │      │      └── other: read + execute
│    │      └── group: read + execute
│    └── user/owner: read + write + execute
└── file type (- = file, d = directory, l = symlink)

Common permission patterns:

Octal String Typical Use
755 rwxr-xr-x Executables, directories
644 rw-r--r-- Regular files (source code, text)
700 rwx------ Private directory
600 rw------- Private file (SSH keys, configs)

chmod syntax:

Form Example Meaning
Symbolic chmod u+x file Add execute for user
Symbolic chmod g-w file Remove write for group
Symbolic chmod o=r file Set other to read only
Symbolic chmod a+r file Add read for all
Octal chmod 755 file Set rwxr-xr-x
Octal chmod 644 file Set rw-r–r–

File vs. directory permissions:

Permission On a File On a Directory
r (read) View contents (cat, less) List contents (ls)
w (write) Modify contents Create/delete files inside
x (execute) Run as a program Enter the directory (cd)

How It Works

Reading Permissions in ls -l

Terminal
student@ubuntu:~/cscd240$ ls -l
-rw-r--r-- 1 student student 78 Mar 30 09:00 hello.c
-rwxr-xr-x 1 student student 8240 Mar 30 09:01 hello
drwxr-xr-x 2 student student 4096 Mar 30 08:45 labs

Changing Permissions

Terminal
# Symbolic: add execute for owner
student@ubuntu:~/cscd240$ chmod u+x script.sh

# Symbolic: remove write for group and others
student@ubuntu:~/cscd240$ chmod go-w hello.c

# Octal: standard executable permissions
student@ubuntu:~/cscd240$ chmod 755 hello

# Octal: private directory
student@ubuntu:~/cscd240$ chmod 700 private_dir

# Verify
student@ubuntu:~/cscd240$ ls -l hello
-rwxr-xr-x 1 student student 8240 Mar 30 09:01 hello

Octal is Binary in Disguise

rwx = 111 = 7    r-x = 101 = 5    r-- = 100 = 4
rw- = 110 = 6    -wx = 011 = 3    --- = 000 = 0

Common Pitfalls

  • chmod 777 as a fix-all – Gives everyone full access. Find the real problem instead.
  • Forgetting x on directories – A directory needs x to cd into it. r alone lets you see names but not access files.
  • Confusing file and directory permissions – w on a directory means create/delete files inside, not modify the directory itself.
  • Not checking permissions after compilation – gcc sets the execute bit automatically, but if you copy a binary from elsewhere, you may need chmod u+x.

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