Permissions

By the end of this lesson you'll be able to read any file's -rwxr-xr-x permission string at a glance, change permissions with chmod (both ways), reassign ownership with chown , use sudo safely, and turn a plain text file into a script you can actually run.

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1️⃣ Reading the rwx Permission String

Run ls -l and every line begins with 10 characters like -rwxr-xr-x . The first character is the file type ( - for a normal file, d for a directory). The remaining nine are three groups of three — user (the owner), group , and other (everyone else) — and within each group the three slots are always read , write , execute in that order. A letter means the permission is granted; a dash - means it isn't.

Octal notation just adds the values: rwx = 4+2+1 = 7 , r-x = 4+0+1 = 5 , r-- = 4+0+0 = 4 . So -rwxr-xr-- is 754 . Three digits, one per group — that's the whole trick.

2️⃣ Changing Permissions with chmod

chmod ("change mode") re-cuts the keys, and it works two ways. Symbolic mode adjusts one thing relative to what's there: chmod u+x file adds ( + ) execute ( x ) for the user ( u ); g is group, o is other, a is all. Octal mode sets all three groups at once to an exact value: chmod 644 file . Use symbolic for a quick tweak, octal to declare a known final state. Run the worked example and watch the string change with each command.

Now compute one yourself. The program below is almost complete — work out the three-digit octal for rwxr-xr-- using read=4, write=2, execute=1 , then fill in the blank.

3️⃣ Making a Script Executable

When you create a script, it's just text — the execute bit is off, so the shell refuses to run it and you get Permission denied . The fix is one command: chmod u+x script.sh adds execute for the owner, and now ./script.sh works. This is one of the most common things you'll do on the command line, so it's worth doing by hand once.

Your turn again. Make a script runnable for its owner, then lock down an SSH key the way SSH demands. Fill in the two blanks:

4️⃣ Changing Ownership with chown

Permissions decide what each audience may do; ownership decides who the owner and group actually are. chown user file changes the owner, and chown user:group file changes both at once. Because reassigning ownership is a privileged action, it almost always needs sudo — you can't quietly hand your files to someone else, or grab theirs. Add -R to apply it through a whole directory tree.

sudo ("superuser do") runs a single command as root — the all-powerful administrator account that ignores every permission check. You need it to install software, edit system files in /etc , or run chown . Root has no safety net: permissions exist partly to protect you from your own typos, and root removes that protection entirely. The classic disaster is a stray space in sudo rm -rf / some/path , which starts deleting the entire filesystem.

Rule of thumb: if a command works without sudo , don't add it. Use the least privilege that gets the job done, and never paste a sudo command you don't understand.

Your group memberships decide which files' group permissions apply to you, so it helps to know who you are:

If you're in the sudo (or wheel ) group, you're allowed to use sudo . If a file's group is devs and you're a member of devs , the file's group permissions apply to you.

No blanks this time — just a brief and an outline to keep you on track. Write the commands yourself, run them in your terminal, and check your output against the example in the comments.

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