How to change file permissions with chmod

1. How Linux permissions work
2. Symbolic mode
3. Numeric (octal) mode
4. Recursive changes
5. A word on 777

#How Linux permissions work

Every file and directory on Linux and macOS has permissions for three groups: the user (owner), the group, and others (everyone else). Each group can have three permissions: read (r), write (w), and execute (x).

Run ls -l and you'll see them on the left:

ls -l
-rw-r--r--  1 jane staff  1240 Jun 23 10:00 notes.txt

The first character is the file type (- for a file, d for a directory). The next nine are three blocks of rwx: owner, group, others. Above, the owner can read and write, while group and others can only read.

chmod ("change mode") is how you adjust those bits.

#Symbolic mode

Symbolic mode is the readable way to make targeted changes. You name the target (u user, g group, o others, a all), then +, -, or =, then the permission.

chmod u+x deploy.sh    # give the owner execute
chmod g-w report.txt   # remove write from the group
chmod o=r config.ini   # set others to read-only exactly
chmod a+r public.txt   # give everyone read

The + adds, - removes, and = sets exactly (clearing anything not listed). This is my go-to when I just need to flip one bit. Making a script runnable is so common I gave it its own write-up: make a script executable with chmod +x.

#Numeric (octal) mode

Numeric mode sets all three groups at once. Each digit is the sum of read (4), write (2), and execute (1):

• 7 = rwx (4+2+1)
• 6 = rw- (4+2)
• 5 = r-x (4+1)
• 4 = r--

The three digits are owner, group, others, in that order:

chmod 755 deploy.sh    # rwxr-xr-x — owner full, others read+execute
chmod 644 notes.txt    # rw-r--r-- — owner read+write, others read
chmod 600 secret.key   # rw------- — owner only

755 is the standard for scripts and directories; 644 is standard for regular files.

#Recursive changes

To apply a mode to a directory and everything inside it, add -R:

chmod -R 755 ./public

Be careful here. Files and directories usually want different permissions (directories need execute to be enterable), so a blanket recursive chmod can break things. When that matters I use find to target each type separately:

find ./public -type d -exec chmod 755 {} \;
find ./public -type f -exec chmod 644 {} \;

#A word on 777

You'll see chmod 777 suggested as a quick fix all over the internet. It grants read, write, and execute to everyone, which is almost never what you want. On a shared server it's a real security risk. If something isn't working, the fix is usually correct ownership (chown) or a sensible 644/755, not throwing the doors wide open.