Frequently Asked Question

What's the difference between symbolic and octal chmod?

Symbolic mode names who you mean (u user, g group, o other, a all) and what you want to do (+ add, - remove, = set exactly) followed by the bits (r, w, x). chmod u+x script.sh adds execute for the owner; chmod g-w,o-w file removes write from group and others; chmod a=rwx file sets every bit on for everyone. Symbolic mode is great for incremental changes because it leaves the bits you do not mention alone.

Octal mode encodes the same nine bits as a three-digit (or four-digit, with special bits) base-8 number: read is 4, write is 2, execute is 1, summed per triplet. So chmod 755 script.sh sets rwxr-xr-x in one go, replacing whatever was there before. Octal is concise, unambiguous, and what installation guides use ("chmod 600 your private key"). Both forms produce identical bits on disk; the crucial difference is that octal replaces all nine bits while a symbolic form like u+x only flips the one bit you named.