chmod
chmod <who><+|-><rwx> <file> | chmod <755> <file>
Risk: Careful
What it does
Changes the keys on a file: who may read, write or run it.
When to use it
A script says Permission denied, or a secret is open to everyone.
Example
$ chmod +x open.shGive the script its run key.
$ chmod 600 secret.txtOnly you may read and write it.
$ chmod go-r secret.txtTake read away from group and others.
Learn it in: Change permissions: chmod · Official docs