Encrypt a file with a password in Linux

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Encrypt

gpg -c <some-file>

You'll probably be prompted for a password.

Decrypt

gpg -d <some-file>.gpg

You might be prompted for a password here. If not, it means that GPG has cached the password for temporary use. To disable this (or to decrease the cache TTL), create and/or edit the ~/.gnupg/gpg-agent.conf file with these lines:

default-cache-ttl 0
max-cache-ttl 0

The above lines will set the cache TTLs to 0 seconds. Once those changes have been implemented, you'll have to reload the configuration using:

gpgconf --reload all