I hope this is not too obvious a thing to mention, but if you want a really bare bones plaintext password manager on Linux, then use vim with its encryption mode:

To create your list of usernames and passwords, create an encrypted file as follows (you'll be prompted to choose a passphrase):

vim -x /path/to/passwordfile

To access or update your list of usernames and passwords:

vim /path/to/passwordfile

The above autodetects that the file is encrypted (so the -x is no longer needed) and you'll will be prompted for your passphrase.

Modern versions of vim use Bruce Schneier's highly regarded Blowfish algorithm. Type :set cm? while in vim to verify that it reports the algorithm as blowfish or blowfish2.

I haven't tried vim on Android and iOS, so I don't know how convenient this would be on smartphones, but for Linux, I can't think of any simpler way for keeping an encrypted file of freely formatted text.