[greenyoda]

One major problem with this scheme is that if someone steals (or subpoenas) your computer they can discover all your passwords just by looking at your shell history. A password manager that's secured by a strong password doesn't have that vulnerability.

Edit: As jroes pointed out below, this is not a problem.

[jroes]

They would only discover the domains, not the passwords you entered. You type the domain at the command line, but you type the password into stdin.

[MTGandP]

The program outputs your site-specific password to stdout. Is it possible for an attacker to get your stdout history?

[lloeki]

Might be. Some terminal emulators save logs. "Unlimited scrolling" often goes to disk, possibly in a file, possibly in swap (Terminal.app makes it stay in memory, which once consumed 3GB of RAM for my long-lived "rails server" tab). iTerm2 allows you to navigate back in time, while Terminal.app saves some data on quit to display on restore (if OSX autosave is enabled).

[jaseg]

When you allow less common tools to be used, one could use the bash or zsh read-builtin to read the master password without echoing (e.g. dash's read can not do that) and use xclip or similar to directly put the password into the X clipboard. With xclip's -l option you could even automagically "forget" the clipboard after it was pasted once.