This program is a simple utility to get a password from a terminal with echo disabled. It writes the password to standard output so that it can be used to securely provide a password for programs that accept reading a password from the output of a program.

One feature that sets this apart from other utilities is that it optionally supports accepting multiple lines of input, which is explained in detail below.

This program only accepts input from a terminal. If standard input is not a terminal, it terminates with an error.

askpass [--help] [--echo] [--multi-line] [--no-eol-tx]
Option Summary:
  --help                generate help message
  --echo                Echo '*' to terminal for each accepted input character.
                          Note that when echo and multi-line are both enabled, 
                        either a new-line ('\n') or carriage-return ('\r') on 
                        input results in both '\n' and '\r' echoed to the 
                        terminal insted of a '*'.
  --multi-line          only terminate reading from input upon reading 
                        end-of-transmission (EOT), otherwise either ('\n') or 
                        ('\r') also terminates input
  --no-eol-tx           By default, askpass translates '\r' to '\n' on output. 
                        This option disables that translation.  Note: this 
                        option has no effect unless multi-line mode is enabled.

The 'Enter' or 'Return' key normally generates a '\r'.  You can also generate a '\r' using ^M.
You can generate a '\n' using ^J
You can generate an EOT using ^D.

Askpass reads from the terminal connected to standard input. When multi-line is disabled (the default), askpass terminates input processing when either a newline (\n) or a carriage-return (\r) is entered, or when an EOT is entered. When multi-line is enabled, only a EOT terminates input processing.

To erase a previously typed character, use either the Backspace or Delete key. Depending on terminal settings, ^H may also work.

Note that some keys generate a sequence of characters, depending on terminal settings. So if you are using a such keys for a password on one terminal, entering the same key sequence on another terminal might result in a different sequence of characters being written to standard output.

The use case that resulted in multi-line mode is the bitcoin-cli RPC based program that can read both the RPC password and sensitive RPC commands that require a password from standard input. In this case, you use askpass in multi-line mode and enter the password as the first line, and RPC commands (that also have passwords as parameters) on subsequent lines. This allows all sensitive input to be entered without echoing it to the terminal.

In mult-line mode, pressing the Enter/Return key generates a \r, which askpass by default translates to a \n on output. You can optionally disable this translation using the --no-eol-tx option.

In echo mode, askpass echos a * for any input character it accepts from standard input, except for \r and \n characters which each echo both a \n and \r to the terminal. Typing Backspace, Delete, or ^H (depending on terminal settings) in echo mode erases the previously echoed * character on the terminal, as well as undoing the previously typed character so that it is not written to stanard output.

This program restores terminal settings upon receiving the following signals:

• SIGHUP
• SIGINT
• SIGTERM
• SIGPIPE

• 0 if no error occurs
• signal number if one of the above signals is received
• 100 for invalid invocation syntax
• 101 if standard input is not a terminal
• 102 if an error occurs setting up the terminal or during input processing

In multi-line mode with echo enabled, if you have already entered a \r or \n, typing backspace multiple times such that you erase more characters than are on the current line, the echoing does not reflect that characters on the previous line have been erased from input. They are in fact erased from input and are not written to standard output, but the echoing just doesn't reflect this.

• CMake >= v2.6 (tested with v3.7.2)
• C++ compiler supporting C++-11 (tested with gcc v6.4.0)
• GNU Make (tested with v4.2.1)
• ncurses (tested with v6.0-r1)
• boost (tested with 1.63.0)

• C++ runtime libraries
• ncurses libraries
• boost_program_options library

1. mkdir -p build/default
2. cd build/default
3. cmake ../..
4. make

Output is single executable askpass. Install in whatever directory you want, or use make install to install in CMake's default bin directory /usr/local/bin.