Question

How would I run a command (this is a command line Java app), read its output, and enter input into it, with a shell script. Is that possible? I heard about something called GNU Screen, but when I search with Google for it, it looks like something that won’t help me? I am very confused. So basically
I need to run a command, get its output, send send it input. Note: You need to input ‘exit’ to stop the command, so yes. I am confused here. Pointing me to a tutorial and explanation of GNU Screen or showing me something would be helpful.

Answer 1

It sounds like what you are looking for is Expect, “[…] a tool for automating interactive applications such as telnet, ftp, passwd, fsck, rlogin, tip, etc.” using the Tcl scripting language.

However, if no command you send to the program depends on previous output of the same program, Bash might work well enough by itself; just use temporary files and I/O redirection (as in Usman Saleem’s answer):

• Save a list of commands into a first file; and
• Redirect the program’s standard input to that file; and
• Redirect the standard output to a second file.

The read built-in command may prove useful for reading the generated output file.

GNU screen is just a “terminal multiplexer” program that a) lets you have multiple terminal “windows” over a single modem or SSH connection and b) allows you to keep programs running even after you disconnect from a server. It is not the program you are looking for.

One thing I would advise against is making your shell script both generate the program’s input and accept the program’s output in real time; data will get caught in the stdio output buffers, and deadlocks can happen. This is why Expect was created.