Igor Kromin |   Consultant. Coder. Blogger. Tinkerer. Gamer.

I've been having to write more than my usual share of shell scripts lately that orchestrate a number of other command line utilities and other shell scripts. Some of the scripts I've had to run from my script behaved well but some required user input and did not give any options to provide that input as command line arguments. Since I was not able to modify the scripts that I was calling, I had to find a different way to invoke them.

The scripts that I was calling used the 'read' command to prompt the user for input like so:
 External Script
read -p "Please enter input 1: " my_var1
read -p "Please enter input 2: " my_var2
read -p "Please enter input 3: " my_var3

These were interactive scripts but with a predictable pattern for input. Luck was on my side and I knew what each of these inputs should be while my script was being run. All I had to do was provide this input so that the external script would run as if it were a non-interactive script.

There are a number of ways to do the above, I went for something really simple, using echo and piping its output to the external script, like so:
 My Script
echo "MyInput1
" | external_script.sh

Note that the echo string is split across multiple lines, this is completely OK to do. Also note that after my 3rd input value there is a new line, that is because I needed that final return character since the 'read' command in the external script would be expecting that.

The same approach worked when the external script prompted for a password using 'read -s' too.

The above approach worked really well in my case, but it's not suitable for every occasion. In more complex situations the 'expect' command would do a better job, but I'll leave that for another article.


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