Script to read user commands from a file for c++ program

Question

I am new to scripting. Facing a problem in trying to do the following. Please help.

My C++ program runs in a while loop, receives commands from the user on a prompt, and executes them. I want to automate this process.

Sample execution

$ ./my_tool

my_prompt> command1 arg1 arg2

result: abc

my_prompt> command2 arg3

result: xyz

my_prompt> command3 abc xyz

result: my_result

my_prompt> exit

$

Now, I want to have a file, input.txt, which will have

command1 arg1 arg2
command2 arg3
command3 <abc> <xyz>
exit

Execution of the script will be something like

./run ./my_tool input.txt

How do I write a simple bash script to achieve this? I will have at least 20 commands.

If I don't have command3, that is, all input parameters are predetermined, then how can I proceed?

Sorry if this is really simple - I am not able to figure out.

Thanks and regards, SB

Nidhoegger · Accepted Answer · 2016-07-17 13:26:33Z

1

You can pass the file line by line to the binary as stdin:

#!/bin/bash

while IFS='' read -r line || [[ -n "$line" ]]; do
    eval $1 < $line
done <<< "$2"

Quick explanation: The IFS thing will make sure the file will be read line by line, otherwise it would create a new iteration on each whitespace. The line ./my_tool < $line will pass $line as stdin to your program so the stdin buffer (where you would read user input from) is filled with the data, so your program reads the "input". done < "$2" will pass the first argument as file to the while loops input, where read will read the input. $2 will be the filename you pass the script upon execution. eval $1 will run the command that is stored in $1 which is your first parameter of the script.

To use it run:

chmod +x myscript.sh # you need to do this only once
./myscript.sh ./my_tool input.txt

edited Jul 17, 2016 at 13:26

answered Jul 16, 2016 at 14:25

Nidhoegger

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3 Comments

Sukanya B Over a year ago

Thanks, but I am getting $line: ambiguous redirect for each line of the input file and exit: No such file or directory on the last line for exit. Am I doing something wrong?

Sukanya B Over a year ago

Changing the last line to done <<< "$2" is working for me. Is that a proper thing to do here?

Sukanya B Over a year ago

Thank you. This is exactly what I needed.

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