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Ubuntu – How to display input sent to console program from a bash script?

My console program runs under Ubuntu 20.04. When run interactively, the input expression (e.g., a+b below) is displayed next to the prompt followed by the output on a new line as follows. I would like to achieve this same format when providing the input to the program from a bash script.

$ ./rec_desc1
Enter expression: a+b
F --> id
R --> eps
T --> FR
F --> id
R --> eps
T --> FR
Q --> eps
Q --> +TQ
E --> TQ
Success.

But if I send the input from a bash script via here-string or piping it (as suggested at How do I provide input to a C program from bash?), the input is not displayed and unfortunately the first line of output is shifted up next to prompt as shown below.

cat rec_desc1_unit_test
#!/bin/bash
echo "Test case 1"
./rec_desc1 <<< a+b

$ ./rec_desc1_unit_test
Test case 1
Enter expression: F --> id
R --> eps
T --> FR
F --> id
R --> eps
T --> FR
Q --> eps
Q --> +TQ
E --> TQ
Success.

Is there a way to pass the input from a bash script so that it displays next to the prompt and is followed by a newline so it matches the interactive case above?

I eventually want to run numerous test cases for different expressions from a bash script and would like to have the self-documentation of the input expression being displayed. I could call set -v prior to invoking the console program in order to display the input if necessary (as shown below). Then I would need to call set +v immediately afterward to avoid displaying the echo statements that describe the next test case. I would like to avoid surrounding each test case with set -v and set +v if possible.

$ cat rec_desc1_unit_test
#!/bin/bash
echo "Test case 1"
set -v
./rec_desc1 <<< a+b

$ ./rec_desc1_unit_test
Test case 1
./rec_desc1 <<< a+b
Enter expression: F --> id
R --> eps
T --> FR
F --> id
R --> eps
T --> FR
Q --> eps
Q --> +TQ
E --> TQ
Success.
Tags:

2

Answers


  1. 0

    You’ll have to post-process the output; making the whole thing a shell function might be a good idea:

    run_rec_desc1() {
    printf '%sn' "$1" |
    ./rec_desc1 2>&1 |
    awk -F': ' -v input="$1" '
        NF==2 {print $1 FS input; $0 = $2}
        {print}
    '
}

    $ run_rec_desc1 a+b
Enter expression: a+b
F --> id
R --> eps
T --> FR
F --> id
R --> eps
T --> FR
Q --> eps
Q --> +TQ
E --> TQ
Success.
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  2. 0

    The fact that rec_desc1 is a C program, and what it does with it’s input is irrelevant to your question assuming you don’t want that to change.

    Here’s a shell script that produces the behavior you describe, i.e. prompts for input and produces some output based on that input (in this case print a series of numbers):

    $ cat rec_desc1
#!/usr/bin/env bash

printf 'Enter a number: ' >&2
if IFS= read -r rsp; then
    seq "$rsp"
    printf 'Success.n' >&2
fi

    Here it is being run manually:

    $ ./rec_desc1
Enter a number: 3
1
2
3
Success.

    Here is the equivalent of your test script calling it:

    $ cat rec_desc1_unit_test
#!/usr/bin/env bash

printf 'Test case 1n'
./rec_desc1 <<< 3

    and the output that produces:

    $ ./rec_desc1_unit_test
Test case 1
Enter a number: 1
2
3
Success.

    Now, here is a modified version of your caller test script:

    $ cat rec_desc1_unit_test
#!/usr/bin/env bash

run_test() {
    local title="$1" cmd="$2" input="$3" output
    printf '%s: %s %sn' "$title" "$cmd" "$input"

    coproc "$cmd"

    printf '%sn' "$input" >&"${COPROC[1]}"
    input="$input"$'n'
    while IFS= read -r output; do
        printf '%s%sn' "$input" "$output"
        input=''
    done <&"${COPROC[0]}"

    wait "$COPROC_PID"

    printf '========n'
}

run_test 'Test Case 1' ./rec_desc1 3

run_test 'Test Case 2' ./rec_desc1 5

    and the output that produces:

    $ ./rec_desc1_unit_test
Test Case 1: ./rec_desc1 3
Enter a number: 3
1
2
3
Success.
========
Test Case 2: ./rec_desc1 5
Enter a number: 5
1
2
3
4
5
Success.
========

    That uses a coprocess to run ./rec_desc1 feeding it input from the calling script and reading it’s output into the calling script. Alternatively you could use expect to do the same.

    The above relies on run_desc1 sending prompts and other status messages to stderr as it should. If it doesn’t do that then you need a little different solution to read the prompt, etc. but that’s complicated by the fact that the prompt doesn’t end in a newline while read only reads newline-terminated lines so you’d need to use a different solution than just read to read the output from run_desc1.

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