Force Bash-Script to wait for a Perl-Script that awaits input

Question

I'm having a Bash-Script that sequentially runs some Perl-Scripts which are read from a file. These scripts require the press of Enter to continue.

Strangely when I run the script it's never waiting for the input but just continues. I assume something in the Bash-Script is interpreted as an Enter or some other Key-Press and makes the Perl continue.

I'm sure there is a solution out there but don't really know what to look for.

My Bash has this while-Loop which iterates through the list of Perl-Scripts (which is listed in seqfile)

while read zeile; do
    if [[ ${datei:0:1} -ne 'p' ]]; then
        datei=${zeile:8}
    else
        datei=$zeile
    fi
    case ${zeile: -3} in
    ".pl")
    perl  $datei  #Here it just goes on...
    #echo "Test 1"
    #echo "Test 2"
    ;;
    ".pm")
        echo $datei "is a Perl Module" 
    ;;
    *)
        echo "Something elso"
    ;;
    esac
done <<< $seqfile;

You notice the two commented lines With echo "Test 1/2". I wanted to know how they are displayed. Actually they are written under each other like there was an Enter-Press:

Test 1
Test 2

The output of the Perl-Scripts is correct I just have to figure out a way how to force the input to be read from the user and not from the script.

what happens is perl script is executed on command line. Does it return the prompt immediately? — LMC
– LMC, Commented Feb 7, 2018 at 17:57
@LuisMuñoz - if the script is executed on the command line it works as it's supposed to be (after executing various Perl-Scripts several times with the same command I decided to do some automation). Actually, the Bash-Script works - it just jumps over the input I want to give. — Qohelet
– Qohelet, Commented Feb 7, 2018 at 18:15

mob · Accepted Answer · 2018-02-07 17:57:05Z

3

Have the perl script redirect input from /dev/tty.

Proof of concept:

while read line ; do
    export line
    perl -e 'print "Enter $ENV{line}: ";$y=<STDIN>;print "$ENV{line} is $y\n"' </dev/tty
done <<EOF
foo
bar
EOF

Program output (user input in bold):

Enter foo: 123 foo is 123 Enter bar: 456 bar is 456

answered Feb 7, 2018 at 17:57

mob

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2 Comments

Qohelet Over a year ago

I had to re-write it a little, but basically just change the one line to perl $zeile </dev/tty . I don't want it to wait for every single input (sometimes no input is required at all). I see your knowledge of bash is bigger than mine, would you mind explaining also why you use the -e Parameter and export the line before instead of just using it directly? (Upvoted and Marked as answer already, but some explanation would be nice, thank you)

mob Over a year ago

The -e and the export probably aren't relevant to your specific problem. -e instructs perl to treat the next command line argument as perl code to execute rather than a file containing Perl code. export makes the bash variable $line available in the Perl process so I could output it.

ErikMD · Accepted Answer · 2018-08-12 22:17:09Z

3

@mob's answer is interesting, but I'd like to propose an alternative solution for your use case that will also work if your overall bash script is run with a specific input redirection (i.e. not /dev/tty).

Minimal working example:

script.perl

#!/usr/bin/env perl
use strict;
use warnings;
{
    local( $| ) = ( 1 );
    print "Press ENTER to continue: ";
    my $resp = <STDIN>;
}
print "OK\n";

script.bash

#!/bin/bash

exec 3>&0 # backup STDIN to fd 3

while read line; do
    echo "$line"
    perl "script.perl" <&3 # redirect fd 3 to perl's input
done <<EOF
First
Second
EOF

exec 3>&- # close fd 3

So this will work with both: ./script.bash in a terminal and yes | ./script.bash for example...

For more info on redirections, see e.g. this article or this cheat sheet.

Hoping this helps

edited Aug 12, 2018 at 22:17

answered Feb 7, 2018 at 18:26

ErikMD

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4 Comments

Qohelet Over a year ago

You were just 8 seconds too late I'm afraid but I find your answer interesting and your links helpful (upvote!), yet I can't make your solution work :/ bash: [[: /: syntax error: operand expected (error token is "/") bash: 3: Bad file descriptor

ErikMD Over a year ago

@Qohelet I cannot reproduce your error. Did you copy the script.bash file as is, and run chmod a+x script.bash && ./script.bash ? If yes, can you give more hints about your config (such as the OS and the shell)? Do you have the same error if you run exec 3>&0 in your terminal?

ErikMD Over a year ago

You may also want to run ls -l /dev/fd/ in a terminal to get the list of opened file descriptors

Qohelet Over a year ago

Overlooked your exec 3>&0 and didn't put it in my code :/ - your idea works perfectly fine.

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