BASH script stop after executing one line but if I use echo instead of exec the output of the script completes

Question

Background I am a newbie to Bash scripting and through Googling I have got as far as I have.

I have two text files: "Filemp3.txt" that contains

songname - artist.mp3

songname2 - artist2.mp3

songname3 - artist3.mp3

and Fileogg.txt that contains

songname - artist.ogg

songname2 - artist2.ogg

songname3 - artist3.ogg

I will be using SOX to convert from MP3 to Ogg.

I have the following bash script:

#!/bin/bash

exec 3<Fileogg.txt

while read mp3; do
        read -u3 ogg
        echo sox "Musicmp3/$mp3" "Musicogg/$ogg"
done <Filemp3.txt

This outputs exactly the commands I want to run line by line.

radio@radio:~$ ./convert-mp3-ogg.sh
sox Musicmp3/songname - artist.mp3 Musicogg/songname - artist.ogg
sox Musicmp3/songname2 - artist2.mp3 Musicogg/songname2 - artist2.ogg
sox Musicmp3/songname3 - artist3.mp3 Musicogg/songname3 - artist3.ogg

But when I edit the script to exec e.g. exec sox "Musicmp3/$mp3" "Musicogg/$ogg" ... the script runs & an ogg file is created but for only the first filename.

I am assuming this is a problem with my Bash script as the ogg file plays fine and Sox does not show any error I am aware of.

Why not just use the one file of sources and do "sox Muxicmp3/$mp3 Musicogg/${mp3%mp3}ogg" - use the input file name but change the extension for the destination? Or do you have some cases where you want to convert `x.mp3` into `y.ogg`? — twalberg, Jan 22 '14 at 14:58

chepner · Accepted Answer · 2014-01-22T18:25:37.693

2

The exec command replaces the command being executed in the current process with a new command. It's like a subroutine call that never returns. In this case, you simply want to call sox, and continue after it returns, so just drop the exec:

while read mp3; do
    read -u3 ogg
    sox "Musicmp3/$mp3" "Musicogg/$ogg"
done < Filemp3.txt

edited Jan 22 '14 at 18:25

answered Jan 22 '14 at 14:46

chepner

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Pedantically, `exec` replaces the current *program*, the process is the same. – cdarke Jan 22 '14 at 15:30
Thank you chepner that worked a treat - also thanks to others with the same answer and explanation – user3223819 Jan 22 '14 at 17:47

score 1 · Answer 2 · answered Jan 22 '14 at 15:29

There are two unrelated meanings of exec, which is probably where you are confused. The first one you use:

exec 3<Fileogg.txt

is fine, it opens file descriptor number 3 for file 'Fileogg.txt', and is read with your read -u3.

There is a second use of exec though, and that is to replace the current program with a different one, in the same process. There is no return from a successful exec. So when you:

exec sox "Musicmp3/$mp3" "Musicogg/$ogg"

that replaces bash with sox, so you never return to the script!

Just remove the exec, you don't need it here:

sox "Musicmp3/$mp3" "Musicogg/$ogg"

BASH script stop after executing one line but if I use echo instead of exec the output of the script completes

2 Answers2