Bash script not working with string variable, runs fine in directly in terminal

Question

I am relatively new to Bash. I wrote a script to generate montage of images using montage utility from imagemagick, by reading list of png files from a text file:

IFS=$'\n'
count=1
for line in `cat pngListGr4`;
do
   montage -tile 4x0 $line $(printf "%03d" $count).png
   ((count=count + 1))
  done
unset IFS

where pngListGr4 file looks like this:

01.png 02.png 03.png 04.png
05.png 06.png 07.png 08.png
...

Hence I was expecting to generate file 001.png and 002.png as the montage of files 1-4 and 5-8. But instead I am getting error:

montage-im6.q16: missing an image filename `001.png' @ error/montage.c/MontageImageCommand/1795.

However following code works fine in terminal:

$ string_="01.png 02.png 03.png 04.png"
$ montage -tile 4x0 $string_ $(printf "%03d" $count).png

Why my string substitution in my bash script giving such issues?

This might help: How can I read a file (data stream, variable) line-by-line (and/or field-by-field)? — Cyrus
– Cyrus, Commented Oct 25, 2019 at 21:51
You are setting IFS to \n to try to read lines, but that means that $string_ will no longer break on spaces. If you copy-paste your script into your terminal, or you copy-paste your terminal command into a script, you'll see that both behave the same way in both places — that other guy
– that other guy, Commented Oct 25, 2019 at 21:54
@thatotherguy I tried unsetting IFS before montage, and then resetting it after montage, indeed my script works after that. Can you please write ur comment as answer? — ipcamit
– ipcamit, Commented Oct 25, 2019 at 22:34

that other guy · Accepted Answer · 2019-10-25 22:38:34Z

1

You are setting IFS to \n to try to read lines, but that means that $string_ will no longer break on spaces in your montage command. The better solution is to use a while read loop which can both get lines and split them up into fields:

count=1
while IFS= read -ra line
do
   montage -tile 4x0 "${line[@]}" "$(printf "%03d" "$count").png"
   ((count++))
done < pngListGr4

answered Oct 25, 2019 at 22:38

that other guy

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