bash script won't loop over lines in file when certain command executed

Question

My script reads a list of ".bag" files from a textfile supplied as an argument. and does something with each one. (for clarity, I have omitted most of the "stuff")

while IFS= read -r bag
do

    echo Extracting from $bag

    # Play the script for saving the images
    python2 extract_imgs.py --image_topics "$image_topics" --basepath "extracted_imgs/$bag_name" &
    extract_imgs_PID=$!

    # play the bag file
    rosbag play -r 10 $bag

    echo rosbag play returned: $?
    echo finished playing $bag

    # kill the subscribe node
    kill $extract_imgs_PID

done < $1

If I comment out the rosbag play -r 10 $bag line, this script behaves as I expect: It reads each line from $1 and runs an iteration of the loop with that line set as $bag.

But with the 'rosbag play -r 10 $bag' line, it works correctly for the first line, but then exits the loop and finishes. Why? the return status of rosbag play is 0. How can I get this script to run the rosbag play command and then continue looping over the lines of the input file?

Probably `rosbag` is reading from stdin, stealing lines from `read`. See [Why redirect stdin inside a while read loop in bash?](https://stackoverflow.com/q/41650892/4518341) and [Shell script while read line loop stops after the first line](https://stackoverflow.com/q/13800225/4518341) — wjandrea, May 18 '20 at 13:55
Also [BashFAQ #89: I'm reading a file line by line and running ssh or ffmpeg, only the first line gets processed!](https://mywiki.wooledge.org/BashFAQ/089) — Gordon Davisson, May 18 '20 at 13:59
Does this answer your question? [Shell script while read line loop stops after the first line](https://stackoverflow.com/questions/13800225/shell-script-while-read-line-loop-stops-after-the-first-line) — wjandrea, May 18 '20 at 14:36
that is the same problem but the Accepted Answer does not apply here, as there is no such "-n" flag for rosbag play. — ezekiel, May 18 '20 at 15:09
See the [second answer](https://stackoverflow.com/a/55334788/4518341) then — wjandrea, May 18 '20 at 20:04

score 0 · Accepted Answer · answered May 18 '20 at 14:24

0

The problem was the same as:

I'm reading a file line by line and running ssh or ffmpeg, only the first line gets processed!

so the solution was to redirect stdin:

rosbag play -r 10 $bag </dev/null

answered May 18 '20 at 14:24

ezekiel

427
5
20

score -1 · Answer 2 · answered May 18 '20 at 14:53

I will offer a solution, with the disclaimer that the solutions which I offer normally don't get very well received around here; but anyway...

#!/bin/sh -x

init() {
cat >> edprint+.txt << EOF
1p
q
EOF

cat >> edpop+.txt << EOF
1d
wq
EOF

next
}

next() {
[[ -s stack ]] && main
end
}

main() {
filename=$(ed -s stack < edprint+.txt)
python2 extract_imgs.py --image_topics "$image_topics" --basepath "extracted_imgs/$bag_name" &
    extract_imgs_PID=$!
rosbag play -r 10 ${filename}
echo "rosbag play returned: $?"
echo "finished playing ${filename}"
ed -s stack < edpop+.txt
next
}

end() {
rm -v ./edpop+.txt
rm -v ./edprint+.txt
exit 0
}

init

The above script uses the text file containing the names as a stack. Ed is used to put each file name into a variable, against which you can then run whatever commands you desire. At the end of the main function, Ed is called again to remove the top line from the file; and a loop between two functions (main and next) iterates until the text file is empty, at which point the two Ed commands files are deleted, and the script terminates.

thanks for reply but the solution was a simple std_in redirect ` — ezekiel, May 18 '20 at 15:06

bash script won't loop over lines in file when certain command executed

2 Answers2