Shell Pipe – How to Pipe Output of a Command if Successful

outputpipeshell

INPUT_FILE=`ls -rt $MY_DIR/FILE.*.xml | head -1 | xargs basename`

I wanted to execute the second command (head -1) only if the first command is successful. How do I improve this command?

Best Answer

Try this:

INPUT_FILE=`ls -rt "$MY_DIR"/FILE.*.xml | head -1 | xargs -r basename`

Passing xargs the -r flag will cause it to only run basename if reads at least one item from standard input (head -1).

head -1 will run but you won't see or capture any output from it.

Also, if you don't want the user to see any error output from ls, you can redirect ls's stderr stream to /dev/null.

INPUT_FILE=`ls -rt "$MY_DIR"/FILE.*.xml 2> /dev/null | head -1 | xargs -r basename`

Also note that I added quotation marks around $MY_DIR. That way, the command will not fail if $MY_DIR contains spaces.

If you're using a modern shell such as bash, you should use a $( ) capture shell instead of backticks. You should also consider changing the style of your variables. You should generally avoid using all-uppercase variable names in scripts. That style is generally reserved for reserved and environmental variables.

input_file=$(ls -rt "$my_dir"/FILE.*.xml 2> /dev/null | head -1 | xargs -r basename)

Related Solutions

Shell – Output of command not in stderr nor stdout

This can happen if the application is writing directly to the TTY instead of STDOUT or STDERR.

You can play with this behavior by comparing the 2 examples below

( echo foo ) &>/dev/null
( echo foo > $(tty) ) &>/dev/null

Notice the first doesn't show anything, but the second does. That's because we sent the output directly to the tty and bypassed the redirection to /dev/null.

You can get around stuff like this by using script

script -c '( echo foo > $(tty) ) &>/dev/null'  >/dev/null

Basically the script utility creates a fake tty and launches the command in that tty. Any output from the command is sent to STDOUT which you can then redirect as normal.

Shell – ny way to use xargs across a pipe

If I understand correctly, you want to fire up one instance flac … | lame … for each input line, and interpolate the input into the arguments to both commands.

Since you need xargs to start a pipeline, you need to make it start a program that's capable of creating pipelines, i.e. a shell.

inotifywait -m -r -q -e moved_to --format "%w%f" ~/test |
xargs -l sh -c 'flac -cd "$0" - | lame -b 320 - "/media/1tb/$0.mp3"'

Alternatively, have the calling shell read lines one by one and run the pipeline.

inotifywait -m -r -q -e moved_to --format "%w%f" ~/test |
while IFS= read -r file; do
  flac -cd "$file" - | lame -b 320 - "/media/1tb/$file.mp3"
done

Note that the format %w%f produces an absolute path, to which you're prepending /media/1tb and appending .mp3. If you want to strip off the directory part of the file in the lame command, change $file to ${file##*/}. If you want to strip off the extension, change $file to ${file%.*}. If you want to do both, you'll have to do it in two steps. If you want to reproduce the directory hierarchy under /media/1tb, you can use mkdir -p.

cd ~/test
inotifywait -m -r -q -e moved_to --format "%w%f" . |
while IFS= read -r file; do
  [ -f "$file" ] || continue; # skip directories and other special files
  dir=${file%/*}; file=${file##*/}
  mkdir -p "/media/1tb/$dir"
  flac -cd "$dir/$file" - | lame -b 320 - "/media/1tb/$dir/${file#.*}.mp3"
done

Best Answer

Related Solutions

Shell – Output of command not in stderr nor stdout

Shell – ny way to use xargs across a pipe

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