Shell Script – Purpose of xargs -I Command

shell-scriptxargs

Can you explain what this command does:

files=(this_is_filename)
for filename in ${files[@]}; do 
  ls -t1 ../htory/$filename* |
    head -1 |
    xargs -I fname cp -p fname ../htory2/somefile.CSV
done

Especially this part of this command

xargs -I fname cp -p fname ../htory2/somefile.CSV

Best Answer

xargs transforms input to arguments. The -I option specifies the string to be used as a placeholder for the argument. Therefore, if the pipeline outputs something like

file1
file2

then the xargs line transforms that to

cp -p file1 ../htory2/somefile.CSV
cp -p file2 ../htory2/somefile.CSV

The head -1 only returns one line, though, so I can see no real benefit of using xargs instead of, say

cp -p "$(ls -t1 ../htory/$filename* | head -1)" ../htory2/somefile.CSV

Moreover, since the target file is always the same, it will be overwriten by the last file in ${files[@]}.

Related Solutions

Xargs -r0 vs xargs -0

-r0 is two flags. It is -r and it is -0.

From the xargs(1) manpage:

-r
If the standard input does not contain any nonblanks, do not run the command. Normally, the command is run once even if there is no input. This option is a GNU extension.

-0
Input items are terminated by a null character instead of by whitespace, and the quotes and backslash are not special (every character is taken literally). Disables the end of file string, which is treated like any other argument. Useful when input items might contain white space, quote marks, or backslashes. The GNU find -print0 option produces input suitable for this mode.

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

Xargs -r0 vs xargs -0

Shell – ny way to use xargs across a pipe

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