170

I want to run a java command once for every match of ls | grep pattern -. In this case, I think I could do find pattern -exec java MyProg '{}' \; but I'm curious about the general case - is there an easy way to say "run a command once for every line of standard input"? (In fish or bash.)

12 Answers 12

93

That's what xargs does.

... | xargs command
  • 26
    Not quite. printf "foo bar\nbaz bat" | xargs echo whee will yield whee foo bar baz bat. Maybe add the -L or -n options? – Jander Feb 17 '11 at 5:17
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    @Jander The question was rather general, so I gave the general tool. True, you will have to adjust its behavior with options depending on the specific circumstances. – Keith Feb 17 '11 at 5:25
  • 4
    ... | tr '\n' '\0' | xargs -0 – vrdhn Feb 17 '11 at 11:12
  • 9
    like, "the specific circumstances which give the right answer to the question". :) – mattdm Feb 17 '11 at 18:17
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    If you want to see the proper way to do this with xargs, see my answer below. – Michael Goldshteyn Jun 30 '15 at 16:08
177

The accepted answer has the right idea, but the key is to pass xargs the -n1 switch, which means "Execute the command once per line of output:"

cat file... | xargs -n1 command

Or, for a single input file you can avoid the pipe from cat entirely and just go with:

<file xargs -n1 command
  • 1
    Also of interest is the ability of xargs to not run if stdin is empty: --no-run-if-empty -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. – Ronan Jouchet Oct 24 '15 at 19:58
  • 6
    How dyou access the line inside command ? – B T Apr 14 '16 at 1:21
  • This is the correct use of xargs. Without -n1, it only works on commands that treat lists of parameters as multiple invocations which not all do. – masterxilo Mar 15 '18 at 9:43
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    printf "foo bar\nbaz bat" | xargs -n1 echo whee splits by words and not by lines – Gismo Ranas Oct 24 '18 at 8:32
114

In Bash or any other Bourne-style shell (ash, ksh, zsh, …):

while read -r line; do command "$line"; done

read -r reads a single line from standard input (read without -r interprets backslashes, you don't want that). Thus you can do either of the following:

$ command | while read -r line; do command "$line"; done  

$ while read -r line; do command "$line"; done <file
  • 6
    When I tried tail -f syslog | grep -e something -e somethingelse| while read line; do echo $line; done it didn't work. It worked with a file piped into the while loop, worked with just the tail -f, worked with just grep, but not with both pipes. Giving the grep the --line-buffered option made it work – user16468 Mar 10 '12 at 20:39
  • This works also when each line needs to be sent to stdin: command | while read -r line; do echo "$line" | command ; done – Den Sep 10 '18 at 9:27
22

I agree with Keith, xargs is the most general tool for the job.

I usually use a 3 step approach.

  • do the basic stuff until you have something you would like to work with
  • prepare the line with awk so it gets the correct syntax
  • then let xargs execute it, maybe with the help of bash.

There are smaller and faster ways, but this ways almost always works.

A simple example:

ls | 
grep xls | 
awk '{print "MyJavaProg --arg1 42 --arg2 "$1"\0"}' | 
xargs -0 bash -c

the 2 first lines selects some files to work with, then awk prepares a nice string with a command to execute and some arguments and $1 is the first column input from the pipe. And finally I make sure that xargs sends this string to bash that just execute it.

It is a little bit overkill, but this recipe has helped me in a lot of places since it is very flexible.

  • 6
    Note, xargs -0 uses the null byte as a record separator, so your awk print statement should be printf("MyJavaProg --args \"%s\"\0",$1) – glenn jackman Feb 17 '11 at 14:30
  • @glenn: Missed the null char, will update the answer – Johan Feb 18 '11 at 12:43
  • @Johan not a big deal, but if you're using awk you can have it do the pattern match and skip the grep e.g., ls | awk '/xls/ {print... – Eric Renouf May 31 '16 at 15:19
15

GNU Parallel is made for that kind of tasks. The simplest usage is:

cat stuff | grep pattern | parallel java MyProg

Watch the intro video to learn more: http://www.youtube.com/watch?v=OpaiGYxkSuQ

  • 1
    No real need for the cat here since grep can directly read the file – Eric Renouf May 31 '16 at 15:20
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  • 1
    Thanks for the link, I don't necessarily agree that it's easier to read, but nice to know it was considered regardless. I would only now slightly quibble that the link doesn't actually apply here since the alternative is not really < stuff grep pattern but is grep pattern stuff with no redirection or cat required at all. Still, that doesn't materially change your argument and if you think it's clearer to always use things in a pipe that starts with cat, then power to you – Eric Renouf Jun 1 '16 at 0:32
8

Also, while read loop in fish shell (I assume you want fish shell, considering you used tag).

command | while read line
    command $line
end

Few points to note.

  • read doesn't take -r argument, and it doesn't interpret your backslashes, in order to make most common use case easy.
  • You don't need to quote $line, as unlike bash, fish doesn't separate variables by spaces.
  • command by itself is a syntax error (to catch such use of placeholder arguments). Replace it with the real command.
  • Doesn't while need to be paired with do & done instead of end? – aff Nov 19 '18 at 7:55
  • @aff This is specifically about fish shell, which has different syntax. – Konrad Borowski Nov 19 '18 at 14:29
  • Ah, so that's what the fish means. – aff Nov 20 '18 at 6:33
6

If you need to control where exactly the input argument is inserted into your command line or if you need to repeat it several times then you need to use xargs -I{}.

EXAMPLE #1

Create an empty folder structure in another_folder that mirrors the subfolders in the current directory:

    ls -1d ./*/ | xargs -I{} mkdir another_folder/{}
EXAMPLE #2

Apply an operation on a file list coming from stdin, in this case make a copy of each .html file by appending a .bak extension:

    find . -iname "*.html" | xargs -I{} cp {} {}.bak

From the xargs man page for MacOS/BSD:

 -I replstr
         Execute utility for each input line, replacing one or more occurrences of
         replstr in up to replacements (or 5 if no -R flag is specified) arguments
         to utility with the entire line of input.  The resulting arguments, after
         replacement is done, will not be allowed to grow beyond 255 bytes; this is
         implemented by concatenating as much of the argument containing replstr as
         possible, to the constructed arguments to utility, up to 255 bytes.  The
         255 byte limit does not apply to arguments to utility which do not contain
         replstr, and furthermore, no replacement will be done on utility itself.
         Implies -x.

Linux xargs man page:

   -I replace-str
          Replace  occurrences of replace-str in the initial-
          arguments with names read from standard input.  Al‐
          so,  unquoted  blanks do not terminate input items;
          instead the separator  is  the  newline  character.
          Implies -x and -L 1.
1

When dealing with potentially unsanitized inputs, I like to see the entire job 'spelled out' line by line for visual inspection before I run it (especially when it's something destructive like cleaning people's mailbox's).

So what I do is generate a list of parameters (ie. usernames), feed it to a file in one-record-per-line fashion, like this:

johndoe  
jamessmith  
janebrown  

Then I open the list in vim, and mangle it with search and replace expressions until I get a list of full commands that need to get executed, like this:

/bin/rm -fr /home/johndoe  
/bin/rm -fr /home/jamessmith 

This way if your regex is incomplete, you will see in what command will have potential problems (ie. /bin/rm -fr johnnyo connor). This way you can undo your regex, and try it again with a more reliable version of it. Name mangling is notorious for this, because it's hard to take care of all the edge cases like Van Gogh, O'Connors, St. Clair, Smith-Wesson.

Having set hlsearch is useful for doing this in vim, as it will highlight all the matches, so you can easily spot if it doesn't match, or matches in an unintended way.

Once your regex is perfect and it catches all the cases you can test for/think of, then I usually convert it to a sed expression so it can be fully automated for another run.

For cases where the number of lines of input prevents you from doing a visual inspection, I highly recommend echoing the command to the screen (or better yet, a log) before it executes, so if it errors out, you know exactly which command caused it to fail. Then you can go back to your original regex and adjust once more.

0

If a program ignores the pipe but accepts files as arguments, then you can just point it to the special file /dev/stdin.

I am not familiar with java, but here is an example of how you would do it for bash:

$ echo $'pwd \n cd / \n pwd' |bash /dev/stdin
/home/rolf
/

The $ is necessary for bash to translate \n into newlines. I'm not sure why.

0

I prefer this - allowing multi-line commands and clear code

find -type f -name filenam-pattern* | while read -r F
do
  echo $F
  cat $F | grep 'some text'
done

ref https://stackoverflow.com/a/3891678/248616

0

Here, a copypaste you can immediately use:

cat list.txt | xargs -I{} command parameter {} parameter

The item from the list will be put where the {} is and the rest of the command and parameters will be used as-is.

0

This is possible with xargs, as other answers indicated. We need to distinguish two specifics in the "once per line" part of the question:

  1. Once: Use -n 1, this ensures that the command is invoked exactly once for each argument. However, by default, xargs assumes that arguments are space-delimited -- this will break once the files contain spaces.
  2. Per line: Use -d '\n', or preprocess the input with tr '\n' '\0' and use -0 . This makes the command robust against spaces in the input.

The final command line then becomes:

.... | xargs -n 1 -d '\n' <command>

or (with tr)

.... | tr '\n' '\0' | xargs -n 1 -0 <command>

If your command can process multiple arguments at once (like grep or sed), you can omit -n 1 to speed up the operation in many cases.

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