Looping a command over a list of arguments in Linux

Question

How to execute the same command with arguments that are delivered by another command via pipe?

As a result of the extraction of file names from a source I get:

    $some_command
    filename1
    filename2
    filename3
    ...
    filenameN

I'd like to create files with these filenames with touch. How can I loop touch over these names?

score 11 · Answer 1 · answered Jul 01 '09 at 17:57

11

You can use xargs with -n1 to run a command once for each piped argument

$some_command | xargs -n 1 touch

In the case of touch however which accepts multiple arguments

touch `$some_command`

will probably work for you.

answered Jul 01 '09 at 17:57

theotherreceive

theotherreceive is correct, you can also use $some_command | xargs -I {} touch -option {} .This is helpful when you need to use options on your xarg execution. – Geo Jul 01 '09 at 20:20

kubanczyk · Accepted Answer · 2011-09-07T10:46:07.067

7

I only use for ... do ... done for very simple cases.

For more complicated/dangerous scenarios:

command | sed 's/^/touch /'

This does nothing but prints intended commands. Review the results, then perform the same thing piping to sh -x (the -x flag is for debugging):

command | sed 's/^/touch /' | sh -x

edited Sep 07 '11 at 10:46

answered Jul 01 '09 at 18:00

kubanczyk

score 4 · Answer 3 · answered Jul 01 '09 at 17:52

4

for i in `$some_command`; do touch $i; done

answered Jul 01 '09 at 17:52

cagenut

...and hope none of the filenames have whitespace. – hark Jul 01 '09 at 18:48
1

Put double quotes around $i to take care of spaces. – Not Now Jul 01 '09 at 20:44
Not quite mr. Bash. First of all, he has to set the IFS variable and later remember to restore it to old value. – kubanczyk Jan 05 '10 at 14:10

Kevin M · Answer 4 · 2010-01-05T16:49:22.697

2

If the filenames contain whitespace, then the following will work around them:

some_command | while read filename ; do touch "$filename" ; done

This will create files named:
file name 1
file name 2
named file 3
etc

Assuming, of course, that it does produce those names.

edited Jan 05 '10 at 16:49

answered Jul 01 '09 at 20:05

Kevin M

It will NOT work with whitespace... You need touch "$filename". – kubanczyk Jan 05 '10 at 14:14

score 1 · Answer 5 · answered Jul 01 '09 at 19:52

"some_command | xargs touch" will probably do the trick, however there are two pitfalls:

If a filename contains any whitespace character it will be treated as a separator, for example a filename "fuh bar" will, this way, be "touch"ed as a file named "fuh" and another one named "bar" instead of a single "fuh bar" one. To alleviate this you may check if your "some_command" is able to produce another delimiter, often NULL (see the "-print0" argument legit with the command "find"), and use the "--null" argument of xargs
If the command does not produce anything xargs will fail miserably. Use its "--no-run-if-empty" argument

5 Answers5