Preparing output for further processing with Xargs

Do Nothing

Sometimes doing nothing is required. Many programs don't appreciate calls without parameters and will report an error. You can make GNU Xargs avoid calling the program if there are parameters (Listing 12). The BSD implementation already does this by default.

Listing 12

Skipping Calls with No Parameters

01 $ find . -name '*.bla'
02 $ find . -name '*.bla' | xargs file
03 Usage: file [-bchikLlNnprsvz0] [--apple] [--mime-encoding] [--mime-type]
04             [-e testname] [-F separator] [-f namefile] [-m magicfiles] file ...
05        file -C [-m magicfiles]
06        file [--help]
07 $ find . -name '*.bla' | xargs -r file

Teamwork

Find is not the only command that works well with Xargs. Xargs is also very useful when combined with grep . With the -l option, the program displays only the filenames that include a match. In Listing 13, for example, the software first searches in the current directory's files scanning for the < symbol. If finds it, it passes the name of the file on to Xargs. Xargs then passes this to file which determines the file type.

Listing 13

Xargs and Grep

01 $ grep -l '<' * | xargs file
02 bar.html: HTML document, ASCII text
03 foo.xml:  XML document text

GNU grep from version 2.4 even recognizes the -Z option (for zero) that ends a filename with a null character instead of a line break, with the advantages mentioned above (Listing 14).

Listing 14

Grep with the -Z Option

01 $ grep -l foo * | xargs file
02 File:               ERROR: cannot open `File' (No such file or directory)
03 with:                 ERROR: cannot open `with' (No such file or directory)
04 linebreaks.txt: ERROR: cannot open `linebreaks.txt' (No such file or directory)
05 bar.html:            HTML document, ASCII text
06 foo.xml:             XML document text
07 $ grep -lZ foo * | xargs -0 file
08 File
09 with
10 linebreaks.txt: ASCII text
11 bar.html:                      HTML document, ASCII text
12 foo.xml:                       XML document text

The Prips [7] program also works wonders together with Xargs. The name stands for "Print IPs" and the tool prints all the IP addresses for a given range of addresses.

Listing 15 shows an example in which I used the -n 1 option one more time. Here, the host command also processes only one IP address per call.

Listing 15

Prips Processed Output

01 $ prips 192.33.96.0/30
02 192.33.96.0
03 192.33.96.1
04 192.33.96.2
05 192.33.96.3
06 $ prips 192.33.96.0/30 | xargs -n 1 host
07 0.96.33.192.in-addr.arpa domain name pointer phys-hpx-dock-1.ethz.ch.
08 1.96.33.192.in-addr.arpa domain name pointer rou-hpx-1-phys-hpx-dock-1.ethz.ch.
09 2.96.33.192.in-addr.arpa domain name pointer floo.ethz.ch.
10 3.96.33.192.in-addr.arpa domain name pointer aragog.ethz.ch.

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