xargs

Is a very useful program to take a list and run commands against that list. xargs will take a list of arguments, loop through them and run a command against 1 or more arguments at a time.

Basic Example

Here is a really straightforward non-destructive example of using xargs to calculate a MD5 hash on every file in the current directory,

ls | xargs -t md5sum

This is how it works,

• -t will show you what xargs is about to execute before it executes it.
• xargs will by default take the output of ls one line at a time and append it to the end of the command

Thanks to the -t the output will be shown on screen,

md5sum planetary.doc
ab5970d50d67bcafe5c554387f76534e = planetary.doc
md5sum Superman.jpg
cdefa50d737dfcf8dc57886ea1a758c4 = Superman.jpg

Substitution to Rename Files

Now let's get more advanced and use -I to allow substitution and explicitly set the location of what xargs receives. First we'll create a some temporary files,

mkdir temp
cd temp
touch files1 file2 file3 # Creates 3 empty files

Now using xargs we will add the txt extension to each file,

ls | xargs -t -I{} mv {} {}.txt
mv file1 file1.txt
mv file2 file2.txt
mv file3 file3.txt

The -I{} specifies that the arguments by ls will be placed in the location of the {} called the replacement string. In fact you can use whatever variable name you want instead of {}. For example, the following will also work,

ls | xargs -t -IvarX md5 varX
md5 file1.txt
MD5 (file1.txt) = d41d8cd98f00b204e9800998ecf8427e
md5 file2.txt
MD5 (file2.txt) = d41d8cd98f00b204e9800998ecf8427e
md5 file3.txt
MD5 (file3.txt) = d41d8cd98f00b204e9800998ecf8427e

Debugging xargs with echo

The echo command is useful to test and see what xargs will be looping through,

ls | xargs -I{} echo "mv {} {}.txt"
mv file1 file1.txt
mv file2 file2.txt
mv file3 file3.txt

Useful Applications of xargs

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