map

Map lines from stdin to commands.

Description

Map lets you process each line from stdin with a command of your choice. For example:

$ ls
LICENSE   README.md makefile  map.1   map.c

$ ls | map f 'echo $f $f'
LICENSE LICENSE
README.md README.md
makefile makefile
map.1 map.1
map.c map.c

Note that the command must be wrapped in single quotes to prevent the variable from being expanded by the shell.

Installation

Install map into /usr/local/bin with the following command:

$ make install

You can use make PREFIX=/some/other/directory install if you wish to use a different destination. If you want to remove map from your system, use make uninstall.

Motivation

There are many ways to accomplish what you can do with map, including find, xargs, awk, and shell for-loops. The approach taken by map is extremely pragmatic and allows me to express concisely what I want. Given the fact that it's designed as a filter, it can operate on any kind of list, not only lists of files.

The problem that prompted me to think about map was the following: given a list of files, I wanted to execute two commands on each. Here's how you can do it with different tools:

With map:

ls *.c | map f 'foo $f; bar $f'

With xargs:

ls *.c | xargs -I % sh -c 'foo %; bar %;'

With awk:

ls *.c | awk '{ system("foo "$0"; bar "$0) }'

With find:

find . -name \*.c -maxdepth 1 -exec foo {} \; -exec bar {} \;

With a bash for-loop:

for f in $(ls *.c)
do
  foo $f
  bar $f
done

With a csh for-loop:

foreach f (*.c)
  foo $f
  bar $f
end

Map's modest claim is that it improves on the ergonomics of existing tools. It's not only that sometimes it allows you to type less, but also the conceptual model needed to operate it is simpler.

Let's consider these tasks:

1. Execute a command foo on each C file:

ls *.c | map f 'foo $f'

ls *.c | xargs foo

find . -name *.c -maxdepth 1 -exec foo {} \;

2. Execute commands foo and bar on each C file:

ls *.c | map f 'foo $f; bar $f'

ls *.c | xargs -I % sh -c 'foo %; bar %;'

find . -name *.c -maxdepth 1 -exec foo {} \; -exec bar {} \;

3. Download files from a list of URLs in a file:

cat urls | map u 'curl -O $u'

cat urls | xargs -n 1 curl -O

4. Sleep three times for one second and say "done" after each elapsed second:

printf "1\n1\n1\n" | map t 'sleep $t && say done'

printf "1\n1\n1\n" | xargs -n 1 -I % sh -c 'sleep % && say done'

5. Same as #4, but run the commands in parallel:

printf "1\n1\n1\n" | map t 'sleep $t && say done &'

printf "1\n1\n1\n" | xargs -n 1 -P 3 -I % sh -c 'sleep % && say done'

The last three examples are not possible with find, because it only operates on file hierarchies.

When using map, the commands don't vary much because the second argument is a template for a well known syntax. On the other hand, there's more variation in the invocations to xargs and find, which means you may need to remember those command line options if you want an idiomatic solution.

As with anything in life, familiarity helps and if you use a tool in a certain way over and over it will seem simple to operate, but we can still analyze the conceptual models and determine how much information is needed in each case. I would say the advantage of map is that it requires less knowledge.

Of course xargs and find are much larger tools, with a bigger feature set. Map has around 20 lines of code. For comparison, here's the source code of GNU xargs. No doubt xargs will offer a lot more features, but so far with map I've completely stopped using xargs and for-loops. Another way to think about map vs xargs: if map had been specified in POSIX and xargs was just released, I'm not sure I would install it unless map proved to be unfit for a given use case.

Something important to keep in mind is that map works on lines, not on files. My reasoning was that in the context of a POSIX environment, a map function can be expected to filter lines from stdin. In that regard, it is very generic because a line can represent anything.

Known issues

In the examples above I frequently use the output of ls. In my projects, filenames don't contain spaces, newlines, or other special characters because I find them inconvenient, even though they are valid. As map works on lines, filenames that contain newlines should be handled separately. For files that contain whitespace, the common solution is to wrap the variable in double quotes, so instead of $f you would use "$f".

Contributing

If you find a bug, please create an issue detailing the ways to reproduce it. If you have a suggestion, create an issue detailing the use case.