Bash process substitution

This post appeared originally in our sysadvent series and has been moved here following the discontinuation of the sysadvent microsite

In bash, we often use redirects (that is < and > ) to get output from a command to a file, or input from a file to a command. But sometimes, commands takes two or more files as input. Then our ordinary scheme does not work anymore.

Classical problem: Diff output from two commands

Let’s say you want to diff(1) the output of two commands. For example, compare the contents of two directories. You may run the two commands, and redirect the output to files, then diff the files, and finally remove the files. Awkward.

ls dir1 | sort > file1
ls dir2 | sort > file2
diff -u file1 file2
rm file1 file2

Since diff can take stdin as one input via the special filename ‘-‘, we might cut down to one file, but this is still awkward.

ls dir1 | sort > file1
ls dir2 | sort | diff -u file1 -
rm file1

Bash has (of course) a better solution: Process substitution, that is, treat the output (or input) of commands as files. Enter the process substitution operators:

>(command list) # Input
<(command list) # Output

Now, let us solve our diff challenge with a simple one-liner:

diff -u <( ls dir1 | sort)  <( ls dir2 | sort )

Neat, isn’t it? I use this all the time!

Bonus: Avoid sub-shell scripting

The following bash shell loop is a pitfall often missed, leading to subtle bugs that are hard to spot. Pipe to a while loop runs in a sub-shell, so global variables goes out of scope when they are changed inside the loop.

#!/bin/bash
global=0

echo "Outside loop, global=$global"

for n in 1 2 3; do echo $n; done | \
while read i; do
    global=$i
    echo "Inside loop: global=$global"
done

echo "Outside loop, global=$global again :-("

Using command substitution, we avoid this elegantly:

#!/bin/bash
global=0

echo "Outside loop, global=$global"

while read i; do
    global=$i
    echo "Inside loop: global=$global"
done < <( for n in 1 2 3; do echo $n; done )

echo "Outside loop, global=$global still :-)"