In bash, we often use redirects (that is
> ) 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 Substition, 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 oneliner:
$ diff -u <( ls dir1 | sort) <( ls dir2 | sort )
Neat, isn’t it? I use this all the time!
Bonus: Avoid subshell 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 subshell, 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 :-)"
Subscribe via RSS