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Discussion on: Bash Brackets Quick Reference

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ferricoxide profile image
Thomas H Jones II

You missed one of the really great BASHisms: <( COMMAND ). Basically, run command in a subshell, then return it's output through a file-descriptor. Meaning that you can do things like:

  • diff two streams
  • run a command within a shell to create an input-"file" for other commands that want input in the form of a file rather than a stream.

Another great one is VAR=($( COMMAND )) ...which takes the output from COMMAND and creates an array-variable from it.

Also useful is $( COMMAND )$? for when you care about how a command exited but not its output (e.g., you want to test to see if grep found a string in a file, [[ $( grep -q PATTERN FILE )$? ]].

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rpalo profile image
Ryan Palo Author

These are great! When I get in front of a keyboard, I’ll add them to the list! Thanks for sharing. What are the most common file-expecting commands you use the sub shell redirect with?

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ferricoxide profile image
Thomas H Jones II

AWS CLI for (particularly for CloudFormation) seems to not quite process STDIN for the --parameters flag as one might expect. Usually have to do something like what I put in my article when I'm doing stack-iterations as part of a quick test-change-test cycle (where "quick" is relative to things like RDS - there's frequently not enough time to wait for one launch to delete before moving on to the next launch).

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rpalo profile image
Ryan Palo Author

Thanks again for the tips! I just wanted to let you know that I updated the article with your suggestions. 😀

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kogans profile image
Stanislav Kogan

the $( COMMAND )$? trick is a very bad one. Please don't advertise it, it basically NEVER works as you intend it to.

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ferricoxide profile image
Thomas H Jones II

Not sure what your experience is, but mine is that it works pretty much exactly as one would reasonably expect. It's functionally equivalent to executing something like:

<COMMAND>

if [[ $? ]]
then
…

But without any shell-linters bitching about using an inefficient execution-form.

That said, you need to be familiar with what the subshelled-command's likely outputs are going to be. Which is to say:

  • If the subshelled-command has an output other than just an exit code, you need to suppress it. Some commands have a built-in "quiet" option; for those that don't, you can suppress by redirecting output to /dev/null. Failing to suppress output will tend to cause evaluation-logic to evaluate as a string of <COMMAND_OUTPUT><COMMAND_EXITCODE> rather than an integer of <COMMAND_EXITCODE>.
  • Similarly, if you care to handle more than a -eq 0 or -ne 0 output, you need to be familiar enough with the given command's possible exit-codes to set up the appropriate handlers you might want/need.
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