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Bash Brackets Quick Reference

Ryan Palo on June 20, 2018

Cover image credit: Fonts.com Bash has lots of different kinds of brackets. Like, many much lots. It adds meaning to doubling up different brack... [Read Full]
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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, [[ $( grep -q PATTERN FILE )$? ]].

 

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?

 

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).

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

 

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

 

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.
 

A couple of things about HEREDOC bits:

  • HEREDOCs when used with a command are fed into stdin for a command. The way you've used it in your first example for assignment to a variable doesn't work, at least for me and my bash 4.4.19.
$ nice_message=<<MESSAGE
Hi there!  I really like the way you look
when you are teaching newbies things
with empathy and compassion!
You rock!
MESSAGE
$ echo $nice_message

$
$ cat <<MESSAGE
Hi there!  I really like the way you look
when you are teaching newbies things
with empathy and compassion!
You rock!
MESSAGE
Hi there!  I really like the way you look
when you are teaching newbies things
with empathy and compassion!
You rock!
$ 
  • If you put single quotes around the initial tag, the HEREDOC will be treated like a single-quoted string, leaving variable expressions uninterpolated.
$ cat <<EOF
$RANDOM
EOF
4578

$ cat <<'EOF'
$RANDOM
EOF
$RANDOM
  • You can also use triple less-than symbols without a tag to just feed in a single simple string, like so:
$ cat <<EOF
beep
EOF
beep

$ cat <<<beep
beep
$
 

I updated the post with your help and extra info! Thanks again!

 

Hm. I’ll take a look at that. Thanks! I was doing all of my local testing in Zsh, and forgot that there might be differences.

 

This is a really great overview. Probably the clearest thing I've ever read about substitutions and their ilk :)

One thing I might add to it would be just to flag which bits are POSIX. Like where you have the single- vs. double-square-brackets rule of thumb, you say that if you need to use test or [ you'll know it - well the main reason you'd know it is if you were wanting to write a script which might be portable, i.e. work in a foreign shell.

 

Thanks! I’ll put something in there about that. I tried to avoid it, because I found during my research that articles that constantly focused on POSIX really distracted from the rest of the info. But you are right that it’s probably good to at least mention that it is a thing.

Thanks for the feedback!

 

Small fix for -
echo ${url%%/*}

It actually prints -
https:

 

Ah! You're totally right. Let me get that fixed. Thanks!

 

Love this post. Bash has remained something I've been content to stumble around with instead of learning, but this got me intrigued.

 

Heh... There's so much I can do with BASH that, when under a time-crunch, it's hard for me to justify figuring out "how do I accomplish 'X' in <INSERT_LANGUAGE_OR_DSL>" ...when I can just bang it out in BASH in like two seconds

 

Yeah, I logically know all of this but have still maintained vast ignorance.

 

Basically until I wrote this, I just guessed and then tried another set of brackets when one didn’t work. I do that with a lot of stuff in Bash, so I’m going back to fill in all the gaps.

 

This is great! I use bash a ton but wasn't aware of several of these. Really useful.

 

Thanks! Glad it was able to help you out!

 

Great summary.

I've written a ton of bash.

My rule for writing bash is to write as little bash as possible.

Anything with any sort of even mildly complex logic, use a modern scripting language - Ruby (my preference) or Python.

I've also written github.com/thewoolleyman/process_h... which makes dealing with subprocesses much nicer in Ruby.

 

My experience has been the exact opposite. Bash, when properly learned, is very powerful and, what's more important, SELF SUFFICIENT.
Ruby and Python either make for very long scripts to accomplish even the basic tasks or pull in a gillion libraries as dependencies, which turns it into a sysadmin nightmare. BTW, the process helper library you linked to is a classic example of that.

My personal preferences for scripting: bash when possible, Perl when necessary, Python if you're adventurous and NEVER Ruby.

 

It's nice to show off some tricks, but the amount of bad and downright DANGEROUS practices in this article is just too much. Really, sometimes it's just best to READ THE F-CKING MAN PAGE!!!

 

Hi there! I found the man pages dense and opaque at times, which is why I put this article together. Are there any especially dangerous practices that you’d like to point out?

 

It's true that bash man page is a very dry reading, but once you get the basics figured out, it is definitely the best source. That's why it's important to use the correct terminology in your article - it will allow people to quickly find the necessary subjects in the man page.

Now, for the mistakes in the article.

  • "(( Double Parentheses ))" is actually a compound command that undergoes arithmetic evaluation. its the equivalent of "let".
# Not correct
a=(( 4 + 1 ))

# Correct
((a=4 + 1))
  • "$( Dollar Single Parentheses )" is actually called "Command substition"

  • "$( Dollar Single Parentheses Dollar Q )$?" is an absoultely HORRIBLE trick, NEVER EVER EVER do this! Oh, and btw, this doesn't actually work, because, the few times that it won't spew out an error, it will always evaluate to true.

# This is very bad
if [[ $( grep -q PATTERN FILE )$? ]]; then
 ...

# Do this instead
if grep -q PATTERN FILE; then
 ...

# oh, and by the way
[[ $(false)$? ]]; echo $?
#=> 0
# thats because  $? is evaluated as a string, and "1" is non empty

[[ $(false) $? ]]; echo $?
#=> bash: conditional binary operator expected
#=> bash: syntax error near `$?'
# LOL
  • "$(( Dollar Double Parentheses ))" is actually called "Arithmetic expansion" with the expression within undergoing "Arithmetic evaluation". An interesting fact: double paratheses is not the only place arithmetic evaluation occurs: it also occurs inside array subscripts, which can be a pretty surprising thing sometimes.

  • [ Single Square Brackets ]. This deserves a special caution: unless you require backward compatibility with classic sh, you should NEVER use it. It's error-prone, requires gillion of quotes to make reliable and spawns a separate process for no good reason. Also, "[[" has more features.

  • "{ Single Curly Braces }" is called "brace expansion".

  • "${dollar braces}" is called "parameter expansion".

Yep, I’m aware of what these are all called. I wrote it this way because the target audience most likely wouldn’t, and I wanted to make it an accessible visual guide. That being said, your other notes are helpful, and, when I get a chance, I’ll do my best to add the warnings and caveats where they’re needed. Thanks for taking the time to walk me through your experiences!

 

I once renegotiated my salary when the CTO asked "But how did you passed standard input to the application?". Got a 30%. lolz ;..;

 

There is also triple < here doc that provides a string as stdin input, e.g.
cat <<<"this string"

 

Cool! Does this give you any benefits over “echo some string | cat”? I guess it saves cat from being run in a sub shell, maybe?

 

A useful list. Thanks for writing this up :)

 

Great article! Thanks for writing it!

 

Thanks! Glad you liked it!

 

Well, after Powershell all this seems not even ugly, but as a product of some extraterrestrial mind.
I really have no idea why would anyone use it in 2018 voluntarily.

 

I know both bash and powershell and i can tell you're just trolling: powershell certainly is the alien language. As for who would use bash in 2018 voluntarily, i guess most developers do, contrary to powershell.

 

I agree, powershell is a complete trainwreck. Even M$ has given up attracting any body outside of the windows sysadmin sect to it, that's where this whole linux under windows bs came from.

this is so funny and embarassing at the same time

 

Nice try.

BTW, I wasn't trolling at all, so you don't need to 'overtroll' anybody.

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