![]() ![]() One thing to remember is that this is strictly integer arithmetic. The last value calculated will be the output. # => I don't want to brag, but I have like 9 friends."ī=$(( a *= 2 )) # You can even do assignments. Message="I don't want to brag, but I have like $(( a / 2 )) friends." Remember how regular (( Double Parentheses )) don’t output anything? Remember how that is kind of annoying? Well, you can use $(( Dollar Double Parentheses )) to perform an Arithmetic Interpolation, which is just a fancy way of saying, “Place the output result into this string.” a=$(( 16 + 2 )) However, in Bash, if statements will process the then branch if the expression after if has an exit code of 0 and the else branch otherwise, so, in this case, Matthew notes that we can drop all of the fancy stuff and simplify to: if grep -q PATTERN FILE then if ]Įcho "Dat pattern was totally in dat file!"Īlthough, really, this isn’t so much a special bracket pattern as it is an interesting use of $?, since the above works even if there is a space between the $( stuff ) and the $?. If you want to interpolate a command, but only the exit code and not the value, this is what you use. $( Dollar Single Parentheses Dollar Q )$? # And just to prove that it's a subshell. Ooooor, you can be a total BAshMF and do it this way: # The baller way This works because the sort command expects one or many filenames as arguments. In this case, for `ls -l`, that is the "file size" # -nr means sort numerically in reverse (descending) order # Just in case you don't magically remember all bash flags, ![]() # => executables on your computer, sorted in order of descending size. Like a billion lines of output that contain many of the # => bash: syntax error near unexpected token '(' i=4Įcho $? # Check the exit code of the last command If the result inside is zero, it returns an exit code of 1. If the result inside is non-zero, it returns a zero (success) exit code. Any variable changes that happen inside them will stick, but don’t expect to be able to assign the result to anything. However, do note that there is no output. You can perform assignments, logical operations, and mathematic operations like multiplication or modulo inside these parentheses. Thanks Davide for bringing up this use case for parentheses. This should give you enough of a feel to not freak out if you see it in somebody’s Bash script, though. ![]() I’ll put it on the list of drafts to write. There’s a whole bunch more to dive into here, and a ton of gotchas to look out for, but that’s a whole nother article. So one way you can split a string on a character is something like this: grade_string='A B F D C A-' In the input inside the parentheses, Bash uses the current environment variable $IFS (field separator) and will split the array string on any character found in $IFS. But, for completeness’s sake: cheeses=('cheddar' 'swiss' 'provolone' 'brie') Now, arrays and associative arrays are only available in newer versions of Bash, and there are a lot of weird edge cases and syntax rules that make it easy to make mistakes using them–so much so that I try to steer Bash newbies clear of their usage unless they’re definitely the right tool to use. Because it’s within a subshell, if you have it inside a loop, it will run a little slower than if you called the commands without the parentheses. Any variables declared or environment changes will get cleaned up and disappeared. This means that they run through all of the commands inside, and then return a single exit code. The first usage for single parenthesis is running commands inside in a subshell. We’ll go through in order of net total squigglyness (NTS score). ![]() It doesn’t like space where curly braces are concerned. So here we go.Ī tiny note on all of these is that Bash generally likes to see a space between round or square brackets and whatever’s inside. So here, I’m going to lay them all out and then print this article out and staple it to the wall by my desk. I constantly find myself doing a 5-second search for which one is the right one to do since I’m not writing Bash scripts all the time. And, the brackets are used differently than many other languages. It adds meaning to doubling up different brackets, and a dollar sign in front means something even more different. Bash has lots of different kinds of brackets. ![]()
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