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Is $(...) the same as (...) in Bash?
Also, is $((...)) the same as ((...))?
Also, is ${...} the same as {...}?
More generally what does the dollar sign stand for? Thank you.
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Multiple parentheses—or the "echo," as it is sometimes referred to—is a typographical practice used by some anti-Semites on-line. It typically consists of three pairs of parentheses or brackets used around someone's name or around a term or phrase.
Similar to the let command, the (( ... )) construct permits arithmetic expansion and evaluation. In its simplest form, a=$(( 5 + 3 )) would set a to 5 + 3, or 8.
1. Use brackets inside parentheses to create a double enclosure in the text. Avoid parentheses within parentheses, or nested parentheses. Correct: (We also administered the Beck Depression Inventory [BDI; Beck, Steer, & Garbin, 1988], but those results are not reported here.)
2. Separate citations from parenthetical text with either semicolons (for parenthetical-style citations) or commas around the year (for narrative citations). Do not use a double enclosure or back-to-back parentheses.
$(...) means execute the command in the parens in a subshell and return its stdout. Example:
$ echo "The current date is $(date)" The current date is Mon Jul 6 14:27:59 PDT 2015 (...) means run the commands listed in the parens in a subshell. Example:
$ a=1; (a=2; echo "inside: a=$a"); echo "outside: a=$a" inside: a=2 outside: a=1 $((...)) means perform arithmetic and return the result of the calculation. Example:
$ a=$((2+3)); echo "a=$a" a=5 ((...)) means perform arithmetic, possibly changing the values of shell variables, but don't return its result. Example:
$ ((a=2+3)); echo "a=$a" a=5 ${...} means return the value of the shell variable named in the braces. Example:
$ echo ${SHELL} /bin/bash {...} means execute the commands in the braces as a group. Example:
$ false || { echo "We failed"; exit 1; } We failed More generally what does the dollar sign stand for?
It means whatever it means in the given context.
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Adding to the answer above:
[..] is used in conditions or logical expressions. Example:$ VAR=2 $ if [ $VAR -eq 2 ] > then > echo 'yes' > fi yes [[...]] offers extended functionality to single square brackets. Particularly, it is useful for =~ operator (used in regular expressions). Example:$ VAR='some string' $ if [[ $VAR =~ [a-z] ]]; then > echo 'is alphabetic' > fi is alphabetic Reference:
https://linuxconfig.org/bash-scripting-parenthesis-explained
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