What is the operator =~ called? Is it only used to compare the right side against the left side?
Why are double square brackets required when running a test?
ie. [[ $phrase =~ $keyword ]]
Thank you
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3 Answers
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What is the operator
=~called?I'm not sure it has a name. The bash documentation just calls it the
=~operator. -
Is it only used to compare the right side against the left side?
The right side is considered an extended regular expression. If the left side matches, the operator returns
0, and1otherwise. -
Why are double square brackets required when running a test?
Because
=~is an operator of the[[ expression ]]compound command.
8 Comments
kojiro
If the left side matches, the operator returns 0 or 1 as you say, but it also sets the
BASH_REMATCH array, which bears mentioning. Also, [[ is not a command – It's a keyword.
Carl Norum
Yup - all further information available at the doc link above.
kojiro
bash is weird.
type [[ says that [[ is a shell keyword, but the manpage does refer to [[ as a conditional command at least once. Meh.
chepner
[[ is a shell keyword which introduces a compound command [[ ... ]], just like if is a shell keyword that introduces the if..then..fi compound command.
serup
perhaps it is called the Equal Tilde operator
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The =~ operator is a regular expression match operator. This operator is inspired by Perl's use of the same operator for regular expression matching.
The [[ ]] is treated specially by bash; consider that an augmented version of [ ] construct:
[ ]is actually a shell built-in command, which, can actually be implemented as an external command. Look at your /usr/bin, there is most likely a program called "[" there! Strictly speaking,[ ]is not part of bash syntax.[[ ]]is a shell keyword, which means it is part of shell syntax. Inside this construct, some reserved characters change meaning. For example,( )means parenthesis like other programming language (not launching a subshell to execute what's inside the parentheses). Another example is that<and>means less than and greater than, not shell redirection. This allow more "natural" appearance of logical expressions, but it can be confusing for novice bash programmers.
Wirawan
4 Comments
LukeSavefrogs
Thanks for the easy and useful explanation of the difference between
[[ ]] and [ ]... It cleared my mind
0xc0de
Though this is focusing on only a part of the question, it is helpful.
SD.
@LukeSavefrogs what did you understand from this? Can you help me to understand this?
LukeSavefrogs
@SD.
[ and [[ are two different commands. Sometimes they are interchangeable but as of now it is always better to use the latter, which is more secureThe =~operator is the pattern match operator. It did not exist in the original Bourne shell, when test, or internally [ ], was used for conditionals.
The let command, or [[ ]] internally, has more functionality than test, including pattern matching capabilities. This is why you have to use [[ ]], instead of [ ], when using =~.
