Question

I have been working with some bash scripting lately and been looking through the man pages. From what I have gathered, does $(( )) mean expr and [ ] mean test?

For $(( )):

echo $(( 5 + 3 ))

has the same output as:

echo $(expr 5 + 3)

For [ ]:

test 'str' = 'str'

has the same success value as:

[ 'str' = 'str' ]

Did I get my understanding right?

Answer 1

the ((...)) construct is equivalent to the bash builtin let. let does mostly the same stuff which expr does.

the $((...)) construct, note the $ at the beginning, will substitute the output of the expression inside just like $(...) does.

the [...] construct is in fact just another name for test.

see the bash help pages for more information.

• help "("
• help let
• help [
• help test

see also:

• http://mywiki.wooledge.org/BashFAQ/031
• http://mywiki.wooledge.org/ArithmeticExpression