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up vote 5 down vote favorite

In shell script we can substitute expr $a*$b with $(($a+$b)).

But why not just with (($a+$b)), because in any resource it is written that (()) is for integer computation.

So we use $(()) when there are variables instead of integer values do we? And what should we use instead of $(()) when variables can receive float values?

share|improve this question
    
See also unix.stackexchange.com/questions/149823/… – Gilles yesterday
up vote 8 down vote accepted

  1. expr is archaic. Don't use it.

  2. $((...)) and ((...)) are very similar. Both do only integer calculations. The difference is that $((...)) returns the result of the calculation and ((...)) does not. Thus $((...)) is useful in echo statements:

    $ a=2; b=3; echo $((a*b))
6

    ((...)) is useful when you want to assign a variable or set an exit code:

    $ a=3; b=3; ((a==b)) && echo yes
yes

  3. If you want floating point calculations, use bc or awk:

    $ echo '4.7/3.14' | bc -l
1.49681528662420382165

$ awk 'BEGIN{print 4.7/3.14}'
1.49682
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1  
If expr is archaic what should we use instead of expr text : '.*' – Stranger 2 days ago
    
If s is a shell variable, its length is ${#s} – John1024 2 days ago
    
${#} means a number of arguments and $(#s} means a number of characters of a variable does it? – Stranger 2 days ago
    
Yes. That's right. – John1024 2 days ago
1  
@Stranger Many uses of expr STRING : REGEX can be written as case STRING in PATTERN). expr is only useful when REGEX can't be expressed with shell wildcards. – Gilles yesterday
up vote 1 down vote

expr is old, but it does have one limited use I can think of. Say you want to search a string. If you want to stay POSIX with grep, you need to use a pipe:

if echo november | grep nov 
then
  : do something
fi

expr can do this without a pipe:

if expr november : nov
then
  : do something
fi

the only catch is expr works with anchored strings, so if you want to match after the beginning you need to change the REGEXP:

if expr november : '.*ber'
then
  : do something
fi

Regarding (( )), this construct is not POSIX, so should be avoided.

Regarding $(( )), you do not have to include the dollar sign:

$ fo=1
$ go=2
$ echo $((fo + go))
3
share|improve this answer
1  
Many uses of expr STRING : REGEX can be written as case STRING in PATTERN). expr is only useful when REGEX can't be expressed with shell wildcards. – Gilles yesterday

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