Tell me more ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.
up vote 1 down vote favorite

How do I assign x the value of x + 1? I can do it in other languages but can't figure it out in bash.

share|improve this question

4 Answers

up vote 7 down vote accepted

I just tested two different ways and both worked for me:

x=$((x+1))

...or...

x=$((++x))
share|improve this answer
Thanks it worked! – JShoe May 22 '11 at 3:41
up vote 2 down vote

this should do the work

let x=$x+1
share|improve this answer
That gets a return of 1. – JShoe May 22 '11 at 3:39
let x=x+1 should be let x=$x+1. That is likely the reason it is returning one. – Bandit May 22 '11 at 3:54
oops, missed the dollar – freethinker May 22 '11 at 4:24
up vote 1 down vote

This might work:

x = `expr $x + 1`
share|improve this answer
Nope. It assigns what's in the quote to x. – JShoe May 22 '11 at 3:36
1  
@JShoe: Those weren't quotes, they were grave accents. On most keyboards, they're located to the left of the 1 key, on the key you press with Shift to enter a tilde (~). – Patches May 22 '11 at 4:30
1  
Also known as backticks in the context of computing. – Ben Alpert May 22 '11 at 5:54
Another reason not to use backticks anymore, but rather $() ;) – slhck♦ May 22 '11 at 8:53
Though a valid answer, expr is old school. It makes you shell out, an unnecessary process. The syntax in @Bandit 's answer is more modern, and is done in-shell. – Rich Homolka May 27 '11 at 21:45
up vote 0 down vote

@Bandit's answer is fine, but I want to highlight the difference that "let" and (( )) make to normal shell syntax:

let x++

causes bash (or ksh, or any POSIX shell) to treat the expression as an "arithmetic evaluation" in which the referenced variables don't need to be preceeded with "$". One advantage of using (( )) is that otherwise-special tokens don't need to be quoted or escaped, e.g. "*" for multiplication as in:

(( x = x * 2 ))

I find this syntax slightly clearer than $(( )) which uses the output of the expression, e.g.

x=$(( x * 2 ))
share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.