Increment variable value by 1 (shell programming)

Question

I am a beginner in shell programming, and it sounds like a very stupid question but I can't seem to be able to increase the variable value by 1. I have looked at tutorialspoint's Unix / Linux Shell Programming tutorial but it only shows how to add together two variables.

I have tried the following methods but they don't work:

i=0  $i=$i+1 # doesn't work: command not found  echo "$i"  $i='expr $i+1' # doesn't work: command not found  echo "$i"  $i++ # doesn't work*, command not found  echo "$i" 

How do I increment the value of a variable by 1??

Answer 1

You can use an arithmetic expansion like so:

i=$((i+1)) 

or declare i as an integer variable and use the += operator for incrementing its value.

declare -i i=0 i+=1 

or use the (( construct.

((i++)) 

Answer 2

There are more than one way to increment a variable in bash, but what you tried is not correct.

You can use for example arithmetic expansion:

i=$((i+1)) 

or only:

((i=i+1)) 

or:

((i+=1)) 

or even:

((i++)) 

Or you can use let:

let "i=i+1" 

or only:

let "i+=1" 

or even:

let "i++" 

See also: http://tldp.org/LDP/abs/html/dblparens.html.