Arithmetic

• Using (( )):

n=123
(( n+=1 ))
echo $n
#n=124

And also echo $(( n+1 ))

This construct can also be used to force a base, or trim leading zeroes: - $((10#0000123)) returns 123. Without 10# a number with leading zeroes could be interpreted as octal.

• Using let:

n=123
let n+=1
echo $n
#n=124

NOTE about let

If the last ARG evaluates to 0, let returns 1; let returns 0 otherwise." ['help let']

So a better alternative for scripts with set -e is to use (( )). Otherwise let will just stop running the script silently!!!

let var=0
# $? = 1

let var++
# $? = 1

let var=1
# $? = 1

let var=2
# $? = 0

let var++
# $? = 0