Try it:
$ a=0
$ let a++
$ echo $?
1 # Did the world just go mad?
$ echo $a
1 # Yes, it did.
$ let a++
$ echo $?
0 # We have normality.
$ echo $a
2
Contrast with this:
$ b=0
$ let b+=1
$ echo $?
0
And this (from Sirex):
$ c=0
$ let ++c
$ echo $?
0
What is going on here?
$ bash --version
GNU bash, version 4.1.5(1)-release (x86_64-pc-linux-gnu)
let a++
does not work) – rahmu Feb 21 '12 at 10:42