Bash incrementation breaking scripts

Question

I'm having a weird issue incrementing a bash variable that seems to be breaking after my first attempt at incremntation that I cannot pin down, here is a sample of what I am doing and the debug output, anyone see any reason this should NOT work?

I am currently on GNU bash, version 4.2.45(1)-release (i686-pc-linux-gnu)

#!/bin/bash
set -ex 
declare -i elem=0
echo $elem # 0
    (( elem++ )) # breaks
echo $elem # 1 but never encountered
while IFS=$'\n' read -r line || [[ -n "$line" ]]; do
    (( elem++ ))
    echo $elem
done <"${1}" # foo\nbar\nbaz

Output

./incr.sh test
+ declare -i elem=0
+ echo 0
0
+ ((  elem++  ))

The weirdest part is by changing the initial incrementor to (( elem+=1 )) the entire program increments correctly, this seems extremely buggy to the eye...

#!/bin/bash
set -ex 
declare -i elem=0
echo $elem
    (( elem+=1 ))
echo $elem
while IFS=$'\n' read -r line || [[ -n "$line" ]]; do
    (( elem++ ))
    echo $elem
done <"${1}" # foo\nbar\nbaz

Output

+ declare -i elem=0
+ echo 0
0
+ ((  elem+=1  ))
+ echo 1
1
+ IFS='
'
+ read -r line
+ ((  elem++  ))
+ echo 2
2
+ IFS='
'
+ read -r line
+ ((  elem++  ))
+ echo 3
3
+ IFS='
'
+ read -r line
+ ((  elem++  ))
+ echo 4
4
+ IFS='
'
+ read -r line
+ [[ -n '' ]]

Where is filename=test coming from? I don't see that in the script. — Barmar
– Barmar, Commented Nov 20, 2013 at 18:57
Sorry I changed the code to just use $1 as the input file, if you look again I changed the input, sry I was using a older versions output — ehime
– ehime, Commented Nov 20, 2013 at 18:58
Why did you delete your code? The question is nonsensical without the code. — John Kugelman
– John Kugelman, Commented Dec 30, 2013 at 19:23

that other guy · Accepted Answer · 2013-11-20 19:05:30Z

3

set -e makes your script exit when any command returns failure.

(( 0 )), and equivalently elem=0; (( elem++ )) returns failure.

Therefore, the script exits.

If you set -e and want to run commands whose status you don't care, about, you can use

(( elem++ )) || true

answered Nov 20, 2013 at 19:05

that other guy

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5 Comments

ehime Over a year ago

Awesome anwer, waiting to accept =) thank you, great catch, didn't see that

Barmar Over a year ago

Why does ((elem++)) return failure?

ehime Over a year ago

because elem evaluates as bool(0) ;)

Barmar Over a year ago

Another fix would be to use pre-increment (( ++elem )), since this will evaluate to 1?

that other guy Over a year ago

@Barmar that happens to work in this case, but if elem=-1 you're back to the same problem.

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