Comparing an integer variable in the Bash shell

Question

I know I'm missing something very obvious, but I just can't see it. In the code below, new_i variable gets incremented properly, however, when I get to the if statement, I think my syntax is off. I looked at some examples, but none show variable set to zero when if statement is greater than (in this case 15) a number.

#!/bin/sh
i=$(cat /etc/hour.conf)
new_i=$((i+1))
if [[ "$new_i" -gt 15 ]]; then
 new_i=0
fi
echo "$new_i">/etc/hour.conf
echo "$new_i"

When I run this script I get the following error:

./loops: 3: ./loops: Illegal number: new_i

Thanks in advance for your help!

Ah, the problem is on line **3**... What is content of `/etc/hour.conf`? — grundic, Jun 16 '17 at 14:27
Hm.. Can you cat that file? `echo 0>x.txt && cat x.txt` gives me empty file. To fix it you have to add space before `>`: **echo 0 >/etc/hour.conf** — grundic, Jun 16 '17 at 14:30
@user3308131, why do you think `new_i` is greater than 15? The code is syntactically correct. Try to replace `i=$(cat /etc/hour.conf)` with `i=4`, for instance — Ruslan Osmanov, Jun 16 '17 at 14:31
Apart from the space in the assignment you had earlier, that seems to work ok. As long as the file contains a number, and not for example the string `new_i`. Which you could get if the echo to the file was typoed as `echo "new_i" > /etc/hour.conf`. But not from Bash. It would take the string as a variable name (yes, really, try `bash -c 'y=123; x=y; echo $(( x+1 ))' `. But then your hashbang line doesn't say `bash`, it says `/bin/sh`. — ilkkachu, Jun 16 '17 at 14:37
@user3308131, you should fetch the number from file with sed, awk, perl etc. Enclose the command substitution in double quotes: `i="$(....)"` — Ruslan Osmanov, Jun 16 '17 at 14:38
(And of course if your `/bin/sh` just so happens to be, say `dash`, it doesn't support `[[..]]`.) — ilkkachu, Jun 16 '17 at 14:41
@RuslanOsmanov, what difference do you suppose `sed` would make compared to `cat` if the file only contains a number. And while double-quoting is a good habit, it doesn't do a difference in an assignment like that. — ilkkachu, Jun 16 '17 at 14:42
@ilkkachu, how do you know that the file contains only a number? Yes, he can leave it as `i=$(...)`. — Ruslan Osmanov, Jun 16 '17 at 14:45
The code I run is correct if I change the filename `/etc/hour.conf` to `x.txt` and the initial content of the file is empty. — zhenguoli, Jun 16 '17 at 14:51
If you edit your code to remove bugs after they're pointed out, the result is a comment thread that makes no sense to new readers. — Charles Duffy, Jun 16 '17 at 15:49

score 2 · Accepted Answer · answered Jun 16 '17 at 14:50

2

This works - the $ in front of the i in the new_i=$(( line and the removal of the quotes and one set of the brackets fixed the errors and now the scripts works as intended. Thanks to everyone for their help!

#!/bin/sh
i=$(cat /etc/hour.conf)
new_i=$(($i+1))
if [ $new_i -gt 15 ]; then
 new_i=0
fi
echo "$new_i">/etc/hour.conf
echo "$new_i"

answered Jun 16 '17 at 14:50

user3308131

159
1
5
16

1

`$((i+1))` works the same as `$(($i+1))` if `i` is a valid number. And the code you posted works if I change the filename to `x.txt` and set it initial content to empty. – zhenguoli Jun 16 '17 at 14:55
Glad we helped... but really not clear how we did it, haha ;) – Paulo Mattos Jun 16 '17 at 14:57
`$((i+1))` is *definitely* valid out-of-the-box. – Charles Duffy Jun 16 '17 at 15:50
As for `[[ ]]` vs `[ ]`, the former requires a different shebang -- it'll work with `#!/bin/bash`, for example; just not `#!/bin/sh`. – Charles Duffy Jun 16 '17 at 15:52

Paulo Mattos · Answer 2 · 2017-06-16T14:50:33.050

0

Try without the quotes:

if [ $new_i -gt 15 ]; then
    ...
fi

or, better yet, use arithmetic evaluation instead:

if (( $new_i > 15 )) ; then
    ...
fi

edited Jun 16 '17 at 14:50

answered Jun 16 '17 at 14:24

Paulo Mattos

18,845
10
77
85

different error ./loops Illegal number:new_i – user3308131 Jun 16 '17 at 14:29
1

-get i think it is a typo – D'Arcy Nader Jun 16 '17 at 14:29
I used -gt when I tested. – user3308131 Jun 16 '17 at 14:33
(( $new_i > 15) gave the same error.... ./loops: Illegal number: new_i – user3308131 Jun 16 '17 at 14:44
What difference does removing the quotes make if the variable doesn't contain a valid number? – ilkkachu Jun 16 '17 at 14:45
@user3308131 set `new_i=20` and comment out the first two lines. If that works as expected, then your problem is elsewhere ;) – Paulo Mattos Jun 16 '17 at 14:48
@ilkkachu it wasn't clear initially that the nasty `new_i` didn't contain a valid integer... ;) – Paulo Mattos Jun 16 '17 at 14:56

Comparing an integer variable in the Bash shell

2 Answers2