3

What's wrong with this code? If I input the number 25, it output failed instead of lol. Am I missing something?

read -p "Enter number : " num
if [ `expr $num > 5` ] 
then 
echo "lol" 
else 
echo "failed" 
fi
Baptiste Mille-Mathias
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user8422062
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  • See: [How to use double or single brackets, parentheses, curly braces](https://stackoverflow.com/q/2188199/6862601). – codeforester Sep 01 '18 at 17:16
  • Also see: [What is the difference between the Bash operators [[ vs [ vs ( vs ((?](https://unix.stackexchange.com/q/306111/201820) on Unix & Linux SE and [Is double square brackets preferable over single square brackets in Bash?](https://stackoverflow.com/q/669452/6862601). – codeforester Sep 01 '18 at 17:18

4 Answers4

4

The code 

if [ `expr $num > 5` ]

actually does not do want you think. It will run expr $num > 5, so evaluate parameter and redirect the out to a file named 5 ("put 25 in a file named 5") and if will evaluate the return code of the previous expression.

If the code if meant to check evaluate if a number is bigger than 5, replace 

if [ `expr $num > 5` ]

with

[ "$num" -gt 5 ]

-gt stands for greater than

Baptiste Mille-Mathias
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    `if` does not evaluate the return code of the previous expression. What makes it fail here is that the `expr` output is redirected to the filename `5` thus `expr` does not output anything to stdout. If the call were `expr $num + 5` the code would echo `"lol"` – vdavid Sep 01 '18 at 11:19
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    You are missing a space: `[ "$num" -gt 5 ]`. – PesaThe Sep 01 '18 at 11:30
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    Only `$num` is the parameter; the output of `expr 25` will be written to a file named `5`. – chepner Sep 01 '18 at 12:07
4

@babtistemm's answer gives you the suggested solution but in case you insist on using (the bit oldish) expr for some reason:

read -p "Enter number : " num
if expr "$num" '>' 5 >/dev/null
    then 
        echo "lol" 
    else 
        echo "failed" 
fi

Notes:

  1. You need to quote > so that the shell does not interpret it as redirecting the stdout. You could also use \>.

  2. It is good practice to add double quotes to $num as well, so that expr will interpret it as one expression, thus limiting the chances of a very bad bug or a malicious user hacking your program. (Best would be to do a sanity-check on $num before using it, e.g. checking if it is an integer.)

  3. This solution necessitates calling a new process, expr, which costs a lot more resource from the OS than using the test shell command only.

  4. If you omit the >/dev/null, you will also get a 0 or 1 printed (meaning false or true), the stdout of expr. But independently of that, expr sets its exit status, $? according to the result of the expression, which is tested then by the if. (A side remark: if you try to echo $? after calling expr, it may come at a surprise first that $? = 0 means true/success as exit status, and $? != 0 means false by convention.)

tif
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    It might also be a good idea to sanity check your input, so that user-supplied data can't generate syntactically invalid code (which might be vulnerable to exploits). For example: `if expr "${num}" : '[0-9][0-9]*$' && expr "$num" '>' 5 >/dev/null; then` – ghoti Sep 01 '18 at 11:46
  • For sure, the above code is 100% vulnerable. I fear that a malevolent user could execute any code under the script's privileges even if you quote the first operand as `"$num"`. – tif Sep 01 '18 at 11:59
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    @ghoti You’re right about the quotes but I think the regex is overkill for OP's issue. – vdavid Sep 01 '18 at 14:27
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    @tif unless `expr` exposes a way to execute code, if the parameter is quoted than you are safe from attacks. – vdavid Sep 01 '18 at 14:29
3

you can use an arithmetic expression in bash:

if (( num > 5 )); then ...

In the manual, see https://www.gnu.org/software/bash/manual/bash.html#Conditional-Constructs

glenn jackman
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1

Same in a short line:

read -p 'Enter a number: ' num
(( num > 5 )) && echo lol || echo fail

could be condensed:

read -p 'Enter a number: ' num;((num>5))&&echo lol||echo fail

This syntaxe will work if first command success!

read -p 'Enter a number: ' num
((num > 5)) && {
      echo lol
      /bin/wrongcommand
      :
 } || echo fail

Could output:

lol
bash: /bin/wrongcommand: No such file or directory

But no fail because of : (is an alias for true) will alway success.

You could of course group command for fail:

read -p 'Enter a number: ' num
((num > 5)) && {
      echo lol
      /bin/wrongcommand
      :
 } || {
      echo fail
      other command
 }

Could be written:

read -p 'Enter a number: ' num;((num>5))&&{ echo lol;/bin/wrongcommand;:;}||{ echo fail;other command;}

You could group commands between {  and ;} (care about the space after first {!)

Enter a number: 4
fail
bash: other: command not found

Enter a number: 7
lol
bash: /bin/wrongcommand: No such file or directory
F. Hauri - Give Up GitHub
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