• Uncategorized

About bash : How-can-I-compare-numbers-in-Bash

Question Detail

I’m unable to get numeric comparisons working:

echo "enter two numbers";
read a b;

echo "a=$a";
echo "b=$b";

if [ $a \> $b ];
then
    echo "a is greater than b";
else
    echo "b is greater than a";
fi;

The problem is that it compares the number from the first digit on, i.e., 9 is bigger than 10, but 1 is greater than 09.

How can I convert the numbers into a type to do a true comparison?

Question Answer

In Bash, you should do your check in an arithmetic context:

if (( a > b )); then
    ...
fi

For POSIX shells that don’t support (()), you can use -lt and -gt.

if [ "$a" -gt "$b" ]; then
    ...
fi

You can get a full list of comparison operators with help test or man test.

Like this:

#!/bin/bash

a=2462620
b=2462620

if [ "$a" -eq "$b" ]; then
  echo "They're equal";
fi

Integers can be compared with these operators:

-eq # Equal
-ne # Not equal
-lt # Less than
-le # Less than or equal
-gt # Greater than
-ge # Greater than or equal

See this cheatsheet.

There is also one nice thing some people might not know about:

echo $(( a < b ? a : b ))

This code will print the smallest number out of a and b

In Bash I prefer doing this as it addresses itself more as a conditional operation unlike using (( )) which is more of arithmetic.

[[ n -gt m ]]

Unless I do complex stuff like

(( (n + 1) > m ))

But everyone just has their own preferences. Sad thing is that some people impose their unofficial standards.

You can also do this:

[[ 'n + 1' -gt m ]]

Which allows you to add something else which you could do with [[ ]] besides arithmetic stuff.

The bracket stuff (e.g., [[ $a -gt $b ]] or (( $a > $b )) ) isn’t enough if you want to use float numbers as well; it would report a syntax error. If you want to compare float numbers or float number to integer, you can use (( $(bc <<< "...") )).

For example,

a=2.00
b=1

if (( $(bc <<<"$a > $b") )); then 
    echo "a is greater than b"
else
    echo "a is not greater than b"
fi

You can include more than one comparison in the if statement. For example,

a=2.
b=1
c=1.0000

if (( $(bc <<<"$b == $c && $b < $a") )); then 
    echo "b is equal to c but less than a"
else
    echo "b is either not equal to c and/or not less than a"
fi

That’s helpful if you want to check if a numeric variable (integer or not) is within a numeric range.

This code can also compare floats. It is using AWK (it is not pure Bash). However, this shouldn’t be a problem, as AWK is a standard POSIX command that is most likely shipped by default with your operating system.

$ awk 'BEGIN {return_code=(-1.2345 == -1.2345) ? 0 : 1; exit} END {exit return_code}'
$ echo $?
0
$ awk 'BEGIN {return_code=(-1.2345 >= -1.2345) ? 0 : 1; exit} END {exit return_code}'
$ echo $?
0
$ awk 'BEGIN {return_code=(-1.2345 < -1.2345) ? 0 : 1; exit} END {exit return_code}'
$ echo $?
1
$ awk 'BEGIN {return_code=(-1.2345 < 2) ? 0 : 1; exit} END {exit return_code}'
$ echo $?
0
$ awk 'BEGIN {return_code=(-1.2345 > 2) ? 0 : 1; exit} END {exit return_code}'
$ echo $?

To make it shorter for use, use this function:

compare_nums()
{
   # Function to compare two numbers (float or integers) by using AWK.
   # The function will not print anything, but it will return 0 (if the comparison is true) or 1
   # (if the comparison is false) exit codes, so it can be used directly in shell one liners.
   #############
   ### Usage ###
   ### Note that you have to enclose the comparison operator in quotes.
   #############
   # compare_nums 1 ">" 2 # returns false
   # compare_nums 1.23 "<=" 2 # returns true
   # compare_nums -1.238 "<=" -2 # returns false
   #############################################
   num1=$1
   op=$2
   num2=$3
   E_BADARGS=65

   # Make sure that the provided numbers are actually numbers.
   if ! [[ $num1 =~ ^-?[0-9]+([.][0-9]+)?$ ]]; then >&2 echo "$num1 is not a number"; return $E_BADARGS; fi
   if ! [[ $num2 =~ ^-?[0-9]+([.][0-9]+)?$ ]]; then >&2 echo "$num2 is not a number"; return $E_BADARGS; fi

   # If you want to print the exit code as well (instead of only returning it), uncomment
   # the awk line below and comment the uncommented one which is two lines below.
   #awk 'BEGIN {print return_code=('$num1' '$op' '$num2') ? 0 : 1; exit} END {exit return_code}'
   awk 'BEGIN {return_code=('$num1' '$op' '$num2') ? 0 : 1; exit} END {exit return_code}'
   return_code=$?
   return $return_code
}

$ compare_nums -1.2345 ">=" -1.2345 && echo true || echo false
true
$ compare_nums -1.2345 ">=" 23 && echo true || echo false
false

If you have floats, you can write a function and then use that. For example,

#!/bin/bash

function float_gt() {
    perl -e "{if($1>$2){print 1} else {print 0}}"
}

x=3.14
y=5.20
if [ $(float_gt $x $y) == 1 ] ; then
    echo "do stuff with x"
else
    echo "do stuff with y"
fi

One-line solution.

a=2
b=1
[[ ${a} -gt ${b} ]] && echo "true" || echo "false"

I solved this by using a small function to convert version strings to plain integer values that can be compared:

function versionToInt() {
  local IFS=.
  parts=($1)
  let val=1000000*parts[0]+1000*parts[1]+parts[2]
  echo $val
}

This makes two important assumptions:

  1. The input is a “normal SemVer string”
  2. Each part is between 0-999

For example

versionToInt 12.34.56  # --> 12034056
versionToInt 1.2.3     # -->  1002003

Example testing whether npm command meets the minimum requirement…

NPM_ACTUAL=$(versionToInt $(npm --version))  # Capture npm version
NPM_REQUIRED=$(versionToInt 4.3.0)           # Desired version
if [ $NPM_ACTUAL \< $NPM_REQUIRED ]; then
  echo "Please update to [email protected]"
  exit 1
fi

Just adding to all the above answers:

If you have more than one expression in single if statement, you can do something like this:

if (( $a % 2 == 0 )) && (( $b % 2 != 0));
  then
  echo "What you want to do"
fi

Hope this helps!

You may also like...

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.