Weird result in bash elif ladder [duplicate] - linux

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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

Answer 4

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.

Answer 5

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.

Answer 6

One-line solution.
a=2
b=1
[[ ${a} -gt ${b} ]] && echo "true" || echo "false"
gt reference: https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html
&& reference: https://www.gnu.org/software/bash/manual/html_node/Shell-Arithmetic.html
[[...]] construct reference: https://www.gnu.org/software/bash/manual/bash.html#index-_005b_005b
${} reference: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02 (2.6.2)
The format for parameter expansion is as follows:
${expression}
where expression consists of all characters until the matching '}'.
Any '}' escaped by a or within a quoted string, and
characters in embedded arithmetic expansions, command substitutions,
and variable expansions, shall not be examined in determining the
matching '}'.
The simplest form for parameter expansion is:
${parameter}

Answer 7

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

Answer 8

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

Answer 9

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 $val
}
This makes two important assumptions:
The input is a "normal SemVer string"
Each part is between 0-999
For example
versionToInt 12.34.56 # --> 12034056
versionToInt 1.2.3 # --> 1002003
Example testing whether npm command meets the minimum requirement...
NPM_ACTUAL=$(versionToInt $(npm --version)) # Capture npm version
NPM_REQUIRED=$(versionToInt 4.3.0) # Desired version
if [ $NPM_ACTUAL \< $NPM_REQUIRED ]; then
echo "Please update to npm#latest"
exit 1
fi

Answer 10

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!