Why does < (less than) not give the same result as -lt in bash [[ ]]?

Question

I was writing a bash script and suddenly this behaviour started:

[[ 1 < 2 ]]; echo $?  # outputs 0
[[ 2 < 13 ]]; echo $? # outputs 1

but -lt works soundly:

[[ 1 -lt 2 ]]; echo $?  # outputs 0
[[ 2 -lt 13 ]]; echo $? # outputs 0

did I accidentally overwrite < somehow?

here is a script I wrote to test this behaviour:

#!/bin/bash

for a in {1..5}
do
    for b in {1..20}
    do
        [[ $a < $b ]] && echo $a $b
    done

    echo
done

here is the output:

1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
1 10
1 11
1 12
1 13
1 14
1 15
1 16
1 17
1 18
1 19
1 20

2 3
2 4
2 5
2 6
2 7
2 8
2 9
2 20

3 4
3 5
3 6
3 7
3 8
3 9

4 5
4 6
4 7
4 8
4 9

5 6
5 7
5 8
5 9

changing < to -lt in the script gives normal output (5 10 shows up for example).

Rebooting did not change anything.

My bash version is GNU bash, version 4.3.42(1)-release (x86_64-pc-linux-gnu). I am on Ubuntu 15.10. I don't know what other information is relevant here.

Answer 1

From the bash man page.

When used with [[, the < and > operators sort lexicographically using the current locale.

From the output, it appears to be working as designed.

In bash use:

• [[ $a < $b ]] for lexicographical comparison as per the current locale (uses strcoll(3))
• [ "$a" -lt "$b" ] for decimal integer comparison.
• (( a < b )) or [[ $a -lt $b ]] for arithmetic expression comparisons. Beware that as the arithmetic expressions stored in $a and $b are being evaluated, that can have side effects including running arbitrary commands. For instance, x=5 a='++x' bash -c '[[ $a -lt $a ]]; echo "$? $x"' outputs 0 7 and a='x[$(reboot)]' b=a bash -c '[[ $a -lt $b ]]' reboots the system (twice). Also note that for those, numbers with leading 0 are interpreted as octal, not decimal.

Answer 2

How about:

for a in {1..5}; 
do     
  for b in {1..20};     
  do         
    (( $a < $b )) && echo $a $b
  done      
  echo
done

According to http://www.tldp.org/LDP/abs/html/dblparens.html

Similar to the let command, the (( ... )) construct permits arithmetic expansion and evaluation. In its simplest form, a=$(( 5 + 3 )) would set a to 5 + 3, or 8. However, this double-parentheses construct is also a mechanism for allowing C-style manipulation of variables in Bash, for example, (( var++ )).

Answer 3

Firstly, [[ is not POSIX and should be avoided.

Secondly, if you wish to use < as part of an arithmetic test you can do this, but with different syntax:

if [ "$((2 < 13))" -ne 0 ]
then
  echo '2 is less than 13'
else
  echo '2 is greater or equal to 13'
fi

Or:

if expr 2 '<' 13
then
  echo '2 is less than 13'
else
  echo '2 is greater or equal to 13'
fi