The multiple constructs that are available in BASH can lead to confusion.

• Single brackets [ ] are generally OK, but might allow some syntax to be interpreted as commands (thus causing errors)
• Double brackets [[ ]] have less parsing, so are generally safer to use.

You must use the flag operators, or the symbolic operators inside double parentheses, to correctly compare values numerically. For example, say a=2 and b=10:

• [ 2 -lt 10 ] is TRUE
• [[ 2 -lt 10 ]] is TRUE
• [[ 2 -ge 10 ]] is FALSE
• (( 2 < 10 )) is TRUE

What results in an alphanumeric comparison (which has unexpected behavior), or a syntax error (which you may overlook) is the following:

• [[ $a < $b ]], alphanumerically FALSE
• [ $a < $b ], error; also FALSE.

Whenever I have a question about a confusing syntax issue, I write a short test script to flesh out all the possibilities. To distinguish between the correct, incorrect, and error-producing constructs, I wrote a script that produces this table:

This script is:

#!/usr/bin/env bash
# numeric_comparisons.bash - 
# Distinguish valid numeric versus 
# lexicographic comparison constructs.
###
 
a=2
b=10
echo "Numeric comparisons: $a less than $b should be TRUE"
echo "------------------------------------"
echo -ne "[ $a < $b ]\t"
[ $a < $b ] && echo "TRUE" || echo "FALSE"
 
echo -ne "[ ! $a -lt $b ]\t"
[ ! $a -lt $b ] && echo "TRUE" || echo "FALSE"
 
echo -ne "[ $a \< $b ]\t"
[ $a \< $b ] && echo "TRUE" || echo "FALSE"
 
echo -ne "[[ $a < $b ]]\t"
[[ $a < $b ]]  && echo "TRUE" || echo "FALSE"
 
echo -ne "[[ $a -lt $b ]]\t"
[[ $a -lt $b ]]  && echo "TRUE" || echo "FALSE"
 
echo -ne "[ $a -lt $b ]\t"
[ $a -lt $b ] && echo "TRUE" || echo "FALSE"
 
echo -ne "[ \"$a\" -lt \"$b\" ]\t"
[ "$a" -lt "$b" ] && echo "TRUE" || echo "FALSE"
 
echo -ne "test $a -lt $b\t"
test $a -lt $b  && echo "TRUE" || echo "FALSE"
 
echo -ne "(( $a < $b ))\t"
(( $a < $b ))  && echo "TRUE" || echo "FALSE"
 
echo -ne "(( $a -lt $b ))\t"
(( $a -lt $b ))  && echo "TRUE" || echo "FALSE"
 
# Reverse the values of a and b. 
# This should reverse the outcomes
# of the above commands.
a=10
b=2
echo "Numeric comparisons: $a less than $b should be FALSE"
echo "------------------------------------"
echo -ne "[ $a < $b ]\t"
[ $a < $b ] && echo "TRUE" || echo "FALSE"
 
echo -ne "[ $a \< $b ]\t"
[ $a \< $b ] && echo "TRUE" || echo "FALSE"
 
echo -ne "[[ $a < $b ]]\t"
[[ $a < $b ]]  && echo "TRUE" || echo "FALSE"
 
echo -ne "[[ $a -lt $b ]]\t"
[[ $a -lt $b ]]  && echo "TRUE" || echo "FALSE"
 
echo -ne "[ $a -lt $b ]\t"
[ $a -lt $b ] && echo "TRUE" || echo "FALSE"
 
echo -ne "test $a -lt $b\t"
test $a -lt $b  && echo "TRUE" || echo "FALSE"
 
echo -ne "(( $a < $b ))\t"
(( $a < $b ))  && echo "TRUE" || echo "FALSE"
 
echo -ne "(( $a -lt $b ))\t"
(( $a -lt $b ))  && echo "TRUE" || echo "FALSE"