Bash operators

'[...]' vs '[[...]]'

This is kind of frustrating, especially when starting out.

There are two types of expressions in bash, and each uses different operators.

[ 123 -eq 123 ]    # equal
[ 123 -ne 321 ]    # not equal

[ 1 -le 2 ]        # less than or equal
[ 1 -lt 2 ]        # less than

[ 2 -ge 1 ]        # greater than or equal
[ 2 -gt 1 ]        # greater than

See man test.

[[ "123" == "123" ]]  # equal
[[ "123" != "321" ]]  # not equal

[[ "ab" <= "abc" ]]   # less characters or equal
[[ "ab" <  "abc" ]]   # less characters

[[ "abc" >= "ab" ]]   # more characters or equal
[[ "abc" >  "ab" ]]   # more characters