I am doing integer comparison in bash (trying to see if the user is running as root), and I found two different ways of doing it:
Double equals:
if [ $UID == 0 ]
then
fi
-eq
if [ $UID -eq 0 ]
then
fi
I understand that there's no >= or <= in bash, only -ge and -le, so why is there a == if there's a -eq?
Is there a difference in the way it compares both sides?
== is a bash-specific alias for =, which performs a string (lexical) comparison instead of the -eq numeric comparison. (It's backwards from Perl: the word-style operators are numeric, the symbolic ones lexical.)
-eq tells bash to interpret the strings as integers (producing 0 without a warning if a string isn't numeric).
[ 01 -eq 1 ] but [ 01 != 1 ].
== as a [ operator is non-standard and should not be used, it is not bash-specific. It was introduced by ksh and is also supported by zsh (though the first = needs to be quoted), yash and the GNU [ utility (and any such utilities implemented as ksh scripts on some systems) at least).
$ if [ "hello" -eq 0 ]; then echo true; fi bash: [: hello: integer expression expected
To elaborate on bollovan's answer...
There is no >= or <= comparison operator for strings. But you could use them with the ((...)) arithmetic command to compare integers.
You can also use the other string comparison operators (==, !=, <, >, but not =) to compare integers if you use them inside ((...)).
Examples
[[ 01 -eq 1 ]] and (( 01 == 1 )) do integer comparisons. Both are true.[[ 01 == 1 ]] and [ 01 = 1 ] do string comparisons. Both are false.(( 01 -eq 1 )) and (( 01 = 1 )) will return an error.Note: The double bracket syntax [[...]] and the double parentheses syntax ((...)) are not supported by all shells.
mksh/zsh (except in POSIX mode (though that's not a POSIX feature))), (( 010 == 10 )) would return false because 010 would be treated as an octal number (8 in decimal).
test/[ implementations don't have >=/<= operators (yash's [ has though), expr has such operators, though it will do arithmetic comparison if the arguments are recognised as numbers (expr 01 '>=' 1 returns true, expr X01 '>=' X1 returns false).
If you want to do integer comparison you will better use (( )), where you can also use >= etc.
Example:
if (( $UID == 0 )); then
echo "You are root"
else
echo "You are not root"
fi
(( UID == 0 )) or (( ! UID )) for that matters. Note that ((...)) is non-standard (a ksh feature also supported by bash and zsh with variations).
! UID not be more like UID != 0.
! UID evaluates to 1 if UID is 0 or to 0 otherwise, so it's exactly like UID == 0. There's no such thing as "boolean" in shell arithmetics,
[ $UID -eq 0 ], not[ $UID -eq 0].