Problem with logical operator in bash script - bash

Question

I read in tldp.com that
if [ $condition1 ] && [ $condition2 ]
Same as: if [ $condition1 -a $condition2 ]
Returns true if both condition1 and condition2 hold true..."
but when I tried
if [ $a == 2 ] || [ $b == 4 ]
then
echo "a or b is correct"
else
echo "a and b are not correct"
fi
it gives error. I'm using bash.

Answer 1

Your logic is ok but your comparison operators are incorrect, you should use the '-eq' for comparing integers and '==' for strings. See 'man test' for quick reference, though it's also documented in 'man bash'.
When using integer comparison it is always best to initialise variables to 0 as well otherwise if they remain unset you will get errors.
As mentioned by c00k, use [[ rather than [ if using bash as it is a builtin so bash will not need to shell out to use the /usr/bin/[ command.
i.e.
a=0;b=0
# do something else with a or b
if [[ $a -eq 2 ]] || [[ $b -eq 4 ]]
then
echo "a or b is correct"
else
echo "a and b are not correct"
fi

Answer 2

If you're using Bash, then drop the single [ and use double ones [[.
For arithmetic operations, use ((.
So you'd want to write this:
if (( a == 2 )) || (( b == 4 )); then
echo "foo"
fi # etc

Answer 3

Did you assign a value to a and b? If not you have to (otherwise you definitely should) quote your variables with double quotes:
if [ "$a" == "2" ] || [ "$b" == "4" ]; then
echo "a or b is correct";
else
echo "a and b are not correct";
fi

Answer 4

Correct me if I'm wrong. && and || are bash comparison
#!/usr/bin/ksh
set -x ###### debug mode on
while :
do
dt=`date '+%M'`
if ([ "$dt" -ge "15"] && [ "$dt" -le "17" ]) || ([ "$dt" -ge "25" ] && [ "$dt" -le "27" ])
then
echo "Time-->minutes between 15 to 17 OR 25 to 27"
else
echo "Time--> minutes out of range"
fi
sleep 300 ##### sleep for 5 minutes
done
------------- below lines are debug output
+ + date +%M
dt=17
+ [ 17 -ge 15 ]
+ [ 17 -le 17 ]
+ echo Time--> minutes between 15 to 17 OR 25 to 27
Time--> minutes between 15 to 17 OR 25 to 27
+ date
Sat Mar 3 15:17:23 AST 2012
+ sleep 300
>+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ :
+ + date +%M
dt=22
+ [ 22 -ge 15 ]
+ [ 22 -le 17 ]
+ [ 22 -ge 25 ]
+ echo Time--> minutes out of range
Time--> minutes out of range
+ sleep 300
^C$ ######## breaking out of loop(terminating the execution).
$
$ env |grep -i shell
+ grep -i shell
+ env
SHELL=/bin/ksh
$ set +x ##### ending debug mode.