UNIX for Dummies Questions & Answers

Hi,

Can anyone explain what is going on here:

michael-browns-powerbook-g4-15:~ msb65$ start=-1
michael-browns-powerbook-g4-15:~ msb65$ stop=1
michael-browns-powerbook-g4-15:~ msb65$ if [ $start -o $stop -lt 0 ]; then echo hello; fi
-bash: [: -1: unary operator expected
michael-browns-powerbook-g4-15:~ msb65$ start=1
michael-browns-powerbook-g4-15:~ msb65$ if [ $start -o $stop -gt 0 ]; then echo hello; fi
hello

Are you not allowed to compare negative numbers in if statements?

Try this

Code:

a=-1
b=-3

result=$(( a > b  ))
if [[ $result -eq 1 ]] ; then
     echo "$a is greater than $b"
else
     echo "$a is not greater than $b"
fi

use the $(( )) construct for arithmetic operations.

Hi thanks,

This is working. Why do you use double brackets in your if statement?

if [[ ..]] means that the shell internally evaluates the expression and return a zero status when expression is true.

if [ ... ] can mean use /bin/test to evaluate the expression. Do man [ and you will see that the test man page is displayed.

Thank you very much.

I also have a question regarding the syntax of if statements. I have a variable:

temporal_avg=MONTH

Later on I would like to see if temporal_avg is equal to at least on of the following: D,8D,MO, or NS. I would like to have an if statement that basically is:

if [ $temporal_avg != D or 8D or MO or NS ]; then
exit
fi

What would be the proper syntax inside the []?

What shell are you using? If it's ksh:

Code:

if [[ "$temporal_avg" != ?(D|8D|MO|NS) ]]
then
    exit
fi

Otherwise case is probably neatest:

Code:

case "$temporal_avg" in
    D|8D|MO|NS) ;;
    *) exit ;;
esac

Hi,

I am using bash.