Linux Bash - Comparing two variables in an if statement

Question

I am trying to perform a variable swap where the user enters 2 different numbers and a check is performed to make sure that the second number "end" is bigger then the first number "start". If the starting number is bigger then the end number it will perform a swap so that the later calculations will work.

The error that I am getting is: ./Boot.sh: line 94: [: -ne: unary operator expected

                    if [ $end -gt $start ]
                        then
                            start=$swapper
                            end=$start
                            swapper=$end
                    fi

This is the full code as I seem to have made a few mistakes by the comments, I have just tried double brackets and am still having the same error.

                    {
counter=0
while :
    do
        if [ $counter -gt '0' ]     
            then
                printf "\nWould you like to continue terminating processes?\n"
                echo "Type 1 to contiue or 0 to exit"
                read endProgram
            if [ $endProgram -ne '1' ]
                then
                    break
            fi
        fi      
        echo "Welcome to Max process killer"
        while :
            do
                echo "Enter the first number of the process range"
                read start
                echo "Enter the last number of the process range"
                read end
                if [ $start -le '3' -o $end -le '3' ]
                    then
                        echo "Your first and last pid IDs must be geater then 3"
                        read -n 1
                        break
                    if [[ $end -gt $start ]]
                        then
                            start=$swapper
                            end=$start
                            swapper=$end
                    fi
                fi
                printf "\nAre you sure you would like to terminate these processes?\n"
                echo "Enter 1 to confirm or 0 to cancel"
                read confirm
                if [ $read -ne '1' ]
                    then
                        break
                fi
                while [ $start -le $end ]
                    do
                        kill -9 "$start"
                        echo "Process ID:"$start "has been terminated" 
                        start=$(( $start + 1 ))
                done
                break
        done
        counter=$(($counter + 1))
done
                    }

Have you tried quoting your variables? What are their values at the time of the if? — Biffen
– Biffen, Commented Aug 24, 2015 at 6:54
Oh, and that swapping code looks wrong, but that's an other question. — Biffen
– Biffen, Commented Aug 24, 2015 at 6:55
"-ne"???? Where??? I suspect one or more of your variables might be corrupt. set -v or bash -x is your friend. And yes, that swapping code is wrong ;) — paulsm4
– paulsm4, Commented Aug 24, 2015 at 6:55
I have made an update to the code to show that block which is being effected. The line 94 is the line which has the if statement that I have shown above, excuse some of the formatting of the starting and ending brackets for the block — Max Mcgregor
– Max Mcgregor, Commented Aug 24, 2015 at 7:06

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

Your code:

read confirm
if [ $read -ne '1' ]

You read input to variable confirm and not to variable read. Variable read is empty and you get the error:

./Boot.sh: line 94: [: -ne: unary operator expected

Hint: Use a default value (e.g. 0) to avoid an empty variable:

if [ "${confirm:-0}" -ne '1' ]

From man bash:

${parameter:-word}: Use Default Values. If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

${parameter:=word}: Assign Default Values. If parameter is unset or null, the expansion of word is assigned to parameter. The value of parameter is then substituted. Positional parameters and special parameters may not be assigned to in this way.

edited Jun 20, 2020 at 9:12

CommunityBot

11 silver badge

answered Aug 24, 2015 at 7:14

Cyrus

90.2k15 gold badges112 silver badges173 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

chepner Over a year ago

A better solution would be to use the correct parameter $confirm instead of using the default value expansion to produce a constant 0.

Cyrus Over a year ago

Correct :) My hint was for the case if the user simply presses only return.

chepner Over a year ago

It doesn't matter what the user types; the code is simply using the wrong parameter name in the check. Maybe you meant to write ${confirm:-0} in the answer. (And it should be quoted since it is used with [ ... ].

Cyrus Over a year ago

@chepner: Now I understand. Thank you. I've updated my answer.

Cyrus · Accepted Answer · 2015-08-24 07:01:45Z

0

Try using double brackets. They are generally safer. Here's a good wiki page comparing [[]] to [].

if [[ $end -gt $start ]]
then
    swapper=$start
    start=$end
    end=$swapper
fi

edited Aug 24, 2015 at 7:01

Cyrus

90.2k15 gold badges112 silver badges173 bronze badges

answered Aug 24, 2015 at 6:56

asdf

3,0872 gold badges24 silver badges43 bronze badges

Collectives™ on Stack Overflow

Linux Bash - Comparing two variables in an if statement

2 Answers 2

4 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related