bash script: add two statements with binary operator in conditional statement

Question

I have the following bash script

i=1
while SH= read -r LINE
    do
        if echo $LINE | grep 'Network connection down!'
        then 
            echo "shutdown"
            exit
    elif echo $LINE | grep 'Network connection available'
    then
        echo $LINE
        ((i++))
    fi

done < <(tac /var/log/messages)

This script works so far. I am not an expert at shell scripting. I am trying to add an and operator to the first conditional statement, something like this

if echo $LINE | grep 'Network connection down!' && $i<4

Can anyone help show me how to make this work in shell script, every variation I have tried so far ends with various errors like

conditional binary operator expected

and various other ones that I can describe. Many thanks to everyone in advance :)

Answer 1

Shell programming requires a special command to perform comparisons, as the if statement itself does not work with expressions, only commands. In bash, the command to use is the conditional command, [[ ... ]].

if echo "$line" | grep 'Network connection down!' && [[ $i -lt 4 ]]; then

You can use an arithmetic command (( ... )) as well, which allows a more familiar looking expression:

if echo "$line" | grep 'Network connection down!' && (( i < 4 )); then

You can also replace your call to grep with the =~ operator:

if [[ $line =~ Network\ connection\ down! && $i -lt 4 ]]; then

( or [[ $line =~ Network \connection \down! ]] && (( i < 4)), but you probably get the point).