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 :)

like if echo $LINE | grep 'Network connection down!' && [[$i<4]] ? — lindsaymacvean
– lindsaymacvean, Commented Oct 5, 2015 at 18:25

chepner · Accepted Answer · 2015-10-05 18:33:42Z

4

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).

edited Oct 5, 2015 at 18:33

answered Oct 5, 2015 at 18:29

chepner

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3 Comments

Andrea Corbellini Over a year ago

woops, sorry. Forgot we were talking about numbers

lindsaymacvean Over a year ago

perfect thanks. my script worked with [[ $i <4 ]] does it require (( parentheses when using < instead of -lt ?

chepner Over a year ago

Inside [[ ... ]], -lt is used for integer comparisons and < is used for string comparisons. Inside ((...)), everything is an arithmetic operation, so < can be used for integer comparisons instead of -lt.

Corral · Accepted Answer · 2015-10-05 18:55:03Z

0

The "if" line can be written this way. I've changed "<" by "-lt" since "<" and ">" are supposed to be used with strings, while -lt, -gt, -eq and others are better for numbers.

if [[ `echo $LINE|grep -c 'Network connection down!'` -eq 1 && $i -gt 4 ]]

The execution of this part counts the lines with text 'Network connection down!', always one or zero. Then this result is compared to one (1)

echo $LINE|grep -c 'Network connection down!'

answered Oct 5, 2015 at 18:55

Corral

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