Bash loop control while if else with a return

Question

I need to return a true or false if I find a value within a file. I parse the file and if the value is located once which is enough, I want to break and return a true else return false. On top of it I need to pass the file into this check function. I'm trying to only use bash.

is_file_contains_VAR(){
    VARFILENAME=$1

    while read LINE
    do
        if echo "$LINE" | grep -q "$VAR"; then
            break
            return 0   
        else
            return 1
        fi
    done < $VARFILENAME
}

First thing you might want to do here is ask yourself what a statement immediately after a break might do.... — Ray Toal
– Ray Toal, Commented Feb 23, 2014 at 23:41
A break should get me out of the while at least I'm hoping. I don't want it to continue through the whole file. — MAXGEN
– MAXGEN, Commented Feb 23, 2014 at 23:43
Would it make more sense to have the return 0 after the while? so after the last line done < $VARFILENAME — MAXGEN
– MAXGEN, Commented Feb 23, 2014 at 23:45
Yes but that is still only the first problem. Your code is always returning a value the first time through the loop. You need to wait until you have gone through the whole file without seeing VAR in order to return 1. — Ray Toal
– Ray Toal, Commented Feb 23, 2014 at 23:48

ruakh · Accepted Answer · 2014-02-23 23:55:44Z

4

grep -q already does what you want: it will abort as soon as it finds the string in question. So you can just write:

function is_file_contains_VAR () {
    grep -q -e "$VAR" "$1"
}

(Note: the -e is in case "$VAR" starts with a hyphen.)

But for educational purposes . . . to write this as a loop, what you would want is to return 0 as soon as there's a match, and only return 1 at the very end, if you never find a match:

function is_file_contains_VAR () {
    local VARFILENAME="$1"
    local LINE
    while IFS= read -r LINE ; do
        if grep -q -e "$VAR" <<< "$LINE" ; then
             return 0
        fi
    done < "$VARFILENAME"
    return 1
}

edited Feb 23, 2014 at 23:55

answered Feb 23, 2014 at 23:50

ruakh

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3 Comments

chepner Over a year ago

Also, if using the bash loop shown, use bash's built-in regular expression support instead of forking a grep process: if [[ "$LINE" =~ $VAR ]]; then. You would need to make sure that VAR conforms to the regular expressions understood by bash.

MAXGEN Over a year ago

what do you mean by "forking" a grep? Does this open up a new thread is what you mean? Is grep natively supported in bash, meaning I can use it on any machine with bash?

ruakh Over a year ago

@MAXGEN: Not "forking a grep", but "forking a grep process"; that is, using fork() to create a new process, which will then be used to run grep. (If you're not familiar with forking and processes, then please Google them; I can't/won't explain them in a short comment.) Re: "Is grep natively supported in bash, meaning I can use it on any machine with bash?": No. They are separate programs. If you have bash, you'll usually have grep, but there are plenty of exceptions.

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