Using a variable in a regex

Question

How can I use a variable string in a regular expressions?

while read line
do
    if [[ line =~ *"$key"* ]]; then
        echo line
    fi
done < "$filename"

$key is a string variable and I want to print all the lines that has the value of $key.

Answer 1

You've made a few mistakes there. Here's your script corrected:

while read line
do
    if [[ "$line" =~ $key ]]; then
        echo $line
    fi
done < "$filename"

Though it seems it could be replaced with a simple grep:

grep "$key" "$filename"

Answer 2

First, whenever you're using the value of a variable, you need to write $line.

Also, note that read line strips off leading and trailing whitespace, and allows the sequence backslash-newline as a line continuation. To retain whitespace, use IFS= read. To avoid treating \ specially, use read -r. You can of course combine the two as IFS= read -r.

What do you want to test exactly? If you want to test that the line is exactly $key (except for whitespace unless you use IFS= read), use the == operator, and make sure to put $key in double quotes.

if [[ $line == "$key" ]]; then …

If you want to test that the line contains $key (so for example if the value of key is fu*bar then hellofu*bar and fu*barqux match, but fuubar and gu*bar don't), then use the == operator and again make sure to put $key in double quotes. The right-hand side of the == operator is a shell wildcard pattern; putting the variable in double quotes causes it to be treated literally.

if [[ $line == *"$key"* ]]; then …

If you want to treat $key as a wildcard pattern, so that if $key is fu*bar then the line can be fubar or fuqwebar but not fbar or afubar, then leave off the double quotes:

if [[ $line == $key ]]; then …

If you only want part of the line to match, allowing things like afubar:

if [[ $line == *$key* ]]; then …

Use the regular expression match operator =~ if you want to have an extended regular expression on the right-hand side. A regular expression can match part of the line; if you want to match the whole line, put ^ at the beginning of the regex and $ at the end. For example, [[ $line == *"$key"* ]] is equivalent to [[ $line =~ "$key" ]] while [[ $line == "$key" ]] is equivalent to [[ $line =~ ^"$key"$ ]].