Bash – Printing a string when grep does not get a match

bashgrep

Suppose I have two files which look as follows:

$ cat search_file.txt
This line contains kwd1.
This line contains kwd2.
This line contains no match.
This line contains no match.
This line contains kwd5.

$ cat search_kwd.sh
grep kwd1 search_file.txt
grep kwd2 search_file.txt
grep kwd3 search_file.txt
grep kwd4 search_file.txt
grep kwd5 search_file.txt

When I run search_kwd.sh, I get:

$ sh search_kwd.sh
This line contains kwd1.
This line contains kwd2.
This line contains kwd5.

I want to print a string whenever grep does not get a match. The output would look like:

$ sh search_kwd.sh
This line contains kwd1.
This line contains kwd2.
string
string
This line contains kwd5.

How do I go about doing this in bash?

Best Answer

grep exits with non-zero code when nothing found.

From man grep:

Normally the exit status is 0 if a line is selected, 1 if no lines were selected, and 2 if an error occurred.

So you can use:

grep kwd3 search_file.txt || echo "string"

Related Solutions

Grep string where next line does not contain string

You can use perl compatible regular expressions grep:

$ pcregrep -M '(searchString.*\n)(?!.*excludeString)' file
foo2 searchString bar
foo3 searchString bar
foo4 searchString bar

It searches searchString followed by any char ., repeated zero or more times *, followed by new line \n only if there is not (?!) pattern .*excludeString next to it. Option -M is present in order to match multi lines.

Bash – How to use grep when file does not contain the string

grep will return success if it finds at least one instance of the pattern and failure if it does not. So you could either add an else clause if you want both "does" and "does not" prints, or you could just negate the if condition to only get failures. An example of each:

if grep -q "$user2" /etc/passwd; then
    echo "User does exist!!"
else
    echo "User does not exist!!"
fi

if ! grep -q "$user2" /etc/passwd; then
    echo "User does not exist!!"
fi

Best Answer

Related Solutions

Grep string where next line does not contain string

Bash – How to use grep when file does not contain the string

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