sed: How to extract only first occurance of value

Question

I have file settings.inc.php with the following content:

<?php define('_DB_SERVER_', 'mariadb'); define('_DB_NAME_', 'organic'); define('_DB_USER_', 'prestashop'); define('_DB_PASSWD_', 'prestashop');

I want to extract these values to bash, so I managed to create the following command:

sed -rn 's/^.*_DB_NAME_'\'', '\''(\w+)'\''\);/\1/p' settings.inc.php

This will return organic, just as it should, but I would like to improve it further. Let's say we would have this kind of file:

<?php define('_DB_SERVER_', 'mariadb'); define('_DB_NAME_', 'organic1'); define('_DB_NAME_', 'organic2'); define('_DB_USER_', 'prestashop'); define('_DB_PASSWD_', 'prestashop');

Using above command on this file we would get:

organic1 organic2

The thing is: I want for this command to always return only one value, so let's say the first one. Can I achieve that without piping result into second command?

Eric Renouf · Accepted Answer · 2017-07-26 00:32:42Z

With GNU grep you could do it:

grep -m1 -Po "_DB_NAME_', '\K[^']+" settings.inc.php

The grep arguments are:

-m 1: stops the search after 1 match
-P: turns on Perl style regex (so we can use \K here)
-o: only print the part of the line that matches the pattern

The \K part of the pattern says not to include everything up to that point as part of the match, we then have the rest of the pattern finding everything that's not a '.

If you wanted to stick with sed you could find the _DB_NAME_ line, then quit after you found it:

 sed -rn '/_DB_NAME/ {s/^.*_DB_NAME_'\'', '\''(\w+)'\''\);/\1/;p;q}' settings.inc.php

which matches _DB_NAME_ then goes into the block that does your substitution, prints the line, then quits

Sundeep · Accepted Answer · 2017-07-26 04:21:43Z

Can also use awk

$ awk -F "'" '$2=="_DB_NAME_"{print $4; exit}' settings.inc.php organic1

-F "'" use single quotes as input field separator
$2=="_DB_NAME_" check if second field is _DB_NAME_
print $4 if condition satisfies, print 4th field
exit as only first match is needed

SLePort · Accepted Answer · 2017-07-26 05:15:05Z

Pipe your sed command to head and select the first row as show below:

sed -rn 's/^.*_DB_NAME_'\'', '\''(\w+)'\''\);/\1/p' settings.inc.php | head -1

randomir · Accepted Answer · 2017-07-26 00:35:12Z

If you convert your substitute command followed by print to a command block operating only on lines addressed by pattern (containing) _DB_NAME_, you can quit after the first match/print:

$ sed -rn "/_DB_NAME_/ { s/.*'(\w+)'\);$/\1/p;q }" settings.inc.php organic1

Note the q command after p.

Also, your sed script can be simplified by using outer double quotes and anchoring on the end.

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