Where is this Regex expression not closed in sed (apostrophe parenthesis)? - regex

Question

I'm trying to update some setting for wordpress and I need to use sed. When I run the below command, it seems to think the line is not finished. What am I doing wrong?
$ sed -i 's/define\( \'DB_NAME\', \'database_name_here\' \);/define\( \'DB_NAME\', \'wordpress\' \);/g' /usr/share/nginx/wordpress/wp-settings.php
> ^C
Thanks.

Answer 1

Single quotes in most shells don't support any escaping. If you want to include a single quote, you need to close the single quotes and add the single quote - either in double quotes, or backslashed:
sed 's/define\( '\''DB_NAME'\'', '\''database_name_here'\'' \);/define\( '\''DB_NAME'\'', '\''wordpress'\'' \);/g'
I fear it still wouldn't work for you, as \( is special in sed. You probably want just a simple ( instead.
sed 's/define( '\''DB_NAME'\'', '\''database_name_here'\'' );/define( '\''DB_NAME'\'', '\''wordpress'\'' );/g'
or
sed 's/define( '"'"'DB_NAME'"'"', '"'"'database_name_here'"'"' );/define( '"'"'DB_NAME'"'"', '"'"'wordpress'"'"' );/g'

Answer 2

Normally, using single quotes around the script of a sed script is sensible. This is a case where double quotes would be a better choice — there are no shell metacharacters other than single quotes in the sed script:
sed -e "s/define( 'DB_NAME', 'database_name_here' );/define( 'DB_NAME', 'wordpress' );/g" /usr/share/nginx/wordpress/wp-settings.php
or:
sed -e "s/\(define( 'DB_NAME', '\)database_name_here' );/\1wordpress' );/g" /usr/share/nginx/wordpress/wp-settings.php
or even:
sed -e "/define( 'DB_NAME', 'database_name_here' );/s/database_name_here/wordpress/g" /usr/share/nginx/wordpress/wp-settings.php
One other option to consider is using sed's -f option to provide the script as a file. That saves you from having to escape the script contents from the shell. The downside may be that you have to create the file, run sed using it, and then remove the file. It is likely that's too painful for the current task, but it can be sensible — it can certainly make life easier when you don't have to worry about shell escapes.
I'm not convinced the g (global replace) option is relevant; how many single lines are you going to find in the settings file containing two independent define DB_NAME operations with the default value?
You can add the -i option when you've got the basic code working. Do note that if you might ever work on macOS or a BSD-based system, you'll need to provide a suffix as an extra argument to the -i option (e.g. -i '' for a null suffix or no backup; or -i.bak to be able to work reliably on both Linux (or, more accurately, with GNU sed) and macOS and BSD (or, more accurately, with BSD sed). Appealing to POSIX is no help; it doesn't support an overwrite option.
Test case (first example):
$ echo "define( 'DB_NAME', 'database_name_here' );" |
> sed -e "s/\(define( 'DB_NAME', '\)database_name_here' );/\1wordpress' );/g"
define( 'DB_NAME', 'wordpress' );
$
If the spacing around 'DB_NAME' is not consistent, then you'd end up with more verbose regular expressions, using [[:space:]]* in lieu of blanks, and you'd find the third alternative better than the others, but the second could capture both the leading and trailing contexts and use both captures in the replacement.
Parting words: this technique works this time because the patterns don't involve shell metacharacters like $ or  ` . Very often, the script does need to match those, and then using mainly single quotes around the script argument is sensible. Tackling a different task — replace $DB_NAME in the input with the value of the shell variable $DB_NAME (leaving $DB_NAMEORHOST unchanged):
sed -e 's/$DB_NAME\([^[:alnum:]]\)/'"$DB_NAME"'\1/'
There are three separate shell strings, all concatenated with no spaces. The first is single-quoted and contains the s/…/ part of a s/…/…/ command; the second is "$DB_NAME", the value of the shell variable, double-quoted so that if the value of $DB_NAME is 'autonomous vehicle recording', you still have a single argument to sed; the third is the '\1/' part, which puts back whatever character followed $DB_NAME in the input text (with the observation that if $DB_NAME could appear at the end of an input line, this would not match it).
Most regexes do fuzzy matching; you have to consider variations on what might be in the input to determine how hard your regular expressions have to work to identify the material accurately.