sed replace single/double quoted text?

Question

Having some difficulty in replacing some single/double quoted text with sed and was wondering what\'s the correct method for these 2 examples

to change

Answer 1

To understand how to do this, you need to understand how the shell is messing with your quotes and backslashes as well as whether sed is messing with them.

Generally, it is easiest in the shell to use single quotes - except when the regex must match single quotes. There are no special characters inside single quotes; the next single quote ends it. Double quotes are much harder; backslashes and dollars and back-quotes are all special.

The first requirement is best handled by a single quoted sed expression:

Change:

[server]
server[] = "localhost:11211"

to

[server]
server[] = "localhost:11211"
server[] = "localhost:11212"

For this, we could use:

-e '/^server\[] = "localhost:11211"/{p;s/11211/11212/}'

The shell simply sends everything inside the single quotes to sed, untouched. The only special character to sed is the open square bracket. That would start a character class except for the backslash before it. The close square bracket is only special when you are in a character class, so it does not need escaping. The actions on recognizing that line are to print the original, then do the 11212 for 11211 substitution; the default print will print the modified line.

The other part is a bit trickier. Because the patterns contain single quotes, we're going to be best off using double quotes around the regex.

Change:

define('ADMIN_USERNAME','username');    // Admin Username
define('ADMIN_PASSWORD','password');    // Admin Password

$MEMCACHE_SERVERS[] = 'mymemcache-server1:11211'; // add more as an array
$MEMCACHE_SERVERS[] = 'mymemcache-server2:11211'; // add more as an array

to:

define('ADMIN_USERNAME','myusername');  // Admin Username
define('ADMIN_PASSWORD','mypassword');      // Admin Password

$MEMCACHE_SERVERS[] = 'localhost:11211'; // add more as an array
$MEMCACHE_SERVERS[] = 'localhost:11212'; // add more as an array

The first two lines might be mappable using a single regex:

-e "s/^\(define('ADMIN_[A-Z]\{0,8\}','\)\([^']*'\)/\1my\2/"

This captures two parts - the define('ADMIN_USERNAME',' as \1 and the username' as \2, and the substitution places my between the two parts. This was not too bad; the shell doesn't do anything special with any of the characters, so you just type what you want sed to see inside the double quotes.

The last two lines are trickier:

-e "s/^\(\$MEMCACHE_SERVERS\[] = '\)[^:]*\(:[0-9]*'\)/\1localhost\2/"

However, not much trickier; you just need to stop the shell from expanding the $MEMCACHE_SERVERS as a shell variable, which is why there's a backslash in front of the $. The square bracket needs protection from sed as before. The regex matches everything that isn't a colon and replaces it with localhost.

So, this script should work:

sed -e '/^server\[] = "localhost:11211"/{p;s/11211/11212/}' \
    -e "s/^\(define('ADMIN_[A-Z]\{0,8\}','\)\([^']*'\)/\1my\2/" \
    -e "s/^\(\$MEMCACHE_SERVERS\[] = '\)[^:]*\(:[0-9]*'\)/\1localhost\2/" \
    "$@"

However, I've only explained why it should work; I've not demonstrated that it does work!