sed + un-comment parameter and change value in file

Question

I want to uncomment the wal_level parameter and change the value from minimal to archive

how to perform this action by sed / perl one-liner ?

$ grep wal_level  /var/lib/pgsql/data/postgresql.conf
#wal_level = minimal                    # minimal, archive, or hot_standby

expected results:

  wal_level = archive                    # minimal, archive, or hot_standby

Note that wal_level could be couple of spaces from the "#".

For example:

#     wal_level = minimal   

or

#       wal_level = minimal   

etc.

Answer 1

This should work:

sed 's/^ *# *wal_level *= *[^ ]*/wal_level = archive/'

I also allow for a space before the comment (just in case) and 0 or more spaces everywhere else. Alternatively, in perl:

perl -pe 's/^\s*#\s*wal_level\s*=\s*\S+/wal_level = archive/' file

Answer 2

perl -pi.BAK -aF'/(\h*[#=]\h*)/,$_,4' -le '
   $F[4] = "archive";
   $_ = join $,, @F[2..$#F] if /^\h*#\h*wal_level\h*=/;
' /var/lib/pgsql/data/postgresql.conf

The line is split into fields @F and the delimiters are included as well. The parameter 4 in the field split option -F will constrain the number of fields to that. The fields from 3rd onwards are joined together using empty space, which is the default value of the OFS = $,

Answer 3

You can just change the whole line where wal_level appears, including the # and an arbitrary number of whitespaces:

sed '/^# *wal_level/cwal_level = archive' inputfile