Replace in result of other regex outcome - regex

Question

I am trying to automate some things for my CentOS server. For this automation I need to add, replace or modify file lines. This works fine by doing a global replace or replacing unique lines.
sed -i "s/user \\= apache/user \\= nginx/" file.conf
This replaces user = apache with user = nginx which works fine.
But sometimes I need to replace lines like these within a result of an other regex. For example the file /etc/php-fpm.d/www.conf contains the same configuration lines for a serval times.
With a regex I can easily filter out the part I want to do my replaces in but I don't know how to do that.
This is a part of the file:
...
[base]
name=CentOS-$releasever - Base
baseurl=http://centos.mirrors.ovh.net/ftp.centos.org/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
exclude=postfix
[updates]
name=CentOS-$releasever - Updates
baseurl=http://centos.mirrors.ovh.net/ftp.centos.org/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
exclude=postfix
[extras]
name=CentOS-$releasever - Extras
...
With this regex:
sed -n "/\[base\]/,/\[updates\]/p" /etc/yum.repos.d/CentOS-Base.repo
I can get the lines from [base] until [updates]. Then I can modify for example exclude line. But how can I get these lines back into the main file?
This is what I have so far:
# $BASE contains the lines [base] until [updates]
BASE=`sed -n "/\[base\]/,/\[updates\]/p" /etc/yum.repos.d/CentOS-Base.repo`
# Get te line that holds 'exclude' to the $EXCLUDE parameter
EXCLUDE=`echo "$BASE" | grep "exclude"`
# Modify EXCLUDE
EXCLUDE="$EXCLUDE, dovecot"
# Replace old exclude line in $BASE variable with new one
BASE=`sed "s/^exclude.*/$EXCLUDE/g" $BASE`
# Put $BASE back into /etc/yum.repos.d/CentOS-Base.repo
# ???
In this script I am trying to achieve this with multiple commands which is fine. It would also be nice if there is just one replace expression for this. But that of course is not a must.

Answer 1

sed -i "/\[base\]/,/\[updates\]/s/^exclude.*/&, dovecot/"
sed commands consist of an optional address, the command, then any arguments to the command. The address, if present, is either a single expression indicating the line number or single-line pattern identifying the line to which the command is to be applied, or a pair of comma-separated expressions indicating the starting and ending line numbers or patterns of the range to which the command is to be applied.
Your previous sed script ("/\[base\]/,/\[updates\]/p") used the command p (print) with the address range of /\[base\]/ to /\[updates\]/.
You can just apply your s (substitute) command to that same range, to substitute exclude.* (exclude followed by anything - but gobbling up the rest of the line) with &, dovecot (where & in the replacement string means everything that was matched).
If you need to do something more complicated within that address range, you can use the { sed command - which opens a block of commands that terminates with the } command. This lets you do things like if/else blocks, so you could do:
sed -i -e "/\[base\]/,/\[updates\]/{" \
-e "/\[base\]/h" \
-e "s/^exclude=.*/&, dovecot/" \
-e "/^exclude=/h" \
-e "/\[updates\]/{" \
-e "x" \
-e "/^exclude=/b e" \
-e "i \
exclude=dovecot" \
-e ": e" \
-e "x" \
-e "}" \
-e "}" /etc/yum.repos.d/...
This big hairy sed script does the following:
everything in the script operates between the [base] and [updates] lines.
The [base] line or any line that begins exclude= are placed into the "hold" buffer (via the h command)
Any line that begins exclude= gets , dovecot appended to it
When [updates] is seen:
the hold buffer is temporarily recovered (via the x command which swaps the current line with the hold buffer)
the buffer is checked to see if it begins with exclude=
if it does, skip the next line by branching (command b) to label e
if it doesn't (because we weren't skipped), insert the line exclude=dovecot
after label e (via the command :)...
swap the current line and hold buffer again (via x again)
In other words: Any exclude= line gets , dovecot appended; if no exclude= line was seen, a new exclude=dovecot line is added at the end.
Note that it might be more useful to go from /\[base\]/ to /^$/, if you can guarantee a blank line at the end of each block (which is normally the case), in which case the you would update the first -e expression as well as changing -e "/\[updates\]/{" to -e '/^$/'{ (note also the change in quotes so that the $ isn't parsed by the shell).