Modify a line in a particular section of a document with sed - sed

Question

I'd like to use sed to modify a Debian control file to change the dependencies of a particular package.
The file contains metadata for several packages, where entry looks like this:
Package: linux-image-generic
Architecture: i386 amd64 armhf arm64 powerpc ppc64el s390x
Section: kernel
Provides: ${dkms:zfs-modules} ${dkms:virtualbox-guest-modules} ${dkms:wireguard-linux-compat-modules}
Depends: ${misc:Depends}, linux-image-${kernel-abi-version}-generic, linux-modules-extra-${kernel-abi-version}-generic [i386 amd64 arm64 powerpc ppc64el s390x], linux-firmware, intel-microcode [amd64 i386], amd64-microcode [amd64 i386]
Recommends: thermald [i386 amd64]
Description: Generic Linux kernel image
This package will always depend on the latest generic kernel image
available.
Package: linux-tools-generic
Architecture: i386 amd64 armhf arm64 powerpc ppc64el s390x
Section: kernel
Provides: linux-tools
Depends: ${misc:Depends}, linux-tools-${kernel-abi-version}-generic
Description: Generic Linux kernel tools
This package will always depend on the latest generic kernel tools
available.
I would like to find the line that matches Package: linux-image-generic, then modify the following line that matches Depends:, for instance by performing s/linux-image-/linux-image-unsigned-/.

Answer 1

Here's my solution that modifies the Depends: line, but only within the linux-image-generic section.
This solution for GNU sed, but a slight modification makes it work for BSD sed, noted below.
sed '/^Package: linux-image-generic$/,/^$/{/^Depends:/ s/linux-image-/linux-image-unsigned-/}' debian/control
It starts with a range address that matches from the beginning of the package metadata up to the blank line after the package block.
/^Package: linux-image-generic$/,/^$/
Then it uses a {} to apply a command within this range:
/^Depends:/ s/linux-image-/linux-image-unsigned-/
The first part here, /^Depends:/, is a regular expression address that selects only the line(s) that begin with Depends:.
Lastly, the s command performs the substitution on the selected line.
BSD sed (on macOS, etc.) has an additional syntactic rule for function lists { ... }:
The terminating “}” must be preceded by a newline, and may also be preceded by white space.
We need to insert a newline before the }, for example by using the $'\n' ANSI-C string in Bash:
sed '/^Package: linux-image-generic$/,/^$/{/^Depends:/ s/linux-image-/linux-image-unsigned-/'$'\n''}' debian/control
As an aside, the path to finding this solution was to research sed commands that operate on other file formats with similar syntaxes, like INI files.

Answer 2

This might work for you (GNU sed):
sed '/Package: linux-image-generic/{:a;n;/Depends:/!ba;s/linux-image-/&unsigned-/}' file
Find line containing Package: linux-image-generic then continue reading lines until one containing Depends: and substitute linux-image- with linux-image-unsigned-.
N.B. This assumes the package stanza contains Depends:, if not, then:
sed -E '/Package:/{h;:a;n;/Package:/{h;ba};/Depends/!ba;G
s/(linux-image-)(.*)\n.*Package: linux-image-generic/\1unsigned-\2/}' file

Answer 3

The sed solution you found is perfect. For completeness, with GNU awk instead of sed:
awk -v RS= -v ORS='\n\n' '/^Package: linux-image-generic/ {
$0 = gensub(/(\nDepends:[^\n]*linux-image-)/,"&unsigned-",1)} 1' file
If the input record separator (RS) is the empty string records are separated by empty lines. So each section of your file is a record. gensub does the substitution.