What Is the Best Way to Perform a Search and Replace On only Specific Sections of a File? - bash

Question

I have a markdown file with sections separated by headings. I want to perform a search and replace only on specific sections; however, each section has similar content, so a global search and replace would end up affecting all sections. Because of this, I would need to somehow limit the search and replace to only certain sections of the file.
For example, say I wanted to replace all instances of foo with bar under # Section 1, # Section 3, and # Section 4 leaving # Section 2 and # Section 5 unchanged, as shown below
Sample Input:
# Section 1
- foo
- foo
- Unimportant Item
- foo
- Unimportant Item
# Section 2
- foo
- Unimportant Item
# Section 3
- foo
- Unimportant Item
# Section 4
- foo
- Unimportant Item
- foo
# Section 5
- foo
- foo
Sample Output
# Section 1
- bar
- bar
- Unimportant Item
- bar
- Unimportant Item
# Section 2
- foo
- Unimportant Item
# Section 3
- bar
- Unimportant Item
# Section 4
- bar
- Unimportant Item
- bar
# Section 5
- foo
- foo
If I didn't have to worry about the individual sections, a global search and replace would be trivial by using
sed -i 's/foo/bar/g' <input_file>
but I'm not sure if sed is capable of checking context to allow what I am looking for.

Answer 1

Here's a sed version:
sed -E '/^#[^#]\s*Section\s+[134]\s*$/, // s/foo/bar/' input.md

Answer 2

You may use this awk:
awk 'p {sub(/foo$/, "bar")} /^#/ {p = / (Section [134])$/} 1' file
# Section 1
- bar
- bar
- Unimportant Item
- bar
- Unimportant Item
# Section 2
- foo
- Unimportant Item
# Section 3
- bar
- Unimportant Item
# Section 4
- bar
- Unimportant Item
- bar
# Section 5
- foo
- foo
To make it more readable:
awk 'p { # if p==1 and current line # == n
sub(/foo$/, "bar") # replace foo with bar
}
/^#/ { # if line starts with #
p = / (Section [134])$/ # set p = 1/0 if it matches sections
} 1' file

Answer 3

For completion, this awk answer will do the substitutions in the whole section, including the header:
awk '/^#/ { in_section = /Section [1|3|4]/ } in_section { sub(/foo/, "bar") } 1' input.md
If you want to exclude the headers from the substitution:
awk ' /^#/ { in_section = /Section [1|3|4]/; header_line = NR }
in_section && (NR > header_line) { sub(/foo/, "bar") } 1' input.md
Detail
awk '/^#/ { # if in section header
in_section = /Section [1|3|4]/; # determine if section of interest (1/0)
header_line = NR; # value of header line to exclude
}
in_section && (NR > header_line) { # if in section of interest and after header line
sub(/foo/, "bar"); # substitute text
} 1' input.md # 1 is to print all lines

Answer 4

My usual advice whenever you're considering sed -i is to use its older brother ed instead, as unlike sed, it's intended from the get-go to edit files (It's also POSIX standard, unlike sed -i, and thus more portable.)
Something like
ed -s input.md <<EOF
/Section 1/;/Section/s/foo/bar/g
/Section 3/;/Section/sg
w
EOF
Translated: In the block starting with the first line containing Section 1 and ending with the next Section line, replace foo with bar. Then do the same substitution in the Section 3 block. Finally, write the changes back to disk.

Answer 5

You can always provide multiple commands to sed with the -e option so that substitution occurs even when the sections are one after another:
sed -e '/# Section 1/,/#/ s/foo/bar/' -e '/# Section 2/,/#/ s/foo/bar/' input.md
Multiple commands can also be placed in a "sed script file":
# content of script.sed
/# Section 1/,/#/ s/foo/bar/
/# Section 2/,/#/ s/foo/bar/
And you executed like this:
sed -f script.sed input.md

Answer 6

A solution with sed
The key is in the range. The first addressing pattern matches the header(s) where we want the substitution to begin, and the second, matches all headers except the ones in the first addressing pattern. Note that the substitute command is inclusive of the first and last lines in the range (i.e. the headers).
sed -E '/^# Section [134]/, /^# Section [^134]/ s/foo/bar/' input.md
This one excludes the headers from the substitution:
sed -E '/^# Section [134]/, /^# Section [^134]/ { /^#/!s/foo/bar/ }' input.md