How to edit next line after pattern using sed?

Using sed, how do I search for a line ending with foo and then edit the next line if it starts with #bar?

Or put another way, I want to remove a comment # from the next line if it starts with #bar and the previous line ends in foo.

For example:

This is a line ending in foo
#bar is commented out
There are many lines ending in foo
#bar commented out again

I tried:

sed -i 's/^foon#bar/foonbar/' infile

Answers:

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Method 1

sed '/foo$/{n;s/^#bar/bar/;}'

is a literal translation of your requirement. n is for next.

Now that doesn’t work in cases like:

line1 foo
#bar line2 foo
#bar

Or:

line1 foo
line2 foo
#bar

As the line that is pulled into the pattern space by n is not searched for foo.

You could address it by looping back to the beginning after the next line has been pulled into the pattern space:

sed '
  :1
  /foo$/ {
     n
     s/^#bar/bar/
     b1
  }'

Method 2

Use the N;P;D cycle and attempt to substitute each time:

sed '$!N;s/(foon)#(bar)/12/;P;D' infile

this removes the leading # from #bar only if it follows a line ending in foo otherwise it just prints the pattern space unmodified.

Apparently, you want to uncomment US mirrors in /etc/pacman.d/mirrorlist which is a whole different thing:

sed -e '/United States/,/^$/{//!s/^#//' -e '}' /etc/pacman.d/mirrorlist

This will uncomment all mirrors in the US section in /etc/pacman.d/mirrorlist

Method 3

Try:

sed -e '$!N;/foon#bar/s/(n)#/1/;P;D'

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0