How to only remove duplicate lines if they're immediately after each other in the file

Lets say I have the following file:

$ cat test.txt
a
-----
b
-----
-----
c
-----
-----
-----
d
-----
e
-----
-----

Now I want to remove all the -----, but only if they're repeating after each other. So the result should look like this:

a
-----
b
-----
c
-----
d
-----
e
-----

I tried grep -Pvz -- "-----\n-----", but that didn't work.

Solution 1:

That's exactly what the uniq command is made for:

NAME
       uniq - report or omit repeated lines

SYNOPSIS
       uniq [OPTION]... [INPUT [OUTPUT]]

DESCRIPTION
       Filter adjacent matching lines from INPUT (or standard input), writing to OUTPUT
       (or standard output).

       With no options, matching lines are merged to the first occurrence.

So

$ uniq test.txt 
a
-----
b
-----
c
-----
d
-----
e
-----

Alternatively, you can use this sed one-liner 69. Delete duplicate, consecutive lines from a file (emulates "uniq") from Sed One-Liners Explained, Part III: Selective Deletion of Certain Lines and Special Applications

sed '$!N; /^\(.*\)\n\1$/!P; D' test.txt

which might be preferred if you want to edit test.txt in place (by adding the -i or --in-place option).