I’m afraid I can’t imagine how to write a correct batch script that will solve the following task.
Could you help ?

I have a text file “2024-04-29_22-23log.txt” where each line (except the first) starts with OPEN or CLOSE followed by a time stamp.
To highlight status changes, I want to keep only the lines where this word is different from the previous line.
But I want to keep the very first twoo lines and also the last line.

Example:
2024-04-29_22-23
CLOSE 22:23
CLOSE 22:24
OPEN 22:25
OPEN 22:26
OPEN 22:27
CLOSE 22:28
CLOSE 22:29

should get:
2024-04-29_22-23
CLOSE 22:23
OPEN 22:25
CLOSE 22:28
CLOSE 22:29

Thanks for your help.

Following works only with alphabetical file name and forgets last line !

@echo off
setlocal enabledelayedexpansion

set /p "inputFile=Enter file name : "
set "outputFile=!inputFile!_filtered.txt"

rem Copy the first line to the output file
set "lineNum=0"
for /f "usebackq delims=" %%a in ("%inputFile%") do (
    set /a "lineNum+=1"
    if !lineNum! equ 1 (
        echo %%a>>"%outputFile%"
    ) else (
        set "prevLine=!currentLine!"
        set "currentLine=%%a"
        for /f "tokens=1,* delims= " %%b in ("!prevLine!") do set "prevWord=%%b"
        for /f "tokens=1,* delims= " %%c in ("!currentLine!") do set "currentWord=%%c"
        if "!prevWord!" neq "!currentWord!" echo %%a>>"%outputFile%"
    )
)
endlocal

(for /f %%a in ('type "%inputFile%" ^| find /c /v ""') do set /a lines=%%a) >nul
set /a "lastLine=%lines% - 1"
(for /f "usebackq skip=%lastLine% delims=" %%a in ("%inputFile%") do echo %%a)
(for /f "usebackq skip=%lastLine% delims=" %%a in ("%inputFile%") do echo %%a) >>"%outputFile%"

endlocal