Randomly select a line out of a text file and set that line as a variable -

how randomly select line/ip out of text file, , set variable.

i mean like:

set ip=theline/ipfromofthefile 

the array based solutions seem ought fastest because read file once. variable manipulation becomes slow when environment becomes large. array based solutions give non-linear performance. can painfully slow if file large.

for pure batch based solution gives decent linear performance, regardless of file size, aschipfl's answer. however, has problems lines begin ; due default /f eol value. fails if file empty. here corrected version perfect.

@echo off setlocal set "file=test.txt" set "line=" /f %%n in (   'type "%file%" ^| find /c /v ""' ) set set /a rnd=%random% %% %%n 2>nul || goto :break if %rnd%>0 (set "skip=skip^=%rnd%") else set "skip=" /f usebackq^ %skip%^ delims^=^ eol^= %%l in ("%file%") (   set "line=%%l"   goto :break ) :break set line 

there 1 remaining flaw above - if selected line empty, return next non-empty line instead due design of /f.

the "standard" way fix use findstr prefix each line line number, , use expansion find/replace remove prefix. given presence of line number prefix, use additional findstr select correct line, rather relying on /f

@echo off setlocal set "file=test.txt" set "ln=" /f %%n in (   'type "%file%" ^| find /c /v ""' ) set /a rnd=%random% %% %%n + 1 2>nul || goto :result /f "delims=" %%l in (   'findstr /n "^" "%file%" ^| findstr "^%rnd%:"' ) set "ln=%%l" setlocal enabledelayedexpansion set "ln=!ln:*:=!" :result set ln 

if know lines <= 1021 bytes long, there simpler solution dealing empty lines reads file set /p instead of /f.

@echo off setlocal set "file=\utils\jrepl.bat" set "ln=" /f %%n in (   'type "%file%" ^| find /c /v ""' ) set /a rnd=%random% %% %%n 2>nul || goto :result <"%file%" (   /l %%n in (1 1 %rnd%) set /p "ln="   set "ln="   set /p "ln=" ) :result set ln 

just yucks, decided write solution based on jrepl.bat regular expression text processing utility. optimized hybrid jscript/batch script written, jrepl convenient if have in bag of tricks.

if need print random line, simple 1 liner:

call jrepl "^.*" $0 /c /jmatch /jbeg "rnd=math.floor(math.random()*cnt)+1" /jbegln "skip=(rnd!=ln)" /f test.txt 

a bit more code needed value in variable:

@echo off setlocal set "ln=" /f "delims=" %%l in (   'jrepl "^.*" $0 /c /n 1 /jmatch /jbeg "rnd=math.floor(math.random()*cnt)+1" /jbegln "skip=(rnd!=ln)" /f test.txt' ) set "ln=%%l" setlocal enabledelayedexpansion set "ln=!ln:*:=!" :result set ln 

if lines <= 1021 , don't mind temp file, use set /p

@echo off setlocal call jrepl "^.*" $0 /c /jmatch /jbeg "rnd=math.floor(math.random()*cnt)+1" /jbegln "skip=(rnd!=ln)" /f test.txt /o temp.txt set "ln=" <temp.txt set /p "ln=" del temp.txt :result set ln