diff --git a/find-first-fail/find-first-fail b/find-first-fail/find-first-fail
index 5bbf4b7..af90362 100755
--- a/find-first-fail/find-first-fail
+++ b/find-first-fail/find-first-fail
@@ -5,12 +5,14 @@
 
 =head1 NAME
 
-find-first-fail - find the lowest argument that makes a command fail
+find-first-fail - fine function for finding first failing file fragment
 
 
 =head1 SYNOPSIS
 
-B<find-first-fail> [-2] [-q] [-s I<start>] I<command>
+B<find-first-fail> [-2] [-q] [-s I<start>] [-v] I<command>
+
+B<find-first-fail> -f I<inputfile> [-s I<start>] [-q] [-v] I<command>
 
 
 =head1 DESCRIPTION
@@ -26,24 +28,37 @@ value and the previous value.
 If the I<start> value fails, B<find-first-fail> instead searches for
 the highest value that I<command> fails for.
 
+If given a file with B<-f> B<find-first-fail> will find minimal
+section in the file the command fails for.
+
 
 =head1 OPTIONS
 
 =over 4
 
+=item B<-f> I<inputfile>
+
+Search for the bad line in I<inputfile>. Use B<-s> to tell how many
+lines should be treated as header (e.g. 1 if a normal CSV file with a
+header).
+
+
 =item B<-2>
 
 Instead of passing the command a single argument, give the command 2
 arguments: I<from> I<to>.
 
+
 =item B<-q>
 
 Quiet. Ignore output from I<command>.
 
+
 =item B<-s I<start>>
 
 Start searching from the value I<start>. Normally searching will start from the value 1.
 
+
 =item B<-v>
 
 Verbose. Show the commands being run.
@@ -58,40 +73,43 @@ Verbose. Show the commands being run.
 
 This is a silly way to find the last non-existing file (namely 244):
 
-  touch {245..800}
-  find-first-fail ls
+    touch {245..800}
+    find-first-fail ls
 
 This is a silly way to find the last file (namely 800):
 
-  touch {1..800}
-  find-first-fail ls
+    touch {1..800}
+    find-first-fail ls
+
 
 =head2 Test a bash function
 
 Test how long an argument /bin/echo can take
 
-  . $(which find-first-fail)
-  singleecho() {
-    /bin/echo $(perl -e 'print "x"x'$1) >/dev/null
-  }
-  find-first-fail singleecho
+    . $(which find-first-fail)
+    singleecho() {
+      /bin/echo $(perl -e 'print "x"x'$1) >/dev/null
+    }
+    find-first-fail singleecho
+
 
 =head2 Test a bash function that takes from and to as arguments
 
 Use a function that takes two arguments. It finds the line number
 after HOME=.
 
-  . $(which find-first-fail)
-  greplines() {
+    . $(which find-first-fail)
+    greplines() {
       env | perl -ne "$1..$2 and print" | grep HOME=
-  }
-  find-first-fail -2 -q greplines
+    }
+    find-first-fail -2 -q greplines
+
 
 =head2 Test complex command and show what is run
 
 Complex commands can also be run:
 
-  find-first-fail -v perl -e 'exit(shift > 129)'
+    find-first-fail -v perl -e 'exit(shift > 129)'
 
 
 =head2 Find the second limit of a program
@@ -108,6 +126,24 @@ you find the limits?
     find-first-fail -s 200 myprog
 
 
+=head2 Find minimal failing CSV file
+
+Assume: example.csv
+
+    MyHeader
+    4
+    3
+    5
+    100
+    3
+
+    myparser() { perl -ne 'if($_ > 10) { exit 1 }' "$@"; }
+
+To identify the minimal CSV file that causes myparser to fail:
+
+    find-first-fail -f example.csv -s1 myparser
+
+
 =head1 AUTHOR
 
 Copyright (C) 2020 Ole Tange,
@@ -142,7 +178,6 @@ find-first-fail() {
     _find-first-fail() {
 	local low=$1
 	local high=$2
-	# echo $low-$high
 	if [ $low -gt $(($high - 2)) ]; then
 	    echo $low
 	    return
@@ -169,10 +204,8 @@ find-first-fail() {
 	    # _inner_run is needed if cmd is complex like:
 	    #   perl -e 'exit(  (shift) + (shift) > 10)'
 	    if $opt2 ; then
-		$verbose && echo "${cmd[@]}" "$a" "$b"
 		"${cmd[@]}" "$a" "$b"
 	    else
-		$verbose && echo "${cmd[@]}" "$b"
 		"${cmd[@]}" "$b"
 	    fi
 	}
@@ -180,20 +213,148 @@ find-first-fail() {
 	local b="$2"
 	shift
 	shift
-	# echo "a=$a b=$b $@"
-	local cmd=( "$@" )
+	local cmd=("$@")
+	if $opt2 ; then
+	    $verbose && echo "$a<x<$b: ${cmd[@]}" "$a" "$b" >&2
+	else
+	    $verbose && echo "$a<x<$b: ${cmd[@]}" "$b" >&2
+	fi
 	eval "$not" _inner_run "$quiet"
     }
 
-    local opt2=false
-    local quiet=""
-    local start=1
-    local verbose=false
+    _find_in_arg() {
+	# If function(1) = false: run 'not function()' instead
+	local not
+	if _run "$start" "$start" "$@" ; then
+	    not=''
+	else
+	    not='!'
+	fi
+
+	# exponential search for the first value that is false
+	# low = previous value (function($low) == true)
+	# high = low * 2 (function($high) == false)
+	local high=$(( $start*2 ))
+	local low=$start
+	while _run $start $high "$@" ; do
+	    low=$high
+	    high=$(( $high*2 ))
+	    if [ $high -gt 4611686018427387900 ] ; then
+		echo "find-first-fail: Error: exit value does not change of '$@'" >&2
+		return
+	    fi
+	done
+
+	# low = tested good
+	# high = tested fail
+	# Search low..high
+	# echo "low: $low high: $high not: $not"
+	_find-first-fail $low $high "$@"
+	unset low high start quiet
+    }
+
+    #### find-first-fail-file
+
+    _run_file() {
+	# build $tmpfile as line a..b
+	# run:
+	#   cmd $tmpfile
+	# Output is ignored if $quiet
+	# Exit value is negated if $not
+	_inner_run() {
+	    # _inner_run is needed if cmd is complex like:
+	    #   perl -e 'exit(  (shift) + (shift) > 10)'
+	    "${cmd[@]}" "$tmp"
+	}
+	local a="$1"
+	local b="$2"
+	shift
+	shift
+	local cmd=("$@")
+	local tmp=`tempfile -p fff`
+	$verbose && echo "$a<x<$b: ${cmd[@]}" "$tmp" >&2
+	# Build file of line a..b
+	perl -ne "($start and 1..$start) and print" "$inputfile" > "$tmp";
+	perl -ne "$a..$b and print" "$inputfile" >> "$tmp";
+	eval "$not" _inner_run "$quiet"
+	# Postpone exit code
+	local _exit="$?"
+	rm "$tmp"
+	return "$_exit"
+    }
+
+    _find-low-file() {
+	local low=$1
+	local high=$2
+	if [ $low -gt $(($high - 2)) ]; then
+	    global_low=$low
+	    return
+	fi
+	shift
+	shift
+	local middle=$(( ( $low + $high ) / 2 ))
+	if _run_file $middle $global_high "$@" ; then
+	    low=$middle
+	else
+	    high=$middle
+	fi
+	_find-low-file $low $high "$@"
+    }
+
+    _find-high-file() {
+	local low=$1
+	local high=$2
+	if [ $low -gt $(($high - 2)) ]; then
+	    global_high=$high
+	    return
+	fi
+	shift
+	shift
+	local middle=$(( ( $low + $high ) / 2 ))
+	if _run_file $global_low $middle "$@" ; then
+	    high=$middle
+	else
+	    low=$middle
+	fi
+	_find-high-file $low $high "$@"
+    }
+
     
+    _find_in_file() {
+	# If function(1) = false: run 'not function()' instead
+	local not
+	local global_low=$(($start+1))
+	if [ ! -e "$inputfile" ] ; then
+	    echo "find-first-fail: Error: File not found '$inputfile'" >&2
+	    return 1
+	fi
+	local global_high=$(wc -l < $inputfile)
+	if _run_file "$global_low" "$global_high" "$@" ; then
+	    not=''
+	else
+	    not='!'
+	fi
+	# Binary search for $global_low
+	_find-low-file $global_low $global_high "$@"
+	_find-high-file $global_low $global_high "$@"
+	perl -ne "($start and 1..$start) and print" "$inputfile"
+	perl -ne "$global_low..$global_high and print" "$inputfile"
+	unset low high start quiet
+    }
+
+    local opt2=false
+    local optf=false
+    local inputfile
+    local quiet=""
+    local start
+    local verbose=false
+    unset OPTIND
+
     # Parse and remove options
-    while getopts "2qs:v" options; do
+    while getopts "2f:qs:v" options; do
 	case "${options}" in
 	    (2)   opt2=true;;
+	    (f)   optf=true; inputfile="$OPTARG";;
 	    (q)   quiet=">/dev/null 2>/dev/null";;
 	    (s)   start="$OPTARG";;
 	    (v)   verbose=true;;
@@ -201,35 +362,19 @@ find-first-fail() {
 	esac
     done
     shift $(( OPTIND - 1))
-    
-    # If function(1) = false: run 'not function()' instead
-    local not
-    if _run "$start" "$start" "$@" ; then
-	not=''
-    else
-	not='!'
-    fi
+    unset OPTIND
 
-    # exponential search for the first value that is false
-    # low = previous value (function($low) == true)
-    # high = low * 2 (function($high) == false)
-    local high=$start
-    local low
-    while _run $start $high "$@" ; do
-	low=$high
-	high=$(( $high*2 ))
-	if [ $high -gt 4611686018427387900 ] ; then
-	    echo "$0: Error: exit value does not change of '$@'" >&2
-	    return
+    if $optf; then
+	if [ -z "$start" ] ; then
+	    start=0
 	fi
-    done
-
-    # low = tested good
-    # high = tested fail
-    # Search low..high
-    # echo "low: $low high: $high not: $not"
-    _find-first-fail $low $high "$@" 2>/dev/null
-    unset low high start
+	_find_in_file "$@";
+    else
+	if [ -z "$start" ] ; then
+	    start=1
+	fi
+	_find_in_arg "$@";
+    fi
 }
 
 if [ -z "$*" ] ; then
@@ -240,3 +385,4 @@ else
     # find-first-fail command
     find-first-fail "$@"
 fi
+	
diff --git a/find-first-fail/testsuite b/find-first-fail/testsuite
index 58042c5..8b78f8e 100644
--- a/find-first-fail/testsuite
+++ b/find-first-fail/testsuite
@@ -4,6 +4,7 @@ test_unexported_function() {
     myprog() { perl -e 'exit (shift > 12345678)' "$@"; }
     # myprog is a function, so source find-first-fail first
     . `which find-first-fail`
+    echo Find 12345678 in unexported function
     find-first-fail myprog
 }
 
@@ -11,6 +12,7 @@ test_exported_function() {
     myprog() { perl -e 'exit (shift > 12345678)' "$@"; }
     # myprog is an exported function
     export -f myprog
+    echo Find 12345678
     find-first-fail myprog
 }
 
@@ -19,9 +21,9 @@ test_startvalue() {
     myprog() { perl -e '$a=shift;if($a <= 123) { exit 0; }
     	       	        else { exit ($a <= 12345678) }' "$@"; }
     export -f myprog
-    # Finds 123
+    echo Find 123
     find-first-fail myprog
-    # Finds 12345678
+    echo Find 12345678
     find-first-fail -s 200 myprog
 }
 
@@ -29,10 +31,40 @@ test_s_v_12() {
     # Multiple options
     myprog() { perl -e 'exit (shift > 12)' "$@"; }
     export -f myprog
+    echo Find 12 with progress
     find-first-fail -v -s 10 myprog
+    echo Find 12 with progress quiet
     find-first-fail -v -q -s 10 myprog
 }
 
+test_file() {
+    tmp=`tempfile`
+    echo Header > $tmp
+    seq 100 >> $tmp
+    10_to_15() { grep ^10$ $1 && grep ^15$ $1; }
+    export -f 10_to_15
+    echo 10..15
+    find-first-fail -s1 -qf $tmp 10_to_15
+    echo not 10..15
+    find-first-fail -s1 -qf $tmp not 10_to_15
+    rm $tmp
+}
+
+test_header() {
+    tmp=`tempfile`    
+    echo Header > $tmp
+    seq 10 >> $tmp
+    echo 1000 >> $tmp
+    seq 10 >> $tmp
+
+    myparser() { perl -ne 'if($_ > 100) { exit 1 }' "$@"; }
+    export -f myparser
+    echo Should give:
+    echo Header
+    echo 1000
+    find-first-fail -s1 -f $tmp myparser
+}
+
 export -f $(compgen -A function | grep test_)
 compgen -A function | grep test_ | LC_ALL=C sort |
     parallel --timeout 1000% --tag -k --joblog /tmp/jl-`basename $0` '{} 2>&1'