Replace first line in text file by i-th line from another text file

Question

Can i do the following in the terminal? (written in pseudo-code)

for (int i=1;i<=5;i++) {
replace first line of fileout.text by i-th line of filein.txt 
}

i guess it somehow involves using sed, but i don't know how to sed from one file to another.

EDIT: I frame Htorque's answer inside a loop:

for (( i = 1 ; i <= 10 ; i++ )); do
    line=$(sed -n "${i}p" filein.txt)
    sed -i "1c\\$line" fileout.txt
done

which works like a charm. It is possible to replace the fixed string '10' in the counter by the actual number of lines of filein.txt:

nline=$(sed -n '$=' filein.txt)
for (( i = 1 ; i <= $nline ; i++ )); do
    line=$(sed -n "${i}p" filein.txt)
    sed -i "1c\\$line" fileout.txt
done

Answer 1

To replace the first line of FILE.out with the i-th line of FILE.in I'd do:

 i=<line-number>
 line=$(sed -n "${i}p" FILE.in)
 sed -i "1c\\$line" FILE.out

If i doesn't exist in FILE.in, then the first line of FILE.out would be deleted (empty).

If $line contains any special characters (eg. backslash, dollar), then you'd need to escape those.

Not 100% sure this couldn't break elsewhere.

Answer 2

There are two parts to the program: getting the output you want and then replacing the contents of the original file with that output:

#!/bin/sh
# output the first five lines of the first argument
# followed by all but the first of the second argument
# if successful, replace the second argument with the
# result

# cribbed almost entirely from Kernighan & Pike's
$ "The Unix Programming Environment" script "overwrite"

case $# in
0|1)        echo 'Usage: replace5 filea fileb' 1>&2; exit 2
esac

filea=$1; fileb=$2
new=/tmp/$$.new; old=/tmp/$$.old
trap 'rm -f $new; exit 1' 1 2 15    # clean up files

# collect input
if head -5 $filea >$new && tail -n +2 $fileb >> $new
then
    cp $filea $old   # save original file
    trap 'trap "" 1 2 15; cp $filea $old   # ignore signals
          rm -f $new $old; exit 1' 1 2 15   # during restore
    cp $new $filea
else
    echo "replace5: failed, $filea unchanged" 1>&2
    exit 1
fi
rm -f $new $old