Merging of two files with substitution

Question

I need to merge two files, adding the contain of the file 2 directly to the end of the file 1. Notable the last line of the first file consist of the phrase "END". I need to remove it and add there the content of the file 2:

file 1:

string 1
string 2
string N
END

file 2:

string 3
string 4
string M

should give me the file 3

string 1
string 2
string N
string 3
string 4
string M

I've tried simply using

cat file1 file2 >> file3

but it did not substitution of the END in the file2. May I use a special option for this?

Answer 1

Using sed:

sed -e '${r file2' -e ';$d;}' file1

string 1
string 2
string N
string 3
string 4
string M

If you want to use shell variable instead of file names directly:

sed -i.bak -e "\${r $f2" -e ';$d;}' "$f1"

Answer 2

This might work for you (GNU head & cat):

head -n-1 file1 | cat - file2 > file3

Read all but the last line of file1 and concatenate it with file2.

An alternative:

sed -i -e '$r file2' -e '$d' file1

This will append file2 to the end of file1 less the last line of file1 and then replace file1 with the result.

Answer 3

An approach with awk. It !only! skips ENDs occurring on the last line of file1. It recognizes the last line of file1 by using a helper variable set.

Test with multiple ENDs in a file

% awk 'set==""&&NR!=FNR&&last=="END"{last="";set=1} 
    last!=""{print last} 
    {last=$0} END{print}' file file
string 1
string 2
END
string N
string 1
string 2
END
string N
END

Using file1 and file2

% awk 'set==""&&NR!=FNR&&last=="END"{last="";set=1} 
    last!=""{print last} 
    {last=$0} END{print}' file1 file2
string 1
string 2
string N
string 3
string 4
string M

Data

% cat file
string 1
string 2
END
string N
END

% cat file1
string 1
string 2
string N
END

% cat file2
string 3
string 4
string M

Answer 4

Assumptions:

• file1 just has the one entry labeled END
• the END entry is at the end of file1

Small modification to OP's current cat idea using process substitution to strip the END from file1:

$ cat <(grep -v '^END$' file1) file2 > file3
$ cat file3
string 1
string 2
string N
string 3
string 4
string M

---

A variation on Andre's awk solution whereby we print everything we see except the file1/END entry:

$ awk 'FNR==NR && /^END$/ {next} 1' file1 file2 > file3
$ cat file3
string 1
string 2
string N
string 3
string 4
string M

Answer 5

Adding 1 more variant of tac + awk solution here for FUN.

tac file1 | awk 'FNR==1{system("tac file2");next} 1' | tac

Explanation: Adding detailed explanation for above.

tac file1 |               ##using tac command to read contents from bottom to up and sending its standard output as standard input to awk command here.
awk '                     ##Starting awk program from here.
  FNR==1{                 ##Checking condition if this is first line then do following.
    system("tac file2")   ##Printing file2 bottom to up contents.
    next                  ##next will skip all further contents from here.
  }
  1                       ##Printing rest of lines(passed from tac file1) here.
' | tac                   ##Reversing order again to make it into original order.