combine two files and overwrite original file using cat

Question

I try to combine two files using cat command, but facing a problem.

original.txt
============ 
foo
bar
foo
bar

following is my script.

cat original.txt | wc -l > linecount.txt | cat linecount.txt original.txt > original.txt

This script returns error that says "input file and output file is the same.".

Expected result is like this.

original.txt
============
4
foo
bar
foo
bar

Any idea?

Answer 1

You can probably use:

{ wc -l < original.txt; cat original.txt; } > linecount.txt &&
mv linecount.txt original.txt

Or using awk :

awk 'NR==FNR{++n; next} FNR==1{print n} 1' original.txt{,} > linecount.txt &&
mv linecount.txt original.txt

Or:

awk -v n=$(wc -l < original.txt) 'NR==1{print n} 1' original.txt > linecount.txt &&
mv linecount.txt original.txt 

Answer 2

You can use sponge from the moreutils package. I like it for that:

cat <(wc -l orig.txt) orig.txt | sponge orig.txt

---

If you don't have sponge or cannot install it, you can implement it with awk as a bash function:

function sponge() {
    awk -v o="${1}" '{b=NR>1?b""ORS""$0:$0}END{print b > o}'
}

Keep in mind that this will need to store the whole file in memory. Don't use it for very large files.