Add list of words at the end of the 1st line in a loop

Question

I have a list with the names of several files:

file1
file2

I would like to add at the end of the 1st line of each file the name of the file e.g.:

file 1 first row will be

> ATCGCCfile1

file 2 first row will be

> ATTTCCfile2

My idea is now to create a variable a with the file names' name:

a="file1 file2"

loop over the files and add the word re-calling the index:

for i in $a; do cat $i | awk '{print $0"$i"}' > $i.extended_word.txt; done

however in the output the awk '{print $0"$i"}' does not work and gives me just the $i without the name of the file e.g. in case of the 1st file >ATCGCC$i .

What am i doing wrong? I also tried the parentesis ( awk '{print $0"${i}"}') but without succeed.

Answer 1

To store a list, you want an array:

a=(
  file1
  file2
  'other file with spaces'
  $'even with\nnewlines'
)
awk -- '
  FNR == 1 {
    close(out)
    $0 = $0 FILENAME
    out = FILENAME".extended_word.txt"
  }
  {print > out}' "${a[@]}"

Beware that if file names contain = characters, you'd need to make sure that what's left of the = is not a valid awk variable. For instance, if you have a file=1 file, make it ./file=1.

With GNU awk, a work around is to write it:

gawk -e '
  FNR == 1 {$0 = $0 FILENAME}
  {print > FILENAME ".extended_word.txt"}
  ' -E /dev/null "${a[@]}"

Which works even for files with = in their path but unfortunately still not for a file called -.

Answer 2

Assuming all the files are in the same directory and the list is in a file:

files.txt

file1
file2

Using mapfile and sed:

mapfile -t files < files.txt

for f in "${files[@]}"; do
  if [ ! -f "$f" ]; then
    echo "File '$f' does not exists."
    continue
  fi
  
  sed "1s;.*;&$f;" "$f"
  
  # With option '-i' the file will be written
  # sed -i "1s;.*;&$f;" "$f"
done