How can I remove a specific string from a file only if there are other lines in the file?

Question

How can I remove a specific string from a file ONLY if there are other lines in the file?

For example, don't touch this file:

cat file.txt
ASDF

but remove "ASDF" from this file:

cat file.txt
ASDF
TR422

Because it has other characters than "ASDF".

I'll welcome a solution in bash, perl or any other common tool.

Answer 1

In bash, using sed:

if [[ $(< "$file") != "ASDF" ]]; then
  sed -i '/^ASDF$/d' "$file"
fi

Answer 2

Assuming your file names don't contain newline characters, this will remove lines containing just ASDF from all files in the current directory, unless the file consists of a single line containing just ASDF. If your sed doesn't have the -i option, output to a temporary file and move it in place afterwards.

grep -l ASDF -- * | while IFS= read -r filename; do
  if ! echo 'ASDF' | cmp -s - "$filename"; then
    sed -i -e '/^ASDF$/d' -- "$filename"
  fi
done

Here's another approach, which removes the ASDF lines, and only overwrites the original file if the new file is neither empty nor identical to the original.

tmpfile=$(mktemp -p "$(dirname -- "$filename")")
sed '/^ASDF$/' <"$filename" >"$tmpfile"
if [ -s "$tmpfile" ] && ! cmp -s -- "$filename" "$filename"; then
  mv -f -- "$tmpfile" "$filename"
else
  rm -- "$tmpfile"
fi

Answer 3

You would want to look for the number of lines in the file.

Pseudocode:

if (numberOfLines > 1 && file.contains("ASDF")) {
    file.remove("ASDF");
}

Answer 4

A shorter version of enzotib's answer:

[[ $(< "$file") != "ASDF" ]] && sed -i '/^ASDF$/d' "$file"