Append to file only if it exists

Question

I\'ve seen several answers on SO about how to append to a file if it exists and create a new file if it doesn\'t (echo \"hello\" >> file.txt) or overwrite

Answer 1

I think a simple if as proposed in the other answers would be best. However, here are some more exotic solutions:

Using dd

dd can do the check and redirection in one step

echo hello | dd conv=nocreat of=file.txt

Note that dd prints statistics to stderr. You can silence them by appending 2> /dev/null but then the warning file does not exist goes missing too.

Using a custom Function

When you do these kind of redirections very often, then a reusable function would be appropriate. Some examples:

Run echo and redirect only if the file exists. Otherwise, raise the syntax error -bash: $(...): ambiguous redirect.

ifExists() { [ -f "$1" ] && printf %s "$1"; } 
echo hello >> "$(ifExists file.txt)"

Always run echo, but print a warning and discard the output if the file does not exist.

ifExists() {
  if [ -f "$1" ]; then
    printf %s "$1"
  else
    echo "File $1 does not exist. Discarding output." >&2
    printf /dev/null
  fi
} 
echo hello >> "$(ifExists file.txt)"

Please note that ifExists cannot handle all file names. If you deal with very unusual filenames ending with newlines, then the subshell $( ...) will remove those trailing newlines and the resulting file will be different from the one specified. To solve this problem you have to use a pipe.

Always run echo, but print a warning and discard the output if the file does not exist.

appendIfExists() {
  if [ -f "$1" ]; then
    cat >> "$1"
  else
    echo "File $1 does not exist. Discarding output." >&2
    return 1
  fi
} 
echo hello | appendIfExists file.txt