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

Assuming the file is either nonexistent or both readable and writable, you can try to open it for reading first to determine whether it exists or not, e.g.:

command 3>file

3<&- may be omitted in most cases as it's unexpected for a program to start reading from file descriptor 3 without redirecting it first.

Proof of concept:

$ echo hello 3>file
bash: file: No such file or directory
$ ls file
ls: cannot access 'file': No such file or directory
$ touch file
$ echo hello 3>file
$ cat file
hello
$

This works because redirections are processed from left to right, and a redirection error causes the execution of a command to halt. So if file doesn't exist (or is not readable), 3 fails, the shell prints an error message and stops processing this command. Otherwise, 3<&- closes the descriptor (3) associated with file in previous step, >>file reopens file for appending and redirects standard output to it.