I'm trying to run a command, write that to a file, and then I'm using that file for something else.

The gist of what I need is:

myAPICommand.exe parameters > myFile.txt

The problem is that myAPICommand.exe fails a lot. I attempt to fix some of the problems and rerun, but I get hit with "cannot overwrite existing file". I have to run a separate rm command to cleanup the blank myFile.txt and then rerun myAPICommand.exe.

It's not the most egregious problem, but it is annoying.

How can I avoid writing a blank file when my base command fails?

You must have "noclobber" set, check the following example:

$ echo 1 > 1  # create file

$ cat 1

1

$ echo 2 > 1  # overwrite file

$ cat 1

2

$ set -o noclobber

$ echo 3 > 1  # file is now protected from accidental overwrite

bash: 1: cannot overwrite existing file

$ cat 1

2

$ echo 3 >| 1  # temporary allow overwrite

$ cat 1

3

$ echo 4 > 1

bash: 1: cannot overwrite existing file

$ cat 1

3

$ set +o noclobber

$ echo 4 > 1

$ cat 1

4

"noclobber" is only for overwrite, you can still append though:

$ echo 4 > 1

bash: 1: cannot overwrite existing file

$ echo 4 >> 1

To check if you have that flag set you can type echo $- and see if you have C flag set (or set -o |grep clobber).

Q: How can I avoid writing a blank file when my base command fails?

Any requirements? You could just simply store the output in a variable and then check if it is empty. Check the following example (note that the way you check the variable needs fine adjusting to your needs, in the example I didn't quote it or use anything like ${cmd_output+x} which checks if variable is set, to avoid writing a file containing whitespaces only.

$ cmd_output=$(echo)

$ test $cmd_output && echo yes || echo no

no

$ cmd_output=$(echo -e 'nnn')

$ test $cmd_output && echo yes || echo no

no

$ cmd_output=$(echo -e ' ')

$ test $cmd_output && echo yes || echo no

no

$ cmd_output=$(echo -e 'something')

$ test $cmd_output && echo yes || echo no

yes



$ cmd_output=$(myAPICommand.exe parameters)

$ test $cmd_output && echo "$cmd_output" > myFile.txt

Example without using a single variable holding the whole output:

log() { while read data; do echo "$data" >> myFile.txt; done; }

myAPICommand.exe parameters |log

You can delete the file after running, if the command fails, with

myAPICommand parameters > myFile.txt || rm myFile.txt

But I would suggest clobbering the file instead:

myAPICommand parameters >| myFile.txt

See What are the shell's control and redirection operators? for details.

You could create a script to run the myAPICommand.exe, but have it first remove the myFile.txt if it exists. Then you don't have to constantly do the rm command to clean up.

Like:

if [ -e myFile.txt ]

then

    rm myFile.txt && myAPICommand.exe

else

You could also make it so that your command cleans up after itself. If the file is empty adding something like the following.

Like:

if [ -s myFile.txt ]

then

        EXIT 0

else

        rm myFile.txt && EXIT 1

fi