Question

Is there a way to tell curl to output errors to stderr, and everything else to stdout?

The reason is that I am using curl from the command line (actually a cronjob) to upload a file to an FTP site every evening. Unfortunately because curl outputs status information on stderr, I receive an e-mail about an error when nothing actually went wrong. (I’m redirecting stdout to a log file, but leaving stderr unchanged so that cron will e-mail it to me if there is any output.)

There are options to make curl silent, or output everything to stdout, however both these alternatives prevent errors from appearing on stderr – meaning I won’t get an e-mail when there is actually an error I want to know about.

So is there a way to make curl only output errors on stderr, but leave normal output intact on stdout?

Answer 1

After some more experimentation I have come up with the following workaround, but I’m still open to better alternatives.

It works by temporarily storing all output (stdout and stderr) in a temporary file, and then sending the contents of that file to stderr or stdout depending on curl’s exit code. If curl failed the entire output will go to stderr (and be e-mailed to me thanks to cron), but if curl succeeded the output will go to stdout instead (which is redirected to a log file in the cron command, resulting in no e-mail.)

# Get a temporary filename
CURL_LOG=`tempfile`

(
  # Run curl, and stick all output in the temp file
  /usr/bin/curl --verbose ... > "$CURL_LOG" 2>&1
) || (
  # If curl exited with a non-zero error code, send its output to stderr so that
  # cron will e-mail it.
  cat "$CURL_LOG" > /dev/stderr
  rm "$CURL_LOG"
  exit 1
)

# Otherwise curl completed successfully, so send the output to stdout (which
# is redirected to a log file in crontab)
cat "$CURL_LOG"
rm "$CURL_LOG"