send bash stderr to logfile, but only if an error exists

Question

I am using the following code to send stderr to a file. ``` .script >2 "errorlog.$(date)" ``` The problem is that a blank log file is created every time I run the script, even if an error doesn't exist. I have looked online and in a few books as well, and can't figure out how to create a log file only if errors exist.

Accepted Answer

Output redirection opens the file before the script is run, so there is no way to tell if the file will receive any output. What you can do, however, is immediately delete the file if it winds up being empty:

```
logfile="errorlog.$(date)"
# Note your typo; it's 2>, not >2
script 2> "$logfile"; [ -s "$logfile" ] || rm -f "$logfile"
```

I use `-f` just in case, as `-s` can fail if `$logfile` does not exist, not just if it's empty. I use `;` to separate the commands because whether or not `$logfile` contains anything does not depend on whether or not `script` succeeds.

You can wrap this up in a function to make it easier to use.

```
save_log () {
    logfile=${1:-errorlog.$(date)}
    cat - > "$logfile"
    [ -s "$logfile" ] || rm -f "$logfile"
}

script 2> >( save_log )
script 2> >( save_log my_logfile.txt )
```

Not quite as simple as redirecting to a file, and depends on a non-standard feature (process substitution), but not too bad, either.

send bash stderr to logfile, but only if an error exists

Solution

Problem