One additional tidbit ...
To print a message to the ASE errorlog (eg, from a T-SQL;batch, from a stored proc/trigger):
dbcc printolog('<some_string>')
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You didn't mention *why* you're deleting/truncating the errorlog (it was a mistake? on purpose because it's gotten too large?) nor your ASE version so fwiw ...
If your ASE errorlog is growing too large (eg, filling up the filesystem, getting too large to scan/analyze), and you're running ASE 15.0.3+, you can use sp_errorlog (as mentioned by Bret) to do the following:
NOTE: Keep in mind that the sp_errorlog change only affects the currently running ASE; the next time the RUNserver file is used to startup the ASE, said ASE will write to whatever errorlog file is named in the RUNserver file.
1 - run sp_errorlog to point ASE to a newly named log (creating said file if it doesn't exist); the old errorlog file will still be there ...same name, same contents
2 - move the old log out of the way (eg, move to a directory on a different filesystem)
3 - run sp_errorlog to point ASE back to the previously named log (creating a new file since it won't exist); the short-used errorlog (created in step #1) will still exist so you can keep it or remove it once you've determined it contains nothing of value
This allows you to move the old/too-big errorlog out of the way while not having to update the RUNserver file to point at a different errorlog file next time the ASE is started.