"Strong Scripting Skills" - a definition? - sunday

Sun Jan 25 16:14:45 PST 2004

hi ya jim

it's sunday... time for some fun

On Sun, 25 Jan 2004 jimd at starshine.org wrote:

>  I once had an interviewer put up a wretched fragment of shell code like:
> 
>  	kill `ps wax | grep "foo" | grep -v "grep" | awk '{print $11}'`
> 
>  ... and ask me to explain what was "wrong with it.  I pointed out that
>  I wasn't sure which field number the PID was in in that ps output
>  format, and I always check with 'ps -$FLAGS | head' before code
>  anything like that.  (That was the answer he was looking for).  Then

"ps waxu" has its pid in $11
"ps wax"  has its pid in $5 ...

if foo is named " ThisSillyApp", one needs "grep -i"

if foo is an argument for another-app, one needs to watch out for 
the other's command options that happen to be "foo"

if multiple instances of foo is running, you might be sol and/or
kill might kill all ( 2-n ) instances of "foo" which is good and bad...
	bad if it was intended for "kill particular-sshd"

	other times it's good, "kill all-hung-mozilla" (or hung konqueror)

too many pipes in the above ... bash barfs on "|" commands
	cmd="ps axuw | grep sshd "
	echo "$cmd"
	$cmd  -- sometimes doesnt work .. depends

	I break out each pipe'd command separately in bash scripts
	( think i'm missing an old trick someplace )

more importantly.. did one test that it returns a valid pid
	-- always check that pid is non-zero and valid .. before killing

do you want kill -HUP or kill -SIGHUP or kill -9

what if kill doesnt kill it cause foo is stuck or still being used..

why not simplify it after verifying the pid is correct

	kill `cat /var/run/app.pid`
	kill ` pidof foo `

	killall foo

sometimes grep -v grep is needed, sometimes not... i donno why some
greps does it differently

and always test it afterward to verify it did what its supposed to
in the script ( check the return val or other means to double check )

and the above ps options line is not portable to multiple *nix 

c ya
alvin