Github user woowahan-jaehoon commented on the issue:
https://github.com/apache/zeppelin/pull/2780
If interpreter run normally, `interpreter.sh` wait 259 line(```wait```)
until shuting down interpreter.
After click 'restart interpreter', zeppelin send shutdown call to
interpreter throght thrift.
Then, interpreter shutdown and then 261 line(```rm -f "${ZEPPELIN_PID}" >
/dev/null 2> /dev/null```) is executed.
Before, pid file of interpreter is remained because zeppelin hasn't logic
for remove pid file after shuting down interpreter.
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