How to change niceness of process by name

If you have only one java instance running, simply:

renice -n 5 -p $(pgrep ^java$)

$(pgrep ^java$): command substitution; bash replaces this with the output of pgrep ^java$; pgrep ^java$ returns the list of PIDs of the processes whose name matches the regular expression ^java$, which matches all processes whose name is exactly java

If you have multiple java instances running:

for p in $(pgrep ^java$); do renice -n 5 -p $p; done

for p in $(pgrep ^java$); do renice -n 5 -p $p; done: almost the same as above; $(pgrep ^java$) is a command substitution; bash replaces this with the output of pgrep ^java$; pgrep ^java$ returns the list of PIDs of the processes whose name matches the regular expression ^java$, which matches all processes whose name is exactly java; this is expanded into the for loop, which assigns a new line of the output of pgrep ^java$ to the variable $p and runs renice -n 5 -p $p at each iteration until the output of pgrep ^java$ is consumed

Try this:

pgrep java | xargs -n 1 echo renice -n 5 -p

If output is okay, remove echo.

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