Inside a bash script, how to get PID from a program executed when using the eval command?

Question

I have commands in a bash script that are similar to this:

eval "( java -classpath ./ $classname ${arguments[@]} $redirection_options $file )" &
pid=$!

However if I do a ps $pid it shows the main script process instead of the process of the java program.

It obtains the correct process when I omit the eval, but in order to get some of the complicated arguments to work correctly I need to use it.

Any idea of how I can get the PID of the java program when it's executed within an eval command?

TL;DR: use $! after your line with the ampersand & (such as myprogram &) — i_want_more_edits
– i_want_more_edits, Commented Jul 19, 2022 at 15:01

pilcrow · Accepted Answer · 2012-10-10 19:05:37Z

40

Your ampersand is backgrounding the eval line, causing the (top-level) shell to fork a child, the child shell to eval the string and in turn run your java program as a grandchild of the top-level shell. So, $! reports the pid of the child shell, which is the most recently backgrounded command.

Instead move the backgrounding inside your eval:

eval "(java ...) &"
pid=$!

As long as the parenthetical doesn't get complicated enough to become a subshell, the above will work.

edited Oct 10, 2012 at 19:05

answered Dec 2, 2010 at 21:17

pilcrow

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3 Comments

barti_ddu Over a year ago

Yes, this is it :) EXPR="java -jar test.jar &"; eval $EXPR; echo $! should do

Mike Q Over a year ago

FYI: I noticed this works from a terminal but if you have a script with an icon on the desktop, it doesn't work.

Roel Van de Paar Over a year ago

Make sure to add " double quotes around the expression being evaluated, and also make sure to add the & inside those double quotes, just before the final double quote.

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