将 jps -vl 命令的输出分配给 shell 脚本中的变量

Question

I need to assign the output of a command to a variable. The command I tried is:

 #!/bin/bash
 JAVA_PROCESSES=`jps -vl | grep -v 'sun.tools.jps.Jps' | grep -v 'hudson.remoting.jnlp.Main' | grep -v grep`
 NUMBER_OF_JAVA_PROCESSES=`echo $JAVA_PROCESSES | wc -l`
 echo $NUMBER_OF_JAVA_PROCESSES
 echo $JAVA_PROCESSES
 ..

When I tried as in above, all java processes grepped are assigned to JAVA_PROCESSES variable in one line. Processes are not separated by new line. Therefore $NUMBER_OF_JAVA_PROCESSES always give 1 for me.

Also $NUMBER_OF_JAVA_PROCESSES show 1 even no processes are assigned to JAVA_PROCESSES due to the empty line in $JAVA_PROCESSES.

Please suggest a way to assign grepped processes separated by new line.

Answer 1

If the main thing you want is to know whether or not you got any at all, you could just test if the variable is empty:

java_procs=$(jps -vl | grep -v 'sun.tools.jps.Jps' | grep -v 'hudson.remoting.jnlp.Main' | grep -v grep)
if [ -z "$java_procs" ]; then
    echo "No processes"
fi

Also, we can simplify the grep by using extended regex and just needing a single processes:

java_procs=$(jps -vl | grep -Ev 'sun.tools.jps.Jps|hudson.remoting.jnlp.Main|grep')

Assuming none of the lines output by jps can contain linebreaks themselves, we could get the count after that if we need it:

num_procs=$(printf '%s\n' "$java_procs" | wc -l)

The main problem you were running into is that you weren't quoting your variable, so echo $JAVA_PROCESSES was being expanded and then subject to word splitting, so your newlines were being "eaten" by the shell. You'd always have only one line which would be a space separated list of all the words in your JAVA_PROCESSES variable. To protect from word splitting you can quote the variable, as I did in my code above.

echo will also always add a line break at the end, which is good sometimes, and not so good sometimes, but you should be aware of it happening (that's why you would always get a count of 1 even when there were no processes).