0

I have the below code to find out the number of instances of current script running that is running with same arg1. But looks like the script creates a subshell and executes this command which also shows up in output. What would be the better approach to find the number of instances of running script ?

$cat test.sh
#!/bin/bash

num_inst=`ps -ef | grep $0 | grep $1 | wc -l`
echo $num_inst
$ps aux | grep test.sh | grep arg1 | grep -v grep | wc -l
0
$./test.sh arg1 arg2
3
$

I am looking for a solution that matches all running instance of ./test.sh arg1 arg2 not the one with ./test.sh arg10 arg20

Hipster1206
  • 411
  • 6
  • 13

2 Answers2

0

Modify your script like this:

#!/bin/bash

ps -ef | grep $0 | wc -l

No need to store the value in a variable, the result is printed to standard out anyway.

Now why do you get 3?

When you run a command within back ticks (fyi you should use syntax num_inst=$( COMMAND ) and not back ticks), it creates a new sub-shell to run COMMAND, then assigns the stdout text to the variable. So if you remove the use of $(), you will get your expected value of 2.

To convince yourself of that, remove the | wc -l, you will see that num_inst has 3 processes, not 2. The third one exists only for the execution of COMMAND.

Nic3500
  • 8,144
  • 10
  • 29
  • 40
  • Hi Nic, thanks for your suggestion, actually i need to store it to variable so that I can compare the number of instances running and take action based on that...is it possible to read the output to variable directly without creating a subshell – Hipster1206 Aug 03 '22 at 20:53
0

The reason this creates a subshell is that there's a pipeline inside the command substitution. If you run ps -ef alone in a command substitution, and then separately process the output from that, you can avoid this problem:

#!/bin/bash

all_processes=$(ps -ef)
num_inst=$(echo "$all_processes" | grep "$0" | grep -c "$1")
echo "$num_inst"

I also did a bit of cleanup on the script: double-quote all variable references to avoid weird parsing, used $() instead of backticks, and replaced grep ... | wc -l with grep -c.

You might also replace the echo "$all_processes" | ... with ... <<<"$all_processes" and maybe the two greps with a single grep -c "$0 $1":

...
num_inst=$(grep -c "$0 $1" <<<"$all_processes")
...
Gordon Davisson
  • 118,432
  • 16
  • 123
  • 151