How to attach gdb with running process

How to attach gdb with running process ?
gdb <PROCESS NAME> <PROCESS ID>
 we can grab the process id with ps command.
Suppose your program name is a.out
so command to get the process id would be?
ps -elf | grep a.out
usr1   4473  3529  0 16:42 pts/0    00:00:00 grep --color=auto a.out
here process name is a.out and process id is 4473
so command to attach the running process a.out with process id a.out would be
gdb a.out 4473
Note: You might require to execute as sudo user.

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