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to clear all the history entries. Regarding the zombie processes, they are child processes whose parent does not receive notification about their fate anymore. The only way to kill them (without rebooting) is to quit or kill the parent process. To verify the hierarchy of the processes, you can try the command
ps -eO user -H
find the processes in a <defunct> status and quit or kill their parent (if possible).
... Regarding the zombie processes, they are child processes whose parent does not receive notification about their fate anymore. The only way to kill them (without rebooting) is to quit or kill the parent process. To verify the hierarchy of the processes, ..
Almost, but not quite. The parent is sent a SIGCHLD when the child dies/exits. The parent must perform a wait() system call to collect the child's status. The defunct zombie process contains essentially that status, but little more; resources have been freed. The signal is sent; the parent may or may not handle the signal, and may or may not reap the child's status. When a parent exits before handling its children via wait(), the init process inherits the child status and usually cleans up properly.
Go to your home directory and type this in terminal
echo > .bash_history
This will delete the history that will be loaded when the next shell starts, but exiting the current shell will place its current history list back into the file.
The key point is that there is both an in core history for each shell process, and on disk history loaded by newly started shells. Unsetting the HISTFILE shell variable will prevent saving to .bash_history. And preceding a command with a space will prevent that command from entering the history (if HISTCONTROL is set to ignorespace). See the variables in man bash that start with HIST.
Best keep to one topic per thread, else we gets confuzzled.
What is "the history command" that you want o clear the output from? The answers you've been given tell you how to clear the file that records the history but I'm not sure that's what you were asking for.
If you get an old command on your screen, typically by using the up arrow, you can get rid of it by Ctrl+C.