It is noticeable that I’ve been spending a bit of time with Elixir and Erlang, and therefore using iex and Erlang’s shell a lot.
On REPLs like this, I’m used to press
ctrl+d to exit them (bash, scala, python, irb, etc), and I was a bit annoyed that this doesn’t work on iex and erl. While playing with elixir-mines, a minesweeper game that you can play through telnet, I ended up learning another shortcut that can be used in these cases…
When using the telnet command, you can use
ctrl+] to escape a session, taking you to telnet’s prompt (which, BTW, also supports
ctrl+d). By mistake, I tried to do the same thing on iex, but pressed
\ instead of
]. Turns out
ctrl+\ sends a SIGQUIT UNIX signal, which makes both iex and Erlang to stop their process’.
I then wondered what would that make on other REPLs. This also works with python and irb, albeit returning a non-zero exit code. But on Scala’s, something completely different happens. A SIGQUITs causes the JVM to generate a thread dump.
Later on the week, while listening to a podcast, I also found out that SIGHUP, which originally was created to signal a process that a user physically hang up the modem (removing the phone from it, just so you know how old its origins are), is nowadays commonly used to signal a process that it should reload its configuration files.
Those learnings got me curious about how signals work at all. I found a nice overview on this StackExchange article and this post. In a nutshell, SIGINT and SIGKILL will not reach the target process, but instead will be handled directly by the kernel. Other signals are sent to the target process, which in turn decides what to do with them (that means even ignoring them). BTW, you can get a brief overview of existing signals through
Digging a bit more, I found out about SIGSTOP and SIGCONT, and how they can be used to pause and continue a process. It might be useful for debugging purposes in the future. For instance, connect VisualVM to a running JVM, do a Heap dump, pause the process (so nothing changes in between), check what is going on, continue it.
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