entry: foreground and background proc

zk/Foreground_and_background_processes.md

@@ -0,0 +1,105 @@
+---
+tags:
+  - Linux
+---
+
+## Use `&` to send a process to background
+
+Here is a dummy [process](./Processes.md):
+
+```sh
+bash -c 'while sleep 5; do echo "Still running... $(date +%T)"; done'
+```
+
+If I run this normally, it will continue to print every 5 seconds and I can't
+use the terminal.
+
+If I append `&` it will run in the background:
+
+```sh
+
+bash -c 'while sleep 5; do echo "Still running... $(date +%T)"; done' &
+# [1] 13134
+```
+
+It prints the job number (`[1]`) and the PID of the process.
+
+Now `stdout` will continue to interrupt every 5 seconds but I can do other
+things in the foreground, e.g:
+
+```
+ ~ bash -c 'while sleep 5; do echo "Still running... $(date +%T)"; done' &
+[2] 13505
+➜  ~ Still running... 18:20:42
+echo 'i can still use terminal'
+i can still use terminal
+➜  ~ Still running... 18:20:47
+Still running... 18:20:47
+```
+
+> Notice now I have two processes running (the same print script, twice), so the
+> job number has incremented to `[2]`
+
+## Bring a background process back to the foreground
+
+Use `%<job-number>` or `%<pid` to bring a process back to the foreground:
+
+```sh
+
+fg %1
+# [1]  - 13134 running    bash -c 'while sleep 5; do echo "Still running... $(date +%T)"; done'
+# Still running... 18:23:22
+# Still running... 18:23:22
+# Still running... 18:23:27
+```
+
+Now it's back in the foreground and I cannot use the terminal:
+
+```
+Still running... 18:24:17
+ls
+Still running... 18:24:22
+Still running... 18:24:22
+```
+
+## Pausing a job
+
+`Ctrl+Z` does not kill the process. It _pauses_ it. This moves it to the
+background and pauses its execution, returning terminal control back to you.
+
+For example I've stopped `vim` below:
+
+```
+vim minicom.log
+[1]  + 14231 suspended  nvim minicom.log
+```
+
+Again I'm given the job number.
+
+For it to continue in the background:
+
+```sh
+bg %1
+```
+
+Or the foreground:
+
+```sh
+fg %1
+```
+
+(This will reopen `vim`.)
+
+## Terminate a job
+
+```
+kill %1
+```
+
+## View jobs with `jobs`
+
+```
+jobs
+[1]  + suspended  nvim minicom.log
+[2]  - running    bash -c 'while sleep 5; do echo "Still running... $(date +%T)"; done'
+```