Added notes on how to find how long a process ran to the knowledge base

knowledge base/find how long a process has run.md

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+# Find how long a process has run in UNIX
+
+UNIX and Linux have commands for almost everything; if there is no command, you can check some important files in the `/etc` directory or in the `/proc` virtual filesystem to find out some useful information.
+
+## The easy way
+
+If the program started today, `ps` also shows when:
+
+```sh
+$ ps -ef
+UID        PID  PPID  C STIME TTY          TIME CMD
+user     11610 11578  0 Aug24 ?        00:08:06 java -Djava.library.path=/usr/lib/jni:/usr/lib/alpha-linux-gnu/jni...
+user     17057 25803  0 13:01 ?        00:00:24 /usr/lib/chromium-browser/chromium-browser
+```
+
+## The hackish way
+
+Useful if the process started before today:
+
+1. find the process ID
+
+   ```sh
+   $ ps -ef | grep java
+   user 22031 22029   0   Jan 29 ? 24:53 java -Xms512M -Xmx512 Server
+
+   $ pgrep -l java
+   22031 java
+   ```
+
+1. look into the `proc` virtual filesystem for that process and check the creation date, which is when the process was started
+
+   ```sh
+   ls -ld /proc/22031
+   dr-x--x--x   5 user     group           832 Jan 22 13:09 /proc/22031
+   ```
+
+## Sources
+
+- [how to find how long a process has run in unix]
+
+[how to find how long a process has run in unix]: https://dzone.com/articles/how-to-find-how-long-a-process-has-run-in-unix