Lab 3: Linux Processes and Scripting

Exercises:

This lab was an introduction to Bash scripting and linux processes.

1. Learning about linux processes. All linux processes have a PID associated with them, which is a unique id for that process. This can be used then to kill the process from the console.
2. Starting to write bash scripts which execute other programs. This first script runs a infinite loop, which is there to get us to learn how to use pskill to kill the process.

   #!/bin/bash
   javac Loop.java
   java Loop

3. More advanced scripting, using if statements and switch cases. This includes the use of posix pattern matching, which is similar to regex. [1] We also learned about for loops.

   #!/bin/bash
   
   # if statement example
   if [ ... ] then
       echo "Hello world"
       exit 1
   fi
   
   # case statement example
   case $a in
       [0-9]*) ;;
       *) echo "Arguments must be numbers.">&; exit 1;;
   esac

   for a in $*
   do
       echo "$a"
   done
   

   This all in the end culminated in writing a bash script that would only take 2 single digit numbers, outputting "OK" and "Error" otherwise. That can be found here (should that work).

Commands and tidbits:

• ps will list all processes associated with the user (ps -e for everything).
• ps | grep <process> will find processes by that name and print them out in the ps table. Using pgrep will only print out the PIDs and nothing else.
• pkill will kill a process based on name (though bear in mind for applications with multiple processes this can not entirely work), whilst kill takes a PID.
• Running a command/process with an & can free up the console to do other things whilst said process is running. For example, ./runloop &.
• In bash, variables are immediately appended with $, if and case statements are closed by writing if and case backwards (fi and esac) respectively, and for loops are closed by done.
• Bash has a really unintuitive syntax that's hard to get used to.

Reflections

Whilst still followable, this is where the content started increasing in difficulty. First, the processes section was not too hard to get, along with piping outputs from one command to another, however this was when I was first met with the horrible, convoluted, unintuive syntax and style of the bash script. Whitespace rules were incredibly strict, stricter even than a whitespace dependent language like Python, and in ways that were hard to grasp. Examples include things like do, then having to be on a new line to their respective for, if statements, as well as certain things throwing errors if there are no spaces surrounding them: if [ cond ] good, if [cond] bad.

Bash seems to be a language where "if it works it works", and I need to get used to its convoluted syntax, just enough to make a script work.

References

• [1] posix