Bash¶

In high performance computing, we generally work in a terminal, meaning using some shell. Have you ever seen these lines at the top of your “shell scripts?”

#!/bin/sh

or

#!/bin/sh

Those first lines are the interpreter lines - or an instruction for what program to use to run the script. If you look at the variable SHELL in your terminal, you likely will see the shell that you are using as we speak!

echo $SHELL
/bin/bash

There is a rich history of how the Unix shell became the “Bourne again” shell, or what we commonly refer to as bash.

So why bash?¶

The reason to get excited about bash is because there are SO many things you can do with it that you would typically rely on a higher level language (Python, R, etc.) to do. For example, you can trim strings, write functions to otherwise manipulate strings, calculate quantities, read files, count things… it’s a powerful language! Here is a one line function to make a string all lowercase:

lower() {
    printf '%s\n' "${1,,}"
}

$ lower LiKeOmGTaCoS
likeomgtacos

Specifically, if you are interested in little snippets to perform more complex operations, take a look at (or contribute to) the bash bible. So the next time you want to do some special thing from the command line – ask yourself if you can do it with bash first, before you delve into requiring a higher level language dependency.

Loops¶

It’s very easy to do loops in bash! Here is a simple for loop:

for node in compute{1..10}; do
  ssh $node 'hostname -s'
done

And a while loop:

count=1
while /bin/true; do
  echo "looped ${count} times"
  let count=${count}+1
  sleep 10
done

And a classic if-then-else statement

result=0

if [[ "$result" == "0" ]];then
  echo "Success"
else
  echo "Fail"
fi

or maybe you want to loop over files in a directory?

for file in $(ls $PWD); 
    do 
    echo $file; 
done

Another simple loop is literally just a line of space separated strings, like this:

$ for thing in "one" "two";
    do 
        echo $thing 
    done

one
two

Calculator¶

You can add this to your ~/.bashrc file to create a simple to use calculator:

# Results are trimmed to 2 decimal places
calc() { echo "scale=2; $*" | bc -l; }

# Results are not trimmed
calc_full() { echo "$*" | bc -l; }

Here is example usage:

$ calc "1024*8"       # Convert KB into MB
8192

$ calc "40*1.8+32"    # Convert Celsius to Fahrenheit
104.0

$ calc "(2+2)*(2-2)"  # You can also control the order of operation with ( )
0

These examples are referenced from AskCI.