The simplest useful computer program

Sequence

The first idea to grasp is the sequence. A program, not surprisingly, is executed one instruction after the other.

#!/bin/bash

echo "Hello, World!"
echo "My name is Joe."
echo "Goodbye, World!"

Here, the first line (the shebang ) is used to specify that the program is written in bash, while echo is the command that prints the following to screen. The output of this program is, as you might expect,

Hello, World!
My name is Joe.
Goodbye, World!

It would be inconvenient to say goodbye before even saying hello…

Iteration

Another ingredient is the iteration: as long as some condition holds true, keep doing what you are doing. In order to show a concrete example, let’s imagine the following situation. A colleague of yours took about a hundred photos to document some construction work that is taking place close at your office. When you save them on your computer, their naming is something like DSC3162.jpg, DSC3163.jpg, DSC3164.jpg and so on up to DSC3271.jpg. You would like to have a more transparent scheme, like Baustelle_1.jpg, Baustelle_2.jpg and so on. Is there a way to do this automatically, rather than renaming them one by one? It turns out that these few lines do the trick. Comments appearing after # do not affect the program and are written only to explain in common language what each line does. The command mv old_name new_name is used to rename files.

#!/bin/bash

i=3162  # start from 3162
while (( $i <= 3271 ))  # repeat lines between do and done, but stop at 3271
do
  j=$(( i-3161 ))  # renamed photos start from 1 (3162 - 3161 = 1)
    mv "DSC${i}.jpg" "Baustelle_${j}.jpg"  # rename DSC3162.jpg to Baustelle_1.jpg
    i=$(( i+1 ))  # go to next photo
done

In these few lines we have also introduced a convenient entity called variable. These are containers that are used throughout a program to store some information that is needed in different parts of the program. At the beginning we have set the variable i to 3162 (remember that the first photo we want to rename is DSC3162.jpg). After renaming of each file we increment this variable by one with the instruction i=$(( i+1 )) in order to deal with the next photo.

Selection

Let’s now add a third ingredient: the selection. In plain language, do this or that based on whether a condition is true or not. Let’s elaborate on the previous example. After taking photos of the construction site, your colleague uses the same camera to take about two hundred photos at the Christmas party. Original files will follow the same naming scheme, meaning that files DSC3272.jpg to DSC3498.jpg should be renamed Xmas_party_2017_1.jpg, Xmas_party_2017_2.jpg… In code:

#!/bin/bash

i=3162  # start from 3162
while (( $i <= 3498 ))  # rename all photos up to DSC3498.jpg
do
  if (("$i" <= 3271)); then  # the first set of photos follows the old rule
     j=$(( i-3161 ))
       mv "DSC${i}.jpg" "Baustelle_${j}.jpg"
   else  # the second set is renamed to Xmas_party_2017_...
     j=$(( i-3271 ))  # party photos should also start from 1 (3272 - 3271 = 1)
     mv "DSC${i}.jpg" "Xmas_party_2017_${j}.jpg"
   fi
    i=$(( i+1 ))  # go to next photo
done