Scripting

Scripting#

Purpose: Automatise execution of multiple commands at once, so you have less work, make less mistakes and save time.

A bash or shell script is basically a QGIS model but for the command line. It contains one or several commands (e.g. GDAL commands) which are all executed one after the other when running the script.

It is important to follow a certain syntax in the script:

  • Each command needs to be in a new line.

  • You can add any command which you can execute in the command line.

  • Lines marked as comments (# or ::) are not executed. They can be used to describe what the command below does, so others understand it better.

  • The echo command prints out what ever is written after it. Useful to give the user feedback what is being done.

Scripts on Windows and Mac/Linux have different syntax:

Windows: Bash scripts (*.bat)

Bash scripts have the file ending *.bat. Comments are marked using ::. For example, the content of 01_metadata.bat in assignment 2:

:: Prints metadata of the files
:: Author:
:: Date:

echo "Printing info about input files"

:: Get info about gadm36_SVN.gpkg
gdalinfo ./gadm36_SVN.gpkg

:: Get info about N45E014_sub.gpkg
ogrinfo ./N45E014_sub.gpkg

Mac/Linux: Shell scripts (*.sh)

Shell scripts have the file ending *.sh. Comments are marked using #. For example, the content of 01_metadata.sh in assignment 2:

#!/bin/sh
# Prints metadata of the files
# Author:
# Date:

echo "Printing info about input files"

# Get info about gadm36_SVN.gpkg
gdalinfo ./gadm36_SVN.gpkg

# Get info about N45E014_sub.gpkg
ogrinfo ./N45E014_sub.gpkg

How to execute it#

Open your command line and execute the script by calling its file path. For example,

  1. Open your command line and move into the folder .fossgis_assignment_02/scripts of your repo for Assignment 2.

    cd ./fossgis_assignment_02/scripts

  2. Execute the 01_metadata script from the command line using

    Windows: .\01_metadata.bat
    Mac/Linux: ./01_metadata.sh

Important

Mac/Linux: If you get an error message, you might have to make the file executable. To do this run this once, before executing the file: chmod +x ./01_metadata.sh

Variables#

Variables are placeholders for values of all kind, e.g. numbers, file paths or parameter values. Using variables, you can avoid repetitions in the script. In the example below, the age is only set once at the beginning, but used twice in the command below.

Windows

Content of script whatsyourage.bat:

:: Parameters
set MYNAME=Sam
:: The parameter /A is necessary because the value should be numeric
set /A AGE=23
set /A CURRENTYEAR=2024

:: Calculate birth year
SET /A BORNIN = %CURRENTYEAR% - %AGE% 

:: Programm output
echo Hi %MYNAME%!
echo You are %AGE% years old. So you were born in %BORNIN%?

Mac/Linux

Content of script whatsyourage.sh:

:: Parameters
MYNAME=Sam
AGE=23
CURRENTYEAR=2024

:: Calculate birth year
BORNIN=$(($CURRENTYEAR-$AGE))

:: Programm output
echo Hi $MYNAME!
echo You are $AGE years old. So you were born in $BORNIN?

Programm output:

./whatsyourage.bat
Hi Sam!
You are 23 years old. So you were born in 2001?

Command line parameters#

Using command line parameters you can also set the value of a variable based on the input of the user when executing the script. This makes your script more generic and dynamic, so it can be applied to different data sets without always changing the file paths inside of to file.

Windows

Command line parameter values are retrieved using %1, %2, etc. on Windows.

Adapted script whatsyourage.bat:

:: Parameters
set MYNAME=%1
:: The parameter /A is necessary because the value should be numeric
set /A AGE=%2
set /A CURRENTYEAR=2024

:: Calculate birth year
SET /A BORNIN = %CURRENTYEAR% - %AGE% 

:: Programm output
echo Hi %MYNAME%!
echo You are %AGE% years old. So you were born in %BORNIN%?

Mac/Linux

Command line parameter values are retrieved using $1, $2, etc. on Mac/Linux.

Adapted script whatsyourage.bat:

:: Parameters
MYNAME=$1
AGE=$2
CURRENTYEAR=2024

:: Calculate birth year
BORNIN=$(($CURRENTYEAR-$AGE))

:: Programm output
echo Hi $MYNAME!
echo You are $AGE years old. So you were born in $BORNIN?

Programm output:

./whatsyourage.bat Sally 24
Hi Sally!
You are 24 years old. So you were born in 2000?

Assignment tip

This can save you some work in assignment 2.

Trouble shooting#

  • Make sure that the file paths in your script are relative to the location of the script.

  • Make sure you are executing the right kind of script and syntax based on your operating system.

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