Introduction to the Command Line

Prior to learning GDAL or scripting it is important to learn the Command Line and how to use commands from the command prompt.

4. Creating and viewing files

4.2. Display contents of files

Now display the contents of the file on the screen.

1. Type:
type listdir.bat <ENTER>

With the type command you can view the contents of a file.

2. Now type:
listdir <ENTER>

Congratulations! You have created your first script, a batch file. Batch files are scripts that can be used to execute commands in batch. In this case the file executed dir /AD.
Because batch files always have the file extension .bat, the computer knows that this is a batch file and will execute the commands in the file. We'll come back to this later.

3. Let's create another file. Type:
dir > list.txt <ENTER>

This command will not show the result of the dir command on the screen, but saves it to an ASCII file (text file) called list.txt. So we can use > after a command to save its results to a file instead of printing it to the screen.

4. Check the contents of the file:
type list.txt <ENTER>

You can see that the list is larger than your screen.

5. Type:
type list.txt | more <ENTER>

The result of list.txt is given to the more command which displays the results in pages as big as your window. Press   <ENTER> to see the next line. Press <SPACE BAR> to see the next page. Press <CTRL-C> to stop. You can use this last key combination to stop any command if it isn't doing what you like, it crashed or it takes too long.

6. Type:
more <ENTER>

Nothing happens, because the more command expects input from another command. So you can wait forever. In this case you can stop the execution of the more command by using <CTRL-C>.

7. Now try this:
type listdir.bat >> list.txt <ENTER>

8. Display the result:
type list.txt | more <ENTER>

9. What happened?

In summary:

  • > saves the result of a command to a new file. If the file on the right hand of > already exists, it will be overwritten.
  • >> appends the result of a command to an existing ASCII file.
  • | uses the result of the command on the left hand side in the command on the right hand side of the |. This is called a pipe.
  • Use <CTRL-C> to stop the execution of a command.
We can also use these operators to create so called lock files. These are used in scripting. The script will check if a certain file exists. If it exists, the program will wait, if it is removed, the program continues. Therefore, these files can be empty. You can create a lockfile with: type NUL > lockfile.txt <ENTER>