Pitfalls

Unlike other programming languages, Batsh does not type-check, allowing you to assign strings to variables meant to hold integers, for example:

dn = filename("hmpf", 9876);

Listing 2 would then output the file name imagehmpf9876, which is relatively harmless in this case. However, incorrect parameters or parameters with typos can sometimes have drastic effects in more complex functions.

If you define a new variable in a function, it is only visible in that function. Consequently, you cannot access the variable b in Listing 3 outside function goodday. If you try anyway, Batsh thinks you are referring to a new variable b. New variables are empty by default, so println(b) would just output a blank line.

The greeting() function in Listing 4 tries to change variable a inside a function. If you run the finished script, you see the output Hello. If you try to change a, as the greeting() function does, Batsh creates a second variable a that is only visible within the function. To prevent this, you first need to make the variable inside the function global. Listing 5, correspondingly changed, then outputs the value World.

All-Around

Consider the situation in which you need to run an image editing program 10 times to convert 10 TIFF images into the JPEG format. The Batsh while loop can help with this task.

As shown in Figure 4, this process can be achieved more quickly, crisply, and therefore more clearly in Batsh than in Bash scripts. The script performs all the instructions in curly brackets as long as the condition in the parentheses is met.In Figure 4, the numeral in number consequently increases by 1 until number contains the value 11, and you fall out of the loop. The photos can now be converted using this loop.

Figure 4: On the left, a simple Batsh while loop; on the right, the Bash equivalent.

Listing 6 shows the matching programming code. First, the while loop puts together the file names of the original and converted photos using the current number.

The Batsh function call starts an arbitrary program – here, the ImageMagick convert command – that expects the file name of the program to be executed as the first parameter. All remaining parameters are forwarded by call directly to convert.

Depending on which program is executed and how many parameters that program requires, you can pass any number of arguments in the call statement. In this example, convert must be installed on both Linux and Windows so that the resulting scripts work on both systems. Batsh is not yet familiar with the for loop frequently used in Bash scripts.

Reading Circle

Listing 6 assembles the file names, but it would be more elegant to let the script deliver the file names in a directory and then feed them one after another to convert. Batsh provides the built-in function readdir for this:

allfiles = readdir();

In the background, readdir() calls ls in Linux or dir/w in Windows and simply returns the results. However, this leads to two problems. First, you have to be careful with file names provided by ls, because they could contain special characters that Bash later interprets incorrectly. Second, readdir() simply outputs a long character string with the file names, and Batsh does not currently offer a way to split strings into individual parts.