Shell expansion

This exercise is about studying shell expansion. You should run it on your Debian VM in Vagrant.

Create a C program arguments.c with the following contents. You can use nano arguments.c for this, for example.

#include <stdio.h>

int main(int argc, char** argv) {
  for(int i=0; i < argc; i++) {
    printf("Argument #%i: [%s]\n", i, argv[i]);
  }
  return 0;
}

Compile this with gcc -Wall arguments.c -o arguments.

Whitespace

The program prints all its arguments, one per line. The program gets its arguments from the program that started it - in this case the shell. Try running the program with the following commands:

./arguments
./arguments hello
./arguments one two three

Now that you are familiar with what the program does, try the following:

./arguments one       two
./arguments "one two"
./arguments "one      two"

How, based on these examples, does the shell handle whitespace in the line you type?

Pattern matching

Try the following:

• ./arguments * in the folder that contains the arguments program, and its source code arguments.c.
• Make an empty subfolder with mkdir empty, switch to it with cd empty and then run ../arguments *. Since you are now in the subfolder, we need two dots at the start to say "run the program arguments in the folder above". What happens?
• Go back to the folder with the program by running cd .. and then do ls to check you're back in the right folder. In this folder, find three different ways to get the program to produce the following output:

Argument #0: [./arguments]
Argument #1: [*] 

Files with spaces in their names

The command touch FILENAME creates a file. Create a file with a space in its name by typing touch "silly named file". What would happen if you left the quotes off (you can try it, then do ls)?

Start typing ls sill and then press TAB to autocomplete. Assuming you have no other files whose name starts with sill, what happens? Use this method to get the arguments program to print the following:

Argument #0: [./arguments]
Argument #1: [Hello world!] 

The command rm (remove) deletes files again. Use it to remove your file with spaces in its name, using one of several methods to get the shell to pass the spaces through to rm.

Shell variables

In the shell, VARIABLE=VALUE sets a variable to a value and $VARIABLE retrieves its value. For example, to save typing a filename twice:

p=arguments
gcc -Wall $p.c -o $p

which expands to gcc -Wall arguments.c -o arguments. If you want to use a variable inside a word, you can use curly braces: ${a}b means the value of the variable a followed by the letter b, whereas $ab would mean the value of the variable ab.

It is good practice to double-quote variables used like this, because if you tried for example to compile a program called silly name.c with a space in its name, then

program="silly name"
gcc -Wall $program.c -o $program

would expand to

gcc -Wall silly name.c -o silly name

and this would confuse your compiler because you are telling it to compile three source files called silly, name.c and name to a program called silly. Correct would be:

program="silly name"
gcc -Wall "$program.c" -o "$program"

which expands to

gcc -Wall "silly name.c" -o "silly name"

which does what you want - if you indeed want a program with a space in its name!

There is no harm in double-quoting a shell variable every time you want to use it, and this is good practice as it still works if the variable is set to a value that contains spaces.

Note that we also had to quote setting the variable name in the first place, because

program=silly name

would translate as: set the variable program to the value silly, then execute the program name. Variable assignments only apply to the first argument following them, although you can assign more than one variable.

Note that this does not work as expected either:

file=arguments gcc -Wall "$file.c" -o "$file"

The problem here is that the shell first reads the line and substitutes in the value of $file (unset variables expand to the empty string by default) before starting to execute the command, so you are reading the variable's value before writing it. Leaving off the quotes doesn't help: you need to set the variable on a separate line.