axelhodler

Basics and Beyond

Being around for a while each of us might have seen the following lines

if [ -z $1 ]; then
  echo "please provide the input parameter foobar"
  exit 1
fi

# do some work

In the wild it can be found at the beginning of a bash script.

Apparently the script requires an input and fails if the length of the input is zero. If the length is zero it’s not present. Thus the -z.

The author thought without the input it would make no sense to continue with the execution of script. As a result we exit with a non-zero status (exit 1).

After a quick search for “bash check if input is provided” we could grab the above straight from Stackoverflow. No further consideration. It’s fine ;)

Hold on! Let’s have fun with the shell.

Regard > as the prompt and the line which follows afterwards as the output.

> [ -z "not empty"
[: ']' expected
> [ -z "" ] && echo "is empty"
is empty

Hah. Seems like [ is a command. No if required.

Check out the manual

> man [

Where we find the docs of the -z argument stating

-z string True if the length of string is zero.

It also states [ is the utility test. In fact we’re able to interchange [ with test

> test -z "" && echo "is empty"
is empty

We can even build an alternative to test. Formulate the most important requirements into tests. Voila. A home grown testframework which consists of a single function

failed() {
  echo "failed"
  exit 1;
}

./assert-empty "hi" && failed
./assert-empty "" || failed
./assert-empty || failed

So && failed leads to a failure if the command before exits successfully. Which it should not.

And || failed triggers if the command before exits unsuccessfully. Which again, it should not.

Our quick and dirty implementation in C follows. If we have no input or if the input has a length of zero we exit successfully.

#include <string.h>

int main(int argc, char *argv[]) {
  if (argc == 1 || strlen(argv[1]) == 0) {
    return 0;
  } else {
    return 1;
  }
}

We compile the above

gcc -o assert-empty assert-empty.c

And run the tests

sh ./test-assert-empty

Success!

Copy the compiler output to /usr/local/bin to reuse it

cp assert-empty /usr/local/bin/

The script above could now start with

if assert-empty $1; then
  echo "please provide the input parameter foobar"
  exit 1
fi

We should not use it in scripts though. Their portability would be destroyed. Only our system will have the assert-empty utility.

Having a look at the original source of test in C we find it offers a lot more than our assert-empty. It had time to mature. The commit Initial revision of the file dates back to November 1992.

Spelunking around the tests folder of the repository we find how test is used to test other coreutils, such as rm.

Check out some cool parts in one of the files used to test rm

mkdir -p b/a/p
#...
rm -rf b
#...
test -d b/a/p || fail=1

We recognize the classic Arrange-Act-Assert.

Thus what seemed like an innocent bracket [ helps to make sure the foundations we build upon run smoothly.