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James Fenn 485fc8104b introduce the read command, explain exit codes & test constructs

2022-08-20 22:17:58 -07:00

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Intro

Since the days of the first digital computers, interacting with them has been a rather fundamental aspect. Even though we've advanced far past the humble days of thousands of switches, massive control panels, and hundreds of punchcards into an era of smooth, sleek graphical user interfaces (GUIs), the venerable command line interface (CLI) still remains quite useful.

On Unix and Unix-like OSs, such as Linux and macOS, a program known as a shell is responsible for providing the CLI. The most common one today is bash, and it can be found on the majority of Unix-like systems. It supports a shell scripting language dating back to the Thompson shell from 1971, although much of the scripting functionality available in modern shells comes from the Bourne shell.

Compared to many other scripting languages, shell scripting is a bit odd because all of the keywords found in it (such as for, if, while, etc) are actually commands built into the shell, and not special keywords being interpreted separately from the other commands in the script. This means that it's possible to use these commands anywhere, even outside of a script. For years, I didn't even know that common commands like echo were actually being run directly in the shell and not with a separate program!

Before we start, I'd like to say that this post is geared more towards people who already have some prior programming experience, and some existing Linux/Unix command-line experience helps as well.

Setup

In order to follow this post, you'll want to make sure that you have access to a Linux shell of some sort. Thankfully, this is easier than ever to find nowadays. Everything should work the same whether it's running in a Linux VM, macOS, WSL, Termux, or Crostini. All of these include bash by default, but to make sure, run the command which bash. This will print out the location of bash on your system, and as long as the message gives some sort of location, such as /bin/bash, you should be fine.

Although not strictly necessary, you'll probably want to make sure you have a text editor with support for shell script syntax highlighting installed. nano or micro are good options on the command-line, especially if you're new to CLI text editors. With that out of the way, let's get to the actual shell scripting part of this post!

Basic Scripting

A Simple Example

At its core, a shell script is simply a sequence of commands, one per line - if you've used a bash command line before, you already know a lot of the syntax it uses. Each line of the file is interpreted as a single command, and bash will run each line sequentially until it reaches the end of the file, or it gets to an exit command. If you're ever confused about how part of a script works, you can try running it individually outside of the script to see how it behaves.

Let's start out with a simple example: create a file somewhere on your computer called simple.sh and start editing it. In most text editors, you can combine both steps into one by running {editor} simple.sh - nano simple.sh, for example. All you need to put into the file is the text pwd. This is a standard Linux/Unix command that prints out the directory you're currently in. If you save and exit your editor and then run bash simple.sh, it should print out your current directory. In my case, it looks like this:

$ bash pwd.sh
/home/adueppen/scripts

Congratulations! You've just written your first shell script! Sure, it just does the same thing that running pwd in the terminal normally would do, but the real power of shell scripting comes in when using these standard commands along with the scripting features in order to add things like interactivity, loops, and conditions. Now, let's move on to some of those more advanced features.

Conditions With the `if` Command

Conditional execution is one of the most important parts of any programming language. If you couldn't choose whether or not to execute something, things would be ...difficult, to say the least. Thankfully, bash includes if as a built-in command. Note that this is technically different from a typical programming language's if statement, and that's why I've been referring to it as the "if command". In practice, however, it functions basically the same way.

A common way that conditions are used in shell scripts is to check if a particular program is installed before running it. Here's a simple example of it in use:

if command -v cowsay > /dev/null; then
  cowsay "Hello, world!"
fi

Here, an if construct is used with the built-in command command in order to check if the cowsay command exists. It's generally not installed by default (even if it perhaps should be), so we can't assume that it's going to be there. The > /dev/null part is used to throw away the output of command since we don't actually need to know where cowsay is, just that it exists. If it's there, we run cowsay with the argument "Hello, world!", which prints a funky looking cow in the shell.

Although the use of redirection (>) might seem a bit confusing now, I'll get into that later. Right now, if you run this script, it'll either print out ASCII art of a cow saying "Hello, world!", or ...nothing. That's not exactly ideal, so let's add an alternative case for when cowsay isn't available:

if command -v cowsay > /dev/null; then
  cowsay "Hello, world!"
else
  echo "Hello, world! (PS: install cowsay for more fun!)"
fi

Now our script prints out "Hello, world!" even if cowsay isn't installed. If you don't have cowsay on your computer, try installing it to see the output change!

Variables and User Input

Let's make this script a little more interactive - what about asking the user what they want the cow to say? One way to do that is to use the read command. Here's an excerpt from the command's help text, which can be accessed by running read --help in your command line.

read: read [-ers] [-a array] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]

Reads a single line from the standard input, or from file descriptor FD if the -u option is supplied. The line is split into fields as with word splitting, and the first word is assigned to the first NAME, the second word to the second NAME, and so on, with any leftover words assigned to the last NAME. Only the characters found in $IFS are recognized as word delimiters.

-p prompt - output the string PROMPT without a trailing newline before attempting to read

A lot of this isn't useful to us, but we can extract a bit of helpful information:

The (simplified) syntax of the command we want to run is read -p [prompt] [name]
read will allow the user to enter a single line, and store its result in whatever variable we specify as [name]
By passing -p [prompt], we tell the command to output the prompt text before accepting any input.

Now, let's try putting this together - if we want to ask the user a question, we could write something like this:

read -p "What should the cow say? " input

Running this will prompt the user for a line of text, and store its result in a variable named input - but how do we use that variable in our code?

The syntax for accessing variables in bash is to simply write their name starting with a dollar sign ($). With this, variables can be embedded in nearly any bash command or argument - they will simply be replaced with the content of the variable when the command is run. With this in mind, let's rewrite our example with the if command from before...

read -p "What should the cow say? " input

if command -v cowsay > /dev/null; then
  cowsay "$input"
else
  echo "The cow says $input (PS: install cowsay for more fun!)"
fi

Try running this! Your script should now ask you to enter a line of text, which will then be passed to the cowsay command, generating a fun ASCII cow that says whatever you want.

Conditions With the `if` Command - Continued

Now that we've added some user input into our program, we have the ability to get our cow to say some pretty crazy things. I've gotten mine to say "woof," for example - a noise that no real cow should ever be making. Perhaps we should add an extra case in this script to prevent our cow from making such a terrifying sound? In order to accomplish this, we need to learn about test constructs, and how the if command really works.

When any shell command is executed, it implicitly sets a variable named $?, called its "exit code". This is always an integer, and can be seen as a sort of "return value" in bash. Normally, most exit codes should be 0 - by Unix convention, this indicates that the command was a success. Some commands do fail, though - and their exit code can be used to communicate that information to the program that started it.

As an example of this, the command command has the potential to fail if the argument it is given doesn't exist. Running command thiscommanddoesntexist followed by echo $? should print the number 127 in your shell - while running command echo followed by echo $? will print 0. This is how the if statement evaluates its condition - if it returns an exit code of zero, it is interpreted as a success.

With this in mind, we're going to introduce "test constructs" - a shell syntax that evaluates its arguments as a comparison, and returns an exit status indicative of the result (0 for true, 1 for false). Similarly to how if is secretly a command of its own, test constructs are also a command... which is named... "left square bracket" ([). No, I'm not joking.

Here, we want to compare two variables for equality - we want to check whether our $input variable is equal to "woof". The "left square bracket" equivalent of this should then be [ "$input" == "woof" ]. If the comparison is a success (meaning that $input is "woof"), the exit status will be 0, which should be interpreted by an if command as "true." Inside our if statement, we can echo a message to the user, then use the exit 1 command to terminate the script and indicate a failure - if another script wanted to determine the result of ours, it could use this exit code to do so.

read -p "What should the cow say? " input

if [ "$input" == "woof" ]; then
  echo "Your cow sounds like it has a cold! Take it to the vet."
  exit 1
fi

if command -v cowsay > /dev/null; then
  cowsay "$input"
else
  echo "The cow says $input (PS: install cowsay for more fun!)"
fi

One important thing to note about these test constructs: the space between the square brackets and the condition is mandatory. The [ command is interpreted as, well, a command - leaving out that space between [ and "$input" would tell bash to look for a program named ["$input" instead; the same thing would happen if you were to write exit1 instead of exit 1.

This is certainly better, but what if you don't always want to use cowsay to print it out? Two more programs available for printing out text in fun ways are figlet and toilet, so let's make it so that the script will randomly use either cowsay, figlet, or toilet. This time we'll want to use a variable to store the random number we generate. Conveniently, variables in shell scripting are very easy to use. Declaring them doesn't require any special syntax at all, just a value of some sort for the variable. However, when referring to the variable, it has to be prefixed with a dollar sign in order to indicate that it's not a command to be run.

rand=$((RANDOM%3))

if [[ $rand == 0 ]] ; then
  if command -v cowsay > /dev/null; then
    echo "Hello, world!" | cowsay
  else
    echo "cowsay is not installed :("
  fi
elif [[ $rand == 1 ]] ; then
  if command -v figlet > /dev/null; then
    echo "Hello, world!" | figlet
  else
    echo "figlet is not installed :("
  fi
else
  if command -v toilet > /dev/null; then
    echo "Hello, world!" | toilet -t
  else
    echo "toilet is not installed :("
  fi
fi

This time the script uses $RANDOM, which is a "variable" that bash includes as a built-in. Although it looks like a variable, it's actually a function instead (more on that later). It generates a random integer from 0-32767, which isn't quite what we're looking for. Thankfully, we can use the modulo operator (%) to force the value down to either 0, 1, or 2. You've probably noticed by now a few new things in the if statements. In this case, we use double brackets to indicate that we're performing a test, which in this case is the equality of 2 strings of text. We're also now using the elif command (short for else-if) so that we can check for more than just a single condition. Finally, the -t option for toilet is just a way to make sure the text won't wrap too early if your terminal window is wide. Anyway, it's nice that our script has a number of possibilities now, but wouldn't it be nice if it was possible for us to choose the way to print the text ourselves?

Interactivity and input

We can use the built-in command read in order to prompt the user to type something in. This gives us another chance to use variables, this time to store both the user's input as well as the text "Hello, world!" so we don't have to keep writing it out manually. Conveniently, the read command has a handy second argument we can use to store the response in a variable.

echo "1: Print with cowsay"
echo "2: Print with figlet"
echo "3: Print with toilet"
echo "4: Print with echo"
echo "Anything else: Exit"

read -p "Enter a number: " input
hello="Hello, world!"

if [[ $input == "1" ]] ; then
  if command -v cowsay > /dev/null; then
    echo $hello | cowsay
  else
    echo "cowsay is not installed :("
  fi
elif [[ $input == "2" ]] ; then
  if command -v figlet > /dev/null; then
    echo $hello | figlet
  else
    echo "figlet is not installed :("
  fi
elif [[ $input == "3" ]] ; then
  if command -v toilet > /dev/null; then
    echo $hello | toilet -t
  else
    echo "toilet is not installed :("
  fi
elif [[ $input == "4" ]] ; then
  echo $hello
else
  exit
fi

This time, we start with telling the user which options are available to them (always a good idea), and then ask them to enter something. We store this input in a variable called input. After that, we check what the user actually entered. After that we once again check to see whether cowsay, figlet, or toilet is installed, depending on what the user chose, and tell them if they don't have it. We don't need to check if echo is installed since it's a built-in bash command. Lastly, we have an exit command in the else block to make sure the script exits if the user puts in anything beyond the 3 options. In this case it's actually not necessary since the script would already finish and exit automatically, although it's a good idea to include it in case you decide to add more functionality later.

Positional Arguments

(find a way to integrate this with the script?)

Variable	Type	Description
`$0`, `$1`, ...	String	Argument at a specific position.
`$#`	Int	The total number of arguments.
`$@`	Array	All of the arguments passed.
`$*`	String	All arguments passed, as a single string.

Every bash command returns an exit code which can be used to determine a measure of success or state of a script. In this example, we check the exit code of command -v toilet to determine whether toilet is installed. Generally, an exit code of zero implies success, and anything else represents some form of error. There are a few standards for using different error codes for specific purposes, but in this situation we only need to know if it equals zero.

There are a few different ways to use these exit codes in a script. After a command is executed, the $? variable is set to its exit code, which can be used to reference it in later comparisons. Another interesting use is in fail-fast programming; beginning a script with set -e will tell bash to exit the script immediately if any command returns a non-zero exit code.

Functions

In order to write more abstract functionality that can be reused in a script, portions of functionality can be separated into a function. Functions within a script are given the same scope as the rest of the script, but are created with their own positional arguments and can return their own exit code.

function runpipe() {
  if command -v $2 > /dev/null; then
    echo $1 | ${@:2}
  else
    echo "$2 is not installed :("
	return 1
  fi
}

This recreates the functionality in the if statements of the previous script. We can now simplify it as follows:

echo "1: Print with cowsay"
echo "2: Print with figlet"
echo "3: Print with toilet"
echo "4: Print with echo"
echo "Anything else: Exit"

read -p "Enter a number: " input
hello="Hello, world!"

function runpipe() {
  if command -v $2 > /dev/null; then
    echo $1 | ${@:2}
  else
    echo "$2 is not installed :("
  fi
}

if [[ $input == "1" ]] ; then
  runpipe $hello cowsay
elif [[ $input == "2" ]] ; then
  runpipe $hello figlet
elif [[ $input == "3" ]] ; then
  runpipe $hello toilet -t
elif [[ $input == "4" ]] ; then
  echo $hello
else
  exit
fi

Variables and Scope

reference $hello from runpipe()
local VARIABLE=5
export VARIABLE=5

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