The shell's building blocks

Originally posted on my blog.

As we have seen in the previous chapters, the shell is a program allowing you to run other programs. It is an invaluable tool in the life of a software engineer, as it provides you with a simple text-based interface to control your computer and any program you might install or write.

Something I still find striking after years of using a shell almost daily is how simple yet powerful its building blocks are.

Chapter 1 covered commands, I/O streams and pipes. This chapter will cover environment variables, aliases and functions.

Environment variables

Environment variables are key/value pairs that affect how running programs behave. Another way to say that would be that environment variables can allow you to tweak and personalize how certain programs, amongst which your shell, work. They can also define what programs will be called to perform a certain task.

Here are a few examples:

• SHELL defines what shell your terminal runs ('/bin/bash', /bin/zsh, /bin/fish, etc)
• HOME defines where your home directory is located
• EDITOR defines what text editor program should be used to edit text within your terminal (eg nano, vim, emacs, etc)

Displaying an environment variable's value

To display the value of given environment variable, you can use the echo command, followed by a dollar sign and the name of the variable:

$ echo $SHELL
/bin/zsh

You can use the printenv command to list all environment variables along with their value.

$ printenv
USER=br
HOME=/home/br
LC_TERMINAL=terminator
SHELL=/bin/zsh
EDITOR=vim
PWD=/home/br/
PAGER=less

For the sake of brevity, I've only displayed a subset of the environment variables defined on my computer. These variables tell the following story:

• my username is br
• all my personal data is stored in my home directory, located at /home/br
• my default terminal is called terminator
• and whenever I open terminator, it runs the commands via the zsh shell
• my default text editor is vim
• I am currently located in my home directory
• my default pager program is less

Changing an environment variable

What is interesting about these environment variables is that they can be changed, and with them, the behavior of other programs.

For example, let's change the value of our HOME environment variable, defining where our home directory is.

$ HOME=/tmp
$ cd
$ pwd
/tmp

In the first line, I redefined the value of my HOME environment variable from /home/br to /tmp. Remember when you learned that running cd without arguments would take you back to your home directory? Well, it's actually using the HOME environment variable to figure out where your home directory is. Now that HOME has changed, so has cd's behavior.

Another example is PAGER. We saw that my environment had PAGER=less defined by default, which explains why you find yourself reading text within less when you open a man page. man fetches the actual documentation and displays it in a pager, which itself is specified by the PAGER environment variable. If you were to change that variable to something else, like more or bat ¹, it would then change man's behavior.

Note: There is a difference between SHELL and $SHELL. The first one is the name of an environment variable, and the latter represents its value. Consequently, when we executed echo $SHELL, we told our shell to lookup what value was associated with the SHELL environment variable, and then display it to the screen via the echo command. $ is what we call a dereference operator in that context.

Defining new variables

Not only can you change an existing environment variable, but you can also define a new one. If a non-existing variable is echo-ed, it will simply be replaced by an empty string.

$ echo $NEW_VAR

$ NEW_VAR=my-new-env-var
$ echo $NEW_VAR
my-new-env-var

If you define an environment variable this way, it will only be visible by the shell itself, but not by any command executed by your shell (also called subprocesses). To make an environment variable visible by a subprocess, you need to define it after the export keyword.

To illustrate that, we will create our first shell script: a program executing shell commands one after the others.

$ cat <<EOF > echo_var.sh
echo $NEW_VAR
EOF
$ cat echo_var.sh
echo $NEW_VAR

As you can see, the echo_var.sh script only contains one shell command: echo $NEW_VAR.

To execute that bash script, we can run bash echo_var.sh, and all instructions within that script will be executed by bash. Let's have a look at what executing that script displays on the screen with and without export-ing that variable.

$ NEW_VAR=my-new-var
$ echo $NEW_VAR
my-new-var
$ bash echo_var.sh

$ export NEW_VAR=my-new-var
$ echo $NEW_VAR
my-new-var
$ bash echo_var.sh
my-new-var

As you can see, the echo_var.sh subprocess can see the NEW_VAR environment variable after it has been export-ed by its parent shell.

This can very useful if you write programs: some parameters can have a sane default value but can also be overridden by specifying an environment variable. grep does this for example: reading the grep man page, we see:

GREP_OPTIONS May be used to specify default options that will be placed at the beginning of the argument list.

Removing environment variables

You can remove an environment variable by using the unset keyword:

$ unset NEW_VAR
$ bash echo_var.sh

$ echo $NEW_VAR

$

The case of PATH

Until that point, we've executed commands in the shell, and things happened. It was a simple world and it was nice. You might wonder what would happen if I gave the shell a non-existent command though?. Well, I'm glad you asked. Ten points for Gryffindor.

$ cmdnotfound
zsh: command not found: cmdnotfound

The cmdnotfound command, like its name implies, is not found. But what makes a command be found then? What makes the shell happily comply when we type ls, and makes it complain when we type cmdnotfound? It turns out that this is due to an environment variable called PATH, listing all directories in which executable programs can be found.

$ echo $PATH
/home/br/bin:/home/br/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

This means that for any command passed to the shell, it will look into these directories (separated by a colon) in search for the program I'm trying to run.

For example, if I type

$ ls

into my shell, it will look into /home/br/bin, /home/br/.local/bin, /usr/local/sbin, etc, until it finds it in /bin.

$ ls /bin
...
chmod      dd         ed        ls         ps         sh         tcsh       zsh

If the command is not found in any of the directories listed in PATH, then it is not found.

This means that you can also redefine PATH to force your shell to look into new directories. In fact, this is exactly what I've done to make it look into /home/br/bin, where I store tools of my making.

::: Warning
You can mess with up your shell by running unset PATH.

$ unset PATH
$ ls
zsh: command not found: ls

However, calling a command by using its absolute or relative path still works, as PATH is only used to look for commands that only have been invoked by name.

$ unset PATH
$ /bin/ls
bin    code    Documents  ..
...

:::

There is a useful command you can use to know what a program is and where it is found: which.

$ which less
/usr/bin/less
$ which bash
/usr/local/bin/bash
$ which ls
ls: aliased to ls -G

Wait. What? What's an alias?

Aliases

An alias allows you to define custom commands. In the previous example, running ls would actually run ls -G, which enables colorized output.

You can define an alias by using the alias keyword.

$ alias ls='ls -G'

There are a couple of reasons you might want to define aliases:

• redefining a command's behavior (ex: always using ls with the -G option)
• shortening a command's name to make it quicker to type (ex: alias ..='cd ..')
• creating new commands altogether (ex: alias filesize='ls --size --human-readable -1')

To see the underlying command that will be executed by an alias, you can type alias <name>.

$ alias filesize='ls --size --human-readable -1'
$ alias filesize
alias filesize='ls --size --human-readable -1'

Aliases are very simple yet powerful. They allow you to customize your shell to your liking, create new commands without having to remember a lot of options, and decrease the time you spend typing, all of which should make you feel more productive.

:::Note
Aliases can be "nested". If you define ls as an alias of ls -G and filesize as an alias of ls --size --human-readable -1, your shell will unwrap both aliases and execute ls -G --size --human-readable -1 when you type filesize.
:::

When we're executing filesize bin, the shell will see that filesize is an alias for ls --size --human-readable -1 and will actually execute the command ls --size --human-readable -1 bin behind the scenes. This simply is done by replacing the alias by its definition in the command itself. Aliases can however fall short if we want to do something a more complex than this.

For example, one of my favorite productivity tools is mkcd, which creates a directory and steps into it right after. It saves you from typing

$ mkdir new-dir
$ cd new-dir

where you can just type

$ mkcd new-dir

An alias can't really help here, because we are talking about aliasing two commands with a single alias, which does not work. Enter functions.

Functions

According to the bash man page:

A shell function is an object that is called like a simple command and executes a compound command with a new set of positional parameters.

Let's see what that looks like in practice. A function is declared this way.

function name {
    # ...
}

If your function is expecting arguments, these can be accessed by using $n where n is a number. For example, $1 is the first function argument, $2 its second argument, etc. With that in mind, we can now declare our mkcd function.

function mkcd {
    mkdir -p $1
    cd $1
}

Let's now see mkcd in action!

$ function mkcd {
    local target=$1
    mkdir -p $target
    cd $target
}
$ pwd
/home/br
$ mkcd test
$ pwd
/home/br/test

You can use the typeset -f command to see how a function was defined (or which <function-name>, although that only works in zsh).

$ typeset -f mkcd
mkcd () {
    mkdir -p $1
    cd $1
}

Real life examples

These are some of the environment variables, aliases and functions I have defined for myself.

Shorter navigation aliases

alias ..='cd ..'
alias ...='cd ../..'

Colorize commands output

alias ls='ls --color=auto'
alias grep='grep --color=auto'
alias ip='ip --color'

Alias commands I never remember

# https://xkcd.com/1168/
alias untar='tar -zxvf'

Have $HOME/bin be part of PATH

export PATH=$PATH:$HOME/bin

By extending my PATH this way, I can then put every single tool I create into $HOME/bin and have it be usable right-away.

A backup function

function bak {
    cp -r $1 $1.bak
}

This function can be used to backup a file or directory. I regularly use this when I'm about to edit a critical file and I want to make sure I can revert my changes if needed.

Password generation function

This function generate a password composed of alphanumeric characters, of default length 32.

$ function genpass {
    local passlen=${1:-32}
    # Note: LC_ALL=C is needed for macos compatibility
    LC_ALL=C tr -cd '[:alnum:]' < /dev/urandom | fold -w $passlen | head -n1
}
$ genpass
GQROc0tnABqfYH0qpMMwSPYFgcY7OANB
$ genpass 50
WkeQ14E8FIQZN7XlN7yPkYK4yhMOvpAuNzZivKwODNkskh0uq0

The weather in your terminal

function weather {
    curl "wttr.in/${1:-lyon}?m"
}

This function uses curl to send an HTTP request to the http://wttr.in website, that displays weather forecasts in a terminal-friendly way. So I can just type weather mycity and voila:

$ weather lyon
Weather report: lyon

     \   /     Sunny
      .-.      17 °C
   ― (   ) ―   ↖ 6 km/h
      `-’      10 km
     /   \     0.0 mm
                                                       ┌─────────────┐
┌──────────────────────────────┬───────────────────────┤  Sat 04 Apr ├───────────────────────┬──────────────────────────────┐
│            Morning           │             Noon      └──────┬──────┘     Evening           │             Night            │
├──────────────────────────────┼──────────────────────────────┼──────────────────────────────┼──────────────────────────────┤
│    \  /       Partly cloudy  │    \  /       Partly cloudy  │    \  /       Partly cloudy  │    \  /       Partly cloudy  │
│  _ /"".-.     7..8 °C        │  _ /"".-.     13 °C          │  _ /"".-.     13 °C          │  _ /"".-.     10..11 °C      │
│    \_(   ).   ← 5-6 km/h     │    \_(   ).   ↙ 5 km/h       │    \_(   ).   ← 5-10 km/h    │    \_(   ).   ↖ 8-17 km/h    │
│    /(___(__)  10 km          │    /(___(__)  10 km          │    /(___(__)  10 km          │    /(___(__)  10 km          │
│               0.0 mm | 0%    │               0.0 mm | 0%    │               0.0 mm | 0%    │               0.0 mm | 0%    │
└──────────────────────────────┴──────────────────────────────┴──────────────────────────────┴──────────────────────────────┘
                                                       ┌─────────────┐
┌──────────────────────────────┬───────────────────────┤  Sun 05 Apr ├───────────────────────┬──────────────────────────────┐
│            Morning           │             Noon      └──────┬──────┘     Evening           │             Night            │
├──────────────────────────────┼──────────────────────────────┼──────────────────────────────┼──────────────────────────────┤
│     \   /     Sunny          │     \   /     Sunny          │     \   /     Sunny          │    \  /       Partly cloudy  │
│      .-.      10..12 °C      │      .-.      16 °C          │      .-.      14..15 °C      │  _ /"".-.     10..12 °C      │
│   ― (   ) ―   ↖ 14-18 km/h   │   ― (   ) ―   ↑ 23-27 km/h   │   ― (   ) ―   ↑ 15-25 km/h   │    \_(   ).   ↑ 13-26 km/h   │
│      `-’      10 km          │      `-’      10 km          │      `-’      10 km          │    /(___(__)  10 km          │
│     /   \     0.0 mm | 0%    │     /   \     0.0 mm | 0%    │     /   \     0.0 mm | 0%    │               0.0 mm | 0%    │
└──────────────────────────────┴──────────────────────────────┴──────────────────────────────┴──────────────────────────────┘
                                                       ┌─────────────┐
┌──────────────────────────────┬───────────────────────┤  Mon 06 Apr ├───────────────────────┬──────────────────────────────┐
│            Morning           │             Noon      └──────┬──────┘     Evening           │             Night            │
├──────────────────────────────┼──────────────────────────────┼──────────────────────────────┼──────────────────────────────┤
│     \   /     Sunny          │     \   /     Sunny          │     \   /     Sunny          │     \   /     Clear          │
│      .-.      12..13 °C      │      .-.      16 °C          │      .-.      14..15 °C      │      .-.      11 °C          │
│   ― (   ) ―   ↖ 18-22 km/h   │   ― (   ) ―   ↑ 22-28 km/h   │   ― (   ) ―   ↑ 14-24 km/h   │   ― (   ) ―   ↑ 8-16 km/h    │
│      `-’      10 km          │      `-’      10 km          │      `-’      10 km          │      `-’      10 km          │
│     /   \     0.0 mm | 0%    │     /   \     0.0 mm | 0%    │     /   \     0.0 mm | 0%    │     /   \     0.0 mm | 0%    │
└──────────────────────────────┴──────────────────────────────┴──────────────────────────────┴──────────────────────────────┘
Location: Lyon, Métropole de Lyon, Circonscription départementale du Rhône, Auvergne-Rhône-Alpes, France [45.7578137,4.8320114]

Summary

Environment variables, aliases and functions are simple yet powerful to change the shell's behavior into something that feels more intuitive. You feel like nano is not shiny enough and prefer using vim instead? Sure. Define EDITOR=vim. Any command interacting with an editor would then use vim instead of nano.

Aliases are a great way to reduce mental friction in the shell by hiding away complex commands, or just reducing the amount of typing you have to do. When aliases start being not powerful enough because you want to execute multiple commands, you can then have a look at functions instead.

Everything we have seen so far however had an ephemeral effect, as changes you made would disappear when you close your shell session. In the next chapter, we will go dive into how to persistently configure your shell to improve your day-to-day experience and productivity.

Going further

3.1: Write a cat alias that displays meow on screen.

3.2: Write a restorebak function that takes a filename as only argument and renames $1.bak into $1.

3.3: Unset the PATH environment variable and then export it back so that you can use ls again.

---

1. https://github.com/sharkdp/bat ↩