Read the learning objectives listed on this page: Bear in mind what you’re trying to achieve while reading this text. If a topic isn’t making much sense, and isn’t in the objectives, you can probably skip over it. If a topic is listed in the objectives, you should keep studying it until you are confident you’ve met the objective.

Check you have achieved each learning objective listed on this page.

Read the learning objectives listed on this page: Bear in mind what you’re trying to achieve while reading this text. If a topic isn’t making much sense, and isn’t in the objectives, you can probably skip over it. If a topic is listed in the objectives, you should keep studying it until you are confident you’ve met the objective.

Read chapter 7 of How Computers Really Work.

Do every exercise listed in the chapters.

Check you have achieved each learning objective listed on this page.

Through this course, we are going to write lots of programs.

But a lot of programs have already been written by other people. It’s often quicker and easier to use an existing program than to write a new one.

We are going to learn about some commonly used programs. We will learn some common patterns that make them easier to learn them and to combine them.

We run these programs from a terminal. We can either call the programs directly from a terminal, or put instructions to call the programs in a script and run the script.

Conventions

To describe running a program, we will use this syntax:

% ls
file1   file2

Here we are saying: At a terminal prompt (signified by the %), run the command ls.
Below the command is the output (file1 file2) you should expect to see after you run the command. Sometimes this is the exact output you should expect. Sometimes example outputs are given: these could vary according to your filesystem, username, or other variables.

To call a program, we type its name and press enter. Open a terminal and run:

% ls
file1   file2

Most of the programs which take a file as an input take it as an argument to the command.
We list arguments after the command name, separated with a space, e.g. we can pass /tmp as an argument to ls:

% ls /tmp
some-temporary-file some-other-temporary file

Flags and arguments

Some programs take extra arguments. These may be optional or required. Extra arguments generally start with a - or -- then their name. If these extra arguments take a value, expect the value to come after the name.

% grep -r -B 1 "hello" .
./comparison/index.md-```js {linenos=table,linenostart=1}
./comparison/index.md:"hello Mhairi" === `hello ${mhairiName}`;
--
./errors/index.md-```js
./errors/index.md:const result = console.log("hello world");

Here we passed the optional argument -r and the optional argument -B. -r takes no value. We gave -B the value 1.

We call arguments that start with a - or -- flags. We could give flags in any order - grep -B 1 -r "hello" . works the same as grep -r -B 1 "hello" ..

Flags that start with a - not a -- can also be squashed together. grep -rc "Hello" . works the same as grep -r -c "Hello" .. If you are looking up a flag you don’t know, like -rc, remember it may actaully be two flags (-r and -c). You can also specify a value for the last flag in a squashed together list: grep -rcB 1 "hello" ..

"hello" and . are called positional arguments because they don’t have a named flag before them. The program decides how to interpret them based on their position (order) in the command line, rather than a named flag coming before them. Their order matters.

cat is a tool for getting the contents of files.

If you have a file at the path /tmp/about-me.txt, you can output its contents to the terminal by running:

% cat /tmp/about-me.txt
My name is Serina.

I live in Glasgow, and I like the theatre.

man pages are manuals that help us understand tools. Many (but not all) tools on your computer have a man page.

Key take-aways:

• Open a man page for a tool by running man tool-name (e.g. man cat).
• Quit a man page by pressing q (for quit).
• Scroll around a man page by using the up and down arrows.
• The synopsis section has a sample command line. Optional arguments are in []s. Arguments you can repeat more than once have a ... after them. All of the possible single-letter flags are smooshed together into one.
• The description section lists all of the possible flags and arguments in detail.
• Man pages tend to contain lots of information. You probably don’t want to read them top-to-bottom. Read the introduction, but then be searching with specific questions in mind like “How do I reverse the output” or “What does the -r flag do?”.

(Source, including text-only transcript: https://wizardzines.com/comics/cat/)

The comic and the man page contain some of the same information.

The man page contains a lot more information too. The man page lists everything cat can do. The comic has targeted information for specific use-cases.

Both can be useful.

Check you have achieved each learning objective listed on this page.

ls is a tool for listing the files in a directory.

It is named ls because it is used to list files.

Learn about ls from its man page (and the backlog exercises).

Some commonly used flags you should understand:

• -R
• -l
• -t and -r
• -a
• -1
• -h

wc is a tool for counting the bytes, words, or lines in a file.

It is named wc because it is used to find a word count.

Learn about wc from its man page (and the backlog exercises).

Some commonly used flags you should understand:

• -c
• -l
• -w

grep is a tool for searching files for text.

(Source, including text-only transcript: https://wizardzines.com/comics/grep/)

grep is designed to search using regular expressions, which are patterns that describe text.

For instance, the regular expression ^[0-9]*$ searches for a line which has only numbers in it. The ^ means “from the start of the line”, the $ means “until the end of the line”, the [0-9] expresses the characters we’re looking for, and the * means “0 or more of the characters”. ^[0-9] searches for a line which starts with a number (but doesn’t care about what comes after).

You can learn more about Regular Expressions at RegexOne. Many engineers just know the basic concepts, and Google for help when they need something more complicated, and this is fine.

Learn about grep from its man page (and the backlog exercises).

Some commonly used flags you should understand:

• -c
• -F
• -r
• -v
• -l
• -o
• -A, -B, -C

sed is a tool for replacing text in files with other text.

(Source, including text-only transcript: https://wizardzines.com/comics/sed/)

sed executes a list of commands on a file. The most common commands are to replace some text with some other text, or to show or hide or delete lines that meet some criteria.

Sed commands feature both a matcher (called an “address”) specifying which lines to operate on, and an operation (called a command) to perform on those lines.

If you don’t specify an address, your command applies to the whole file:

Learn about sed from its man page (and the backlog exercises).

Some sed commands you should understand and be able to write:

• s/cat/dog/
• s/cat/dog/g
• s/\([0-9]\)/only \1/g
• /dog/d
• 6d

awk is a tiny programming language for manipulating columns of data.

(Source, including text-only transcript: https://wizardzines.com/comics/awk/)

Learn about awk from its man page (and the backlog exercises).

Some awk programs you should understand and be able to write:

• {print $2}
• /Ibrahim/ {print $NF}
• {sum += $2} END {print sum}