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<!DOCTYPE html>
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<title>The Unix Shell: All in One View</title>
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The Unix Shell
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The Unix Shell
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The Unix Shell
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<section id="aio-01-intro"><p>Content from <a href="01-intro.html">Introducing the Shell</a></p>
<hr>
<p>Last updated on 2023-06-09 |
<a href="https://github.com/swcarpentry/shell-novice/edit/main/episodes/01-intro.md" class="external-link">Edit this page <i aria-hidden="true" data-feather="edit"></i></a></p>
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<button role="button" aria-pressed="false" tabindex="0" id="expand-code" class="pull-right" data-expand="Expand All Solutions " data-collapse="Collapse All Solutions "> Expand All Solutions <i aria-hidden="true" data-feather="plus"></i></button>
</div>
<div class="overview card">
<h2 class="card-header">Overview</h2>
<div class="row g-0">
<div class="col-md-4">
<div class="card-body">
<div class="inner">
<h3 class="card-title">Questions</h3>
<ul>
<li>What is a command shell and why would I use one?</li>
</ul>
</div>
</div>
</div>
<div class="col-md-8">
<div class="card-body">
<div class="inner bordered">
<h3 class="card-title">Objectives</h3>
<ul>
<li>Explain how the shell relates to the keyboard, the screen, the
operating system, and users’ programs.</li>
<li>Explain when and why command-line interfaces should be used instead
of graphical interfaces.</li>
</ul>
</div>
</div>
</div>
</div>
</div>
<div class="section level3">
<h3 id="background">Background<a class="anchor" aria-label="anchor" href="#background"></a>
</h3>
<p>Humans and computers commonly interact in many different ways, such
as through a keyboard and mouse, touch screen interfaces, or using
speech recognition systems. The most widely used way to interact with
personal computers is called a <strong>graphical user interface</strong>
(GUI). With a GUI, we give instructions by clicking a mouse and using
menu-driven interactions.</p>
<p>While the visual aid of a GUI makes it intuitive to learn, this way
of delivering instructions to a computer scales very poorly. Imagine the
following task: for a literature search, you have to copy the third line
of one thousand text files in one thousand different directories and
paste it into a single file. Using a GUI, you would not only be clicking
at your desk for several hours, but you could potentially also commit an
error in the process of completing this repetitive task. This is where
we take advantage of the Unix shell. The Unix shell is both a
<strong>command-line interface</strong> (CLI) and a scripting language,
allowing such repetitive tasks to be done automatically and fast. With
the proper commands, the shell can repeat tasks with or without some
modification as many times as we want. Using the shell, the task in the
literature example can be accomplished in seconds.</p>
</div>
<div class="section level3">
<h3 id="the-shell">The Shell<a class="anchor" aria-label="anchor" href="#the-shell"></a>
</h3>
<p>The shell is a program where users can type commands. With the shell,
it’s possible to invoke complicated programs like climate modeling
software or simple commands that create an empty directory with only one
line of code. The most popular Unix shell is Bash (the Bourne Again
SHell — so-called because it’s derived from a shell written by Stephen
Bourne). Bash is the default shell on most modern implementations of
Unix and in most packages that provide Unix-like tools for Windows. Note
that ‘Git Bash’ is a piece of software that enables Windows users to use
a Bash like interface when interacting with Git.</p>
<p>Using the shell will take some effort and some time to learn. While a
GUI presents you with choices to select, CLI choices are not
automatically presented to you, so you must learn a few commands like
new vocabulary in a language you’re studying. However, unlike a spoken
language, a small number of “words” (i.e. commands) gets you a long way,
and we’ll cover those essential few today.</p>
<p>The grammar of a shell allows you to combine existing tools into
powerful pipelines and handle large volumes of data automatically.
Sequences of commands can be written into a <em>script</em>, improving
the reproducibility of workflows.</p>
<p>In addition, the command line is often the easiest way to interact
with remote machines and supercomputers. Familiarity with the shell is
near essential to run a variety of specialized tools and resources
including high-performance computing systems. As clusters and cloud
computing systems become more popular for scientific data crunching,
being able to interact with the shell is becoming a necessary skill. We
can build on the command-line skills covered here to tackle a wide range
of scientific questions and computational challenges.</p>
<p>Let’s get started.</p>
<p>When the shell is first opened, you are presented with a
<strong>prompt</strong>, indicating that the shell is waiting for
input.</p>
<div class="codewrapper sourceCode" id="cb1">
<h3 class="code-label">BASH<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode bash" tabindex="0"><code class="sourceCode bash"><span id="cb1-1"><a href="#cb1-1" tabindex="-1"></a><span class="ex">$</span></span></code></pre>
</div>
<p>The shell typically uses <code>$</code> as the prompt, but may use a
different symbol. In the examples for this lesson, we’ll show the prompt
as <code>$</code>. Most importantly, <em>do not type the prompt</em>
when typing commands. Only type the command that follows the prompt.
This rule applies both in these lessons and in lessons from other
sources. Also note that after you type a command, you have to press the
<kbd>Enter</kbd> key to execute it.</p>
<p>The prompt is followed by a <strong>text cursor</strong>, a character
that indicates the position where your typing will appear. The cursor is
usually a flashing or solid block, but it can also be an underscore or a
pipe. You may have seen it in a text editor program, for example.</p>
<p>Note that your prompt might look a little different. In particular,
most popular shell environments by default put your user name and the
host name before the <code>$</code>. Such a prompt might look like,
e.g.:</p>
<div class="codewrapper sourceCode" id="cb2">
<h3 class="code-label">BASH<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode bash" tabindex="0"><code class="sourceCode bash"><span id="cb2-1"><a href="#cb2-1" tabindex="-1"></a><span class="ex">nelle@localhost</span> $</span></code></pre>
</div>
<p>The prompt might even include more than this. Do not worry if your
prompt is not just a short <code>$</code>. This lesson does not depend
on this additional information and it should also not get in your way.
The only important item to focus on is the <code>$</code> character
itself and we will see later why.</p>
<p>So let’s try our first command, <code>ls</code>, which is short for
listing. This command will list the contents of the current
directory:</p>
<div class="codewrapper sourceCode" id="cb3">
<h3 class="code-label">BASH<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode bash" tabindex="0"><code class="sourceCode bash"><span id="cb3-1"><a href="#cb3-1" tabindex="-1"></a><span class="ex">$</span> ls</span></code></pre>
</div>
<div class="codewrapper">
<h3 class="code-label">OUTPUT<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="output" tabindex="0"><code>Desktop Downloads Movies Pictures
Documents Library Music Public</code></pre>
</div>
<div id="command-not-found" class="callout">
<div class="callout-square">
<i class="callout-icon" data-feather="bell"></i>
</div>
<div id="command-not-found" class="callout-inner">
<h3 class="callout-title">Command not found</h3>
<div class="callout-content">
<p>If the shell can’t find a program whose name is the command you
typed, it will print an error message such as:</p>
<div class="codewrapper sourceCode" id="cb5">
<h3 class="code-label">BASH<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode bash" tabindex="0"><code class="sourceCode bash"><span id="cb5-1"><a href="#cb5-1" tabindex="-1"></a><span class="ex">$</span> ks</span></code></pre>
</div>
<div class="codewrapper">
<h3 class="code-label">OUTPUT<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="output" tabindex="0"><code>ks: command not found</code></pre>
</div>
<p>This might happen if the command was mis-typed or if the program
corresponding to that command is not installed.</p>
</div>
</div>
</div>
</div>
<section><h2 class="section-heading" id="nelles-pipeline-a-typical-problem">Nelle’s Pipeline: A Typical Problem<a class="anchor" aria-label="anchor" href="#nelles-pipeline-a-typical-problem"></a>
</h2>
<hr class="half-width">
<p>Nelle Nemo, a marine biologist, has just returned from a six-month
survey of the <a href="https://en.wikipedia.org/wiki/North_Pacific_Gyre" class="external-link">North Pacific
Gyre</a>, where she has been sampling gelatinous marine life in the <a href="https://en.wikipedia.org/wiki/Great_Pacific_Garbage_Patch" class="external-link">Great
Pacific Garbage Patch</a>. She has 1520 samples that she’s run through
an assay machine to measure the relative abundance of 300 proteins. She
needs to run these 1520 files through an imaginary program called
<code>goostats.sh</code>. In addition to this huge task, she has to
write up results by the end of the month, so her paper can appear in a
special issue of <em>Aquatic Goo Letters</em>.</p>
<p>If Nelle chooses to run <code>goostats.sh</code> by hand using a GUI,
she’ll have to select and open a file 1520 times. If
<code>goostats.sh</code> takes 30 seconds to run each file, the whole
process will take more than 12 hours of Nelle’s attention. With the
shell, Nelle can instead assign her computer this mundane task while she
focuses her attention on writing her paper.</p>
<p>The next few lessons will explore the ways Nelle can achieve this.
More specifically, the lessons explain how she can use a command shell
to run the <code>goostats.sh</code> program, using loops to automate the
repetitive steps of entering file names, so that her computer can work
while she writes her paper.</p>
<p>As a bonus, once she has put a processing pipeline together, she will
be able to use it again whenever she collects more data.</p>
<p>In order to achieve her task, Nelle needs to know how to:</p>
<ul>
<li>navigate to a file/directory</li>
<li>create a file/directory</li>
<li>check the length of a file</li>
<li>chain commands together</li>
<li>retrieve a set of files</li>
<li>iterate over files</li>
<li>run a shell script containing her pipeline</li>
</ul>
<div id="keypoints1" class="callout keypoints">
<div class="callout-square">
<i class="callout-icon" data-feather="key"></i>
</div>
<div class="callout-inner">
<h3 class="callout-title">Key Points</h3>
<div class="callout-content">
<ul>
<li>A shell is a program whose primary purpose is to read commands and
run other programs.</li>
<li>This lesson uses Bash, the default shell in many implementations of
Unix.</li>
<li>Programs can be run in Bash by entering commands at the command-line
prompt.</li>
<li>The shell’s main advantages are its high action-to-keystroke ratio,
its support for automating repetitive tasks, and its capacity to access
networked machines.</li>
<li>A significant challenge when using the shell can be knowing what
commands need to be run and how to run them.</li>
</ul>
</div>
</div>
</div>
</section></section><section id="aio-02-filedir"><p>Content from <a href="02-filedir.html">Navigating Files and Directories</a></p>
<hr>
<p>Last updated on 2024-07-24 |
<a href="https://github.com/swcarpentry/shell-novice/edit/main/episodes/02-filedir.md" class="external-link">Edit this page <i aria-hidden="true" data-feather="edit"></i></a></p>
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</div>
<div class="overview card">
<h2 class="card-header">Overview</h2>
<div class="row g-0">
<div class="col-md-4">
<div class="card-body">
<div class="inner">
<h3 class="card-title">Questions</h3>
<ul>
<li>How can I move around on my computer?</li>
<li>How can I see what files and directories I have?</li>
<li>How can I specify the location of a file or directory on my
computer?</li>
</ul>
</div>
</div>
</div>
<div class="col-md-8">
<div class="card-body">
<div class="inner bordered">
<h3 class="card-title">Objectives</h3>
<ul>
<li>Explain the similarities and differences between a file and a
directory.</li>
<li>Translate an absolute path into a relative path and vice versa.</li>
<li>Construct absolute and relative paths that identify specific files
and directories.</li>
<li>Use options and arguments to change the behaviour of a shell
command.</li>
<li>Demonstrate the use of tab completion and explain its
advantages.</li>
</ul>
</div>
</div>
</div>
</div>
</div>
<p>The part of the operating system responsible for managing files and
directories is called the <strong>file system</strong>. It organizes our
data into files, which hold information, and directories (also called
‘folders’), which hold files or other directories.</p>
<p>Several commands are frequently used to create, inspect, rename, and
delete files and directories. To start exploring them, we’ll go to our
open shell window.</p>
<p>First, let’s find out where we are by running a command called
<code>pwd</code> (which stands for ‘print working directory’).
Directories are like <em>places</em> — at any time while we are using
the shell, we are in exactly one place called our <strong>current
working directory</strong>. Commands mostly read and write files in the
current working directory, i.e. ‘here’, so knowing where you are before
running a command is important. <code>pwd</code> shows you where you
are:</p>
<div class="codewrapper sourceCode" id="cb1">
<h3 class="code-label">BASH<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode bash" tabindex="0"><code class="sourceCode bash"><span id="cb1-1"><a href="#cb1-1" tabindex="-1"></a><span class="ex">$</span> pwd</span></code></pre>
</div>
<div class="codewrapper">
<h3 class="code-label">OUTPUT<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="output" tabindex="0"><code>/Users/nelle</code></pre>
</div>
<p>Here, the computer’s response is <code>/Users/nelle</code>, which is
Nelle’s <strong>home directory</strong>:</p>
<div id="home-directory-variation" class="callout">
<div class="callout-square">
<i class="callout-icon" data-feather="bell"></i>
</div>
<div id="home-directory-variation" class="callout-inner">
<h3 class="callout-title">Home Directory Variation</h3>
<div class="callout-content">
<p>The home directory path will look different on different operating
systems. On Linux, it may look like <code>/home/nelle</code>, and on
Windows, it will be similar to
<code>C:\Documents and Settings\nelle</code> or
<code>C:\Users\nelle</code>. (Note that it may look slightly different
for different versions of Windows.) In future examples, we’ve used Mac
output as the default - Linux and Windows output may differ slightly but
should be generally similar.</p>
<p>We will also assume that your <code>pwd</code> command returns your
user’s home directory. If <code>pwd</code> returns something different,
you may need to navigate there using <code>cd</code> or some commands in
this lesson will not work as written. See <a href="#exploring-other-directories">Exploring Other Directories</a> for
more details on the <code>cd</code> command.</p>
</div>
</div>
</div>
<p>To understand what a ‘home directory’ is, let’s have a look at how
the file system as a whole is organized. For the sake of this example,
we’ll be illustrating the filesystem on our scientist Nelle’s computer.
After this illustration, you’ll be learning commands to explore your own
filesystem, which will be constructed in a similar way, but not be
exactly identical.</p>
<p>On Nelle’s computer, the filesystem looks like this:</p>
<figure><img src="fig/filesystem.svg" alt="The file system is made up of a root directory that contains sub-directories titled bin, data, users, and tmp" class="figure mx-auto d-block"></figure><p>The filesystem looks like an upside down tree. The topmost directory
is the <strong>root directory</strong> that holds everything else. We
refer to it using a slash character, <code>/</code>, on its own; this
character is the leading slash in <code>/Users/nelle</code>.</p>
<p>Inside that directory are several other directories: <code>bin</code>
(which is where some built-in programs are stored), <code>data</code>
(for miscellaneous data files), <code>Users</code> (where users’
personal directories are located), <code>tmp</code> (for temporary files
that don’t need to be stored long-term), and so on.</p>
<p>We know that our current working directory <code>/Users/nelle</code>
is stored inside <code>/Users</code> because <code>/Users</code> is the
first part of its name. Similarly, we know that <code>/Users</code> is
stored inside the root directory <code>/</code> because its name begins
with <code>/</code>.</p>
<div id="slashes" class="callout">
<div class="callout-square">
<i class="callout-icon" data-feather="bell"></i>
</div>
<div id="slashes" class="callout-inner">
<h3 class="callout-title">Slashes</h3>
<div class="callout-content">
<p>Notice that there are two meanings for the <code>/</code> character.
When it appears at the front of a file or directory name, it refers to
the root directory. When it appears <em>inside</em> a path, it’s just a
separator.</p>
</div>
</div>
</div>
<p>Underneath <code>/Users</code>, we find one directory for each user
with an account on Nelle’s machine, her colleagues <em>imhotep</em> and
<em>larry</em>.</p>
<figure><img src="fig/home-directories.svg" alt='Like other directories, home directories are sub-directories underneath "/Users" like "/Users/imhotep", "/Users/larry" or"/Users/nelle"' class="figure mx-auto d-block"></figure><p>The user <em>imhotep</em>’s files are stored in
<code>/Users/imhotep</code>, user <em>larry</em>’s in
<code>/Users/larry</code>, and Nelle’s in <code>/Users/nelle</code>.
Nelle is the user in our examples here; therefore, we get
<code>/Users/nelle</code> as our home directory. Typically, when you
open a new command prompt, you will be in your home directory to
start.</p>
<p>Now let’s learn the command that will let us see the contents of our
own filesystem. We can see what’s in our home directory by running
<code>ls</code>:</p>
<div class="codewrapper sourceCode" id="cb3">
<h3 class="code-label">BASH<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode bash" tabindex="0"><code class="sourceCode bash"><span id="cb3-1"><a href="#cb3-1" tabindex="-1"></a><span class="ex">$</span> ls</span></code></pre>
</div>
<div class="codewrapper">
<h3 class="code-label">OUTPUT<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="output" tabindex="0"><code>Applications Documents Library Music Public
Desktop Downloads Movies Pictures</code></pre>
</div>
<p>(Again, your results may be slightly different depending on your
operating system and how you have customized your filesystem.)</p>
<p><code>ls</code> prints the names of the files and directories in the
current directory. We can make its output more comprehensible by using
the <code>-F</code> <strong>option</strong> which tells <code>ls</code>
to classify the output by adding a marker to file and directory names to
indicate what they are:</p>
<ul>
<li>a trailing <code>/</code> indicates that this is a directory</li>
<li>
<code>@</code> indicates a link</li>
<li>
<code>*</code> indicates an executable</li>
</ul>
<p>Depending on your shell’s default settings, the shell might also use
colors to indicate whether each entry is a file or directory.</p>
<div class="codewrapper sourceCode" id="cb5">
<h3 class="code-label">BASH<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode bash" tabindex="0"><code class="sourceCode bash"><span id="cb5-1"><a href="#cb5-1" tabindex="-1"></a><span class="ex">$</span> ls <span class="at">-F</span></span></code></pre>
</div>
<div class="codewrapper">
<h3 class="code-label">OUTPUT<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="output" tabindex="0"><code>Applications/ Documents/ Library/ Music/ Public/
Desktop/ Downloads/ Movies/ Pictures/</code></pre>
</div>
<p>Here, we can see that the home directory contains only
<strong>sub-directories</strong>. Any names in the output that don’t
have a classification symbol are <strong>files</strong> in the current
working directory.</p>
<div id="clearing-your-terminal" class="callout">
<div class="callout-square">
<i class="callout-icon" data-feather="bell"></i>
</div>
<div id="clearing-your-terminal" class="callout-inner">
<h3 class="callout-title">Clearing your terminal</h3>
<div class="callout-content">
<p>If your screen gets too cluttered, you can clear your terminal using
the <code>clear</code> command. You can still access previous commands
using <kbd>↑</kbd> and <kbd>↓</kbd> to move line-by-line, or by
scrolling in your terminal.</p>
</div>
</div>
</div>
<div class="section level3">
<h3 id="getting-help">Getting help<a class="anchor" aria-label="anchor" href="#getting-help"></a>
</h3>
<p><code>ls</code> has lots of other <strong>options</strong>. There are
two common ways to find out how to use a command and what options it
accepts — <strong>depending on your environment, you might find that
only one of these ways works:</strong></p>
<ol style="list-style-type: decimal">
<li>We can pass a <code>--help</code> option to any command (available
on Linux and Git Bash), for example:</li>
</ol>
<div class="codewrapper sourceCode" id="cb7">
<h3 class="code-label">BASH<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode bash" tabindex="0"><code class="sourceCode bash"><span id="cb7-1"><a href="#cb7-1" tabindex="-1"></a><span class="ex">$</span> ls <span class="at">--help</span></span></code></pre>
</div>
<ol start="2" style="list-style-type: decimal">
<li>We can read its manual with <code>man</code> (available on Linux and
macOS):</li>
</ol>
<div class="codewrapper sourceCode" id="cb8">
<h3 class="code-label">BASH<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode bash" tabindex="0"><code class="sourceCode bash"><span id="cb8-1"><a href="#cb8-1" tabindex="-1"></a><span class="ex">$</span> man ls</span></code></pre>
</div>
<p>We’ll describe both ways next.</p>
<div id="help-for-built-in-commands" class="callout">
<div class="callout-square">
<i class="callout-icon" data-feather="bell"></i>
</div>
<div id="help-for-built-in-commands" class="callout-inner">
<h3 class="callout-title">Help for built-in commands</h3>
<div class="callout-content">
<p>Some commands are built in to the Bash shell, rather than existing as
separate programs on the filesystem. One example is the <code>cd</code>
(change directory) command. If you get a message like
<code>No manual entry for cd</code>, try <code>help cd</code> instead.
The <code>help</code> command is how you get usage information for <a href="https://www.gnu.org/software/bash/manual/html_node/Bash-Builtins.html" class="external-link">Bash
built-ins</a>.</p>
</div>
</div>
</div>
<div class="section level4">
<h4 id="the---help-option">The <code>--help</code> option<a class="anchor" aria-label="anchor" href="#the---help-option"></a>
</h4>
<p>Most bash commands and programs that people have written to be run
from within bash, support a <code>--help</code> option that displays
more information on how to use the command or program.</p>
<div class="codewrapper sourceCode" id="cb9">
<h3 class="code-label">BASH<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode bash" tabindex="0"><code class="sourceCode bash"><span id="cb9-1"><a href="#cb9-1" tabindex="-1"></a><span class="ex">$</span> ls <span class="at">--help</span></span></code></pre>
</div>
<div class="codewrapper">
<h3 class="code-label">OUTPUT<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="output" tabindex="0"><code>Usage: ls [OPTION]... [FILE]...
List information about the FILEs (the current directory by default).
Sort entries alphabetically if neither -cftuvSUX nor --sort is specified.
Mandatory arguments to long options are mandatory for short options, too.
-a, --all do not ignore entries starting with .
-A, --almost-all do not list implied . and ..
--author with -l, print the author of each file
-b, --escape print C-style escapes for nongraphic characters
--block-size=SIZE scale sizes by SIZE before printing them; e.g.,
'--block-size=M' prints sizes in units of
1,048,576 bytes; see SIZE format below
-B, --ignore-backups do not list implied entries ending with ~
-c with -lt: sort by, and show, ctime (time of last
modification of file status information);
with -l: show ctime and sort by name;
otherwise: sort by ctime, newest first
-C list entries by columns
--color[=WHEN] colorize the output; WHEN can be 'always' (default
if omitted), 'auto', or 'never'; more info below
-d, --directory list directories themselves, not their contents
-D, --dired generate output designed for Emacs' dired mode
-f do not sort, enable -aU, disable -ls --color
-F, --classify append indicator (one of */=>@|) to entries
... ... ...</code></pre>
</div>
<div id="when-to-use-short-or-long-options" class="callout">
<div class="callout-square">
<i class="callout-icon" data-feather="bell"></i>
</div>
<div id="when-to-use-short-or-long-options" class="callout-inner">
<h3 class="callout-title">When to use short or long options</h3>
<div class="callout-content">
<p>When options exist as both short and long options:</p>
<ul>
<li>Use the short option when typing commands directly into the shell to
minimize keystrokes and get your task done faster.</li>
<li>Use the long option in scripts to provide clarity. It will be read
many times and typed once.</li>
</ul>
</div>
</div>
</div>
<div id="unsupported-command-line-options" class="callout">
<div class="callout-square">
<i class="callout-icon" data-feather="bell"></i>
</div>
<div id="unsupported-command-line-options" class="callout-inner">
<h3 class="callout-title">Unsupported command-line options</h3>
<div class="callout-content">
<p>If you try to use an option that is not supported, <code>ls</code>
and other commands will usually print an error message similar to:</p>
<div class="codewrapper sourceCode" id="cb11">
<h3 class="code-label">BASH<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode bash" tabindex="0"><code class="sourceCode bash"><span id="cb11-1"><a href="#cb11-1" tabindex="-1"></a><span class="ex">$</span> ls <span class="at">-j</span></span></code></pre>
</div>
<div class="codewrapper">
<h3 class="code-label">ERROR<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="error" tabindex="0"><code>ls: invalid option -- 'j'
Try 'ls --help' for more information.</code></pre>
</div>
</div>
</div>
</div>
</div>
<div class="section level4">
<h4 id="the-man-command">The <code>man</code> command<a class="anchor" aria-label="anchor" href="#the-man-command"></a>
</h4>
<p>The other way to learn about <code>ls</code> is to type</p>
<div class="codewrapper sourceCode" id="cb13">
<h3 class="code-label">BASH<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode bash" tabindex="0"><code class="sourceCode bash"><span id="cb13-1"><a href="#cb13-1" tabindex="-1"></a><span class="ex">$</span> man ls</span></code></pre>
</div>
<p>This command will turn your terminal into a page with a description
of the <code>ls</code> command and its options.</p>
<p>To navigate through the <code>man</code> pages, you may use
<kbd>↑</kbd> and <kbd>↓</kbd> to move line-by-line, or try <kbd>b</kbd>
and <kbd>Spacebar</kbd> to skip up and down by a full page. To search
for a character or word in the <code>man</code> pages, use <kbd>/</kbd>
followed by the character or word you are searching for. Sometimes a
search will result in multiple hits. If so, you can move between hits
using <kbd>N</kbd> (for moving forward) and
<kbd>Shift</kbd>+<kbd>N</kbd> (for moving backward).</p>
<p>To <strong>quit</strong> the <code>man</code> pages, press
<kbd>q</kbd>.</p>
<div id="manual-pages-on-the-web" class="callout">
<div class="callout-square">
<i class="callout-icon" data-feather="bell"></i>
</div>
<div id="manual-pages-on-the-web" class="callout-inner">
<h3 class="callout-title">Manual pages on the web</h3>
<div class="callout-content">
<p>Of course, there is a third way to access help for commands:
searching the internet via your web browser. When using internet search,
including the phrase <code>unix man page</code> in your search query
will help to find relevant results.</p>
<p>GNU provides links to its <a href="https://www.gnu.org/manual/manual.html" class="external-link">manuals</a> including the
<a href="https://www.gnu.org/software/coreutils/manual/coreutils.html" class="external-link">core
GNU utilities</a>, which covers many commands introduced within this
lesson.</p>
</div>
</div>
</div>
<div id="exploring-more-ls-options" class="callout challenge">
<div class="callout-square">
<i class="callout-icon" data-feather="zap"></i>
</div>
<div id="exploring-more-ls-options" class="callout-inner">
<h3 class="callout-title">Exploring More <code>ls</code> Options</h3>
<div class="callout-content">
<p>You can also use two options at the same time. What does the command
<code>ls</code> do when used with the <code>-l</code> option? What about
if you use both the <code>-l</code> and the <code>-h</code> option?</p>
<p>Some of its output is about properties that we do not cover in this
lesson (such as file permissions and ownership), but the rest should be
useful nevertheless.</p>
</div>
</div>
</div>
<div id="accordionSolution1" class="accordion challenge-accordion accordion-flush">
<div class="accordion-item">
<button class="accordion-button solution-button collapsed" type="button" data-bs-toggle="collapse" data-bs-target="#collapseSolution1" aria-expanded="false" aria-controls="collapseSolution1">
<h4 class="accordion-header" id="headingSolution1"> Show me the solution </h4>
</button>
<div id="collapseSolution1" class="accordion-collapse collapse" aria-labelledby="headingSolution1" data-bs-parent="#accordionSolution1">
<div class="accordion-body">
<p>The <code>-l</code> option makes <code>ls</code> use a
<strong>l</strong>ong listing format, showing not only the
file/directory names but also additional information, such as the file
size and the time of its last modification. If you use both the
<code>-h</code> option and the <code>-l</code> option, this makes the
file size ‘<strong>h</strong>uman readable’, i.e. displaying something
like <code>5.3K</code> instead of <code>5369</code>.</p>
</div>
</div>
</div>
</div>
<div id="listing-in-reverse-chronological-order" class="callout challenge">
<div class="callout-square">
<i class="callout-icon" data-feather="zap"></i>
</div>
<div id="listing-in-reverse-chronological-order" class="callout-inner">
<h3 class="callout-title">Listing in Reverse Chronological Order</h3>
<div class="callout-content">
<p>By default, <code>ls</code> lists the contents of a directory in
alphabetical order by name. The command <code>ls -t</code> lists items
by time of last change instead of alphabetically. The command
<code>ls -r</code> lists the contents of a directory in reverse order.
Which file is displayed last when you combine the <code>-t</code> and
<code>-r</code> options? Hint: You may need to use the <code>-l</code>