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<!DOCTYPE html>
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The Unix Shell
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The Unix Shell
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<section id="01-intro"><h2 class="section-heading"><a href="01-intro.html">Introducing the Shell</a></h2>
<hr class="half-width">
<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></section><section id="02-filedir"><h2 class="section-heading"><a href="02-filedir.html">Navigating Files and Directories</a></h2>
<hr class="half-width">
<ul>
<li>The file system is responsible for managing information on the
disk.</li>
<li>Information is stored in files, which are stored in directories
(folders).</li>
<li>Directories can also store other directories, which then form a
directory tree.</li>
<li>
<code>pwd</code> prints the user’s current working directory.</li>
<li>
<code>ls [path]</code> prints a listing of a specific file or
directory; <code>ls</code> on its own lists the current working
directory.</li>
<li>
<code>cd [path]</code> changes the current working directory.</li>
<li>Most commands take options that begin with a single
<code>-</code>.</li>
<li>Directory names in a path are separated with <code>/</code> on Unix,
but <code>\</code> on Windows.</li>
<li>
<code>/</code> on its own is the root directory of the whole file
system.</li>
<li>An absolute path specifies a location from the root of the file
system.</li>
<li>A relative path specifies a location starting from the current
location.</li>
<li>
<code>.</code> on its own means ‘the current directory’;
<code>..</code> means ‘the directory above the current one’.</li>
</ul></section><section id="03-create"><h2 class="section-heading"><a href="03-create.html">Working With Files and Directories</a></h2>
<hr class="half-width">
<ul>
<li>
<code>cp [old] [new]</code> copies a file.</li>
<li>
<code>mkdir [path]</code> creates a new directory.</li>
<li>
<code>mv [old] [new]</code> moves (renames) a file or
directory.</li>
<li>
<code>rm [path]</code> removes (deletes) a file.</li>
<li>
<code>*</code> matches zero or more characters in a filename, so
<code>*.txt</code> matches all files ending in <code>.txt</code>.</li>
<li>
<code>?</code> matches any single character in a filename, so
<code>?.txt</code> matches <code>a.txt</code> but not
<code>any.txt</code>.</li>
<li>Use of the Control key may be described in many ways, including
<code>Ctrl-X</code>, <code>Control-X</code>, and <code>^X</code>.</li>
<li>The shell does not have a trash bin: once something is deleted, it’s
really gone.</li>
<li>Most files’ names are <code>something.extension</code>. The
extension isn’t required, and doesn’t guarantee anything, but is
normally used to indicate the type of data in the file.</li>
<li>Depending on the type of work you do, you may need a more powerful
text editor than Nano.</li>
</ul></section><section id="04-pipefilter"><h2 class="section-heading"><a href="04-pipefilter.html">Pipes and Filters</a></h2>
<hr class="half-width">
<ul>
<li>
<code>wc</code> counts lines, words, and characters in its
inputs.</li>
<li>
<code>cat</code> displays the contents of its inputs.</li>
<li>
<code>sort</code> sorts its inputs.</li>
<li>
<code>head</code> displays the first 10 lines of its input.</li>
<li>
<code>tail</code> displays the last 10 lines of its input.</li>
<li>
<code>command > [file]</code> redirects a command’s output to a
file (overwriting any existing content).</li>
<li>
<code>command >> [file]</code> appends a command’s output to a
file.</li>
<li>
<code>[first] | [second]</code> is a pipeline: the output of the
first command is used as the input to the second.</li>
<li>The best way to use the shell is to use pipes to combine simple
single-purpose programs (filters).</li>
</ul></section><section id="05-loop"><h2 class="section-heading"><a href="05-loop.html">Loops</a></h2>
<hr class="half-width">
<ul>
<li>A <code>for</code> loop repeats commands once for every thing in a
list.</li>
<li>Every <code>for</code> loop needs a variable to refer to the thing
it is currently operating on.</li>
<li>Use <code>$name</code> to expand a variable (i.e., get its value).
<code>${name}</code> can also be used.</li>
<li>Do not use spaces, quotes, or wildcard characters such as ‘*’ or ‘?’
in filenames, as it complicates variable expansion.</li>
<li>Give files consistent names that are easy to match with wildcard
patterns to make it easy to select them for looping.</li>
<li>Use the up-arrow key to scroll up through previous commands to edit
and repeat them.</li>
<li>Use <kbd>Ctrl</kbd>+<kbd>R</kbd> to search through the previously
entered commands.</li>
<li>Use <code>history</code> to display recent commands, and
<code>![number]</code> to repeat a command by number.</li>
</ul></section><section id="06-script"><h2 class="section-heading"><a href="06-script.html">Shell Scripts</a></h2>
<hr class="half-width">
<ul>
<li>Save commands in files (usually called shell scripts) for
re-use.</li>
<li>
<code>bash [filename]</code> runs the commands saved in a file.</li>
<li>
<code>$@</code> refers to all of a shell script’s command-line
arguments.</li>
<li>
<code>$1</code>, <code>$2</code>, etc., refer to the first
command-line argument, the second command-line argument, etc.</li>
<li>Place variables in quotes if the values might have spaces in
them.</li>
<li>Letting users decide what files to process is more flexible and more
consistent with built-in Unix commands.</li>
</ul></section><section id="07-find"><h2 class="section-heading"><a href="07-find.html">Finding Things</a></h2>
<hr class="half-width">
<ul>
<li>
<code>find</code> finds files with specific properties that match
patterns.</li>
<li>
<code>grep</code> selects lines in files that match patterns.</li>
<li>
<code>--help</code> is an option supported by many bash commands,
and programs that can be run from within Bash, to display more
information on how to use these commands or programs.</li>
<li>
<code>man [command]</code> displays the manual page for a given
command.</li>
<li>
<code>$([command])</code> inserts a command’s output in place.</li>
</ul></section>
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