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Simple Command
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Command Substitution
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Loops
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<h1 id="bash-scripting">Bash Scripting</h1>
<p><a href="https://www.gnu.org/software/bash/manual/">Bash Manual</a></p>
<p><code>Ctrl+Shift+C</code>: copy<br />
<code>Ctrl+Shift+C</code>: paste</p>
<h2 id="bash-use-modes">Bash Use Modes</h2>
<p>Interactive mode &rarr; shell waits for user's commands
Non-interactive mode &rarr; shell runs scripts</p>
<h2 id="file-directories-permissions">File &amp; Directories Permissions</h2>
<p>File:</p>
<ul>
<li><code>r</code>: Read. Can see file content</li>
<li><code>w</code>: Write. Can modify file content</li>
<li><code>x</code>: Execute. Can execute file</li>
</ul>
<p>Directory:</p>
<ul>
<li><code>r</code>: Read. Can see dir contents</li>
<li><code>w</code>: CRD. Can create, rename and delete files</li>
<li><code>x</code>: Search. Can access and navigate inside the dir. Necessary to operate on files</li>
</ul>
<h2 id="file-descriptors">File Descriptors</h2>
<p><code>FD 0</code> "standard input" &rarr; Channel for standard input (default keyboard)<br />
<code>FD 1</code> "standard output" &rarr; Channel for the default output (default screen)<br />
<code>FD 2</code> "standard error" &rarr; Channel for error messages, info messages, prompts (default keyboard)<br />
File descriptors chan be joined to create streams that lead to files, devices or other processes.</p>
<p>Bash gets commands by reading lines.<br />
As soon as it's read enough lines to compose a complete command, bash begins running that command.<br />
Usually, commands are just a single line long. An interactive bash session reads lines from you at the prompt.<br />
Non-interactive bash processes read their commands from a file or stream.<br />
Files with a shebang as their first line (and the executable permission) can be started by your system's kernel like any other program.</p>
<h3 id="first-line-of-bash">First Line Of Bash</h3>
<p><code>#!/bin/env bash</code><br />
shebang indicating which interpreter to use</p>
<h3 id="simple-command">Simple Command</h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-0-1">1</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-0-1" name="__codelineno-0-1"></a><span class="o">[</span><span class="w"> </span><span class="nv">var</span><span class="o">=</span>value<span class="w"> </span>...<span class="w"> </span><span class="o">]</span><span class="w"> </span><span class="nb">command</span><span class="w"> </span><span class="o">[</span><span class="w"> </span>arg<span class="w"> </span>...<span class="w"> </span><span class="o">]</span><span class="w"> </span><span class="o">[</span><span class="w"> </span>redirection<span class="w"> </span>...<span class="w"> </span><span class="o">]</span><span class="w"> </span><span class="c1"># [.] is optional component</span>
</code></pre></div></td></tr></table></div>
<h3 id="pipelines-commands-concatenation">Pipelines (commands concatenation)</h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-1-1">1</a></span>
<span class="normal"><a href="#__codelineno-1-2">2</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-1-1" name="__codelineno-1-1"></a><span class="nb">command</span><span class="w"> </span><span class="p">|</span><span class="w"> </span>file.ext<span class="w"> </span><span class="c1"># link the first process&#39; standard output to the second process&#39; standard input</span>
<a id="__codelineno-1-2" name="__codelineno-1-2"></a><span class="nb">command</span><span class="w"> </span><span class="p">|&amp;</span><span class="w"> </span>file.ext<span class="w"> </span><span class="c1"># link the first process&#39; standard output &amp; standard error to the second process&#39; standard input</span>
</code></pre></div></td></tr></table></div>
<h3 id="lists-sequence-of-commands">Lists (sequence of commands)</h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-2-1">1</a></span>
<span class="normal"><a href="#__codelineno-2-2">2</a></span>
<span class="normal"><a href="#__codelineno-2-3">3</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-2-1" name="__codelineno-2-1"></a>command_1<span class="p">;</span><span class="w"> </span>command_2<span class="p">;</span><span class="w"> </span>...<span class="w"> </span><span class="c1"># execute command in sequence, one after the other</span>
<a id="__codelineno-2-2" name="__codelineno-2-2"></a>command_1<span class="w"> </span><span class="o">||</span><span class="w"> </span>command_2<span class="w"> </span><span class="o">||</span><span class="w"> </span>...<span class="w"> </span><span class="c1"># execute successive commands only if preceding ones fail</span>
<a id="__codelineno-2-3" name="__codelineno-2-3"></a>command_1<span class="w"> </span><span class="o">&amp;&amp;</span><span class="w"> </span>command_2<span class="w"> </span><span class="o">&amp;&amp;</span><span class="w"> </span>..<span class="w"> </span><span class="c1"># execute successive commands only if preceding ones succeeds</span>
</code></pre></div></td></tr></table></div>
<h3 id="compound-commands-multiple-commands-as-one">COMPOUND COMMANDs (multiple commands as one)</h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-3-1">1</a></span>
<span class="normal"><a href="#__codelineno-3-2">2</a></span>
<span class="normal"><a href="#__codelineno-3-3">3</a></span>
<span class="normal"><a href="#__codelineno-3-4">4</a></span>
<span class="normal"><a href="#__codelineno-3-5">5</a></span>
<span class="normal"><a href="#__codelineno-3-6">6</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-3-1" name="__codelineno-3-1"></a><span class="c1"># block of commands executed as one</span>
<a id="__codelineno-3-2" name="__codelineno-3-2"></a>&lt;keyword&gt;
<a id="__codelineno-3-3" name="__codelineno-3-3"></a><span class="w"> </span>command_1<span class="p">;</span><span class="w"> </span>command_2<span class="p">;</span><span class="w"> </span>...
<a id="__codelineno-3-4" name="__codelineno-3-4"></a>&lt;end_keyword&gt;
<a id="__codelineno-3-5" name="__codelineno-3-5"></a>
<a id="__codelineno-3-6" name="__codelineno-3-6"></a><span class="o">{</span>command_1<span class="p">;</span><span class="w"> </span>command_2<span class="p">;</span><span class="w"> </span>...<span class="o">}</span><span class="w"> </span><span class="c1"># sequence of commands executed as one</span>
</code></pre></div></td></tr></table></div>
<h3 id="functions-blocks-of-easily-reusable-code">Functions (blocks of easily reusable code)</h3>
<p><code>function_name () {compound_command}</code><br />
Bash does not accept func arguments, parentheses must be empty</p>
<h2 id="command-names-running-programs">Command names &amp; Running programs</h2>
<p>To run a command, bash uses the name of your command and performs a search for how to execute that command.<br />
In order, bash will check whether it has a function or builtin by that name.<br />
Failing that, it will try to run the name as a program.<br />
If bash finds no way to run your command, it will output an error message. </p>
<h2 id="the-path-to-a-program">The path to a program</h2>
<p>When bash needs to run a program, it uses the command name to perform a search.<br />
Bash searches the directories in your PATH environment variable, one by one, until it finds a directory that contains a program with the name of your command.<br />
To run a program that is not installed in a PATH directory, use the path to that program as your command's name.</p>
<h2 id="command-arguments-quoting-literals">Command arguments &amp; Quoting literals</h2>
<p>To tell a command what to do, we pass it arguments. In bash, arguments are tokens, that are separated from each other by blank space.<br />
To include blank space in an argument's value, you need to either quote the argument or escape the blank space within.<br />
Failing that, bash will break your argument apart into multiple arguments at its blank space.<br />
Quoting arguments also prevents other symbols in it from being accidentally interpreted as bash code. </p>
<h2 id="managing-a-commands-input-and-output-using-redirection">Managing a command's input and output using redirection</h2>
<p>By default, new commands inherit the shell's current file descriptors.<br />
We can use redirections to change where a command's input comes from and where its output should go to.<br />
File redirection allows us to stream file descriptors to files.<br />
We can copy file descriptors to make them share a stream. There are also many other more advanced redirection operators. </p>
<h3 id="redirections">Redirections</h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-4-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-4-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-4-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-4-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-4-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-4-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-4-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-4-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-4-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-4-10">10</a></span>
<span class="normal"><a href="#__codelineno-4-11">11</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-4-1" name="__codelineno-4-1"></a><span class="o">[</span>x<span class="o">]</span>&gt;file<span class="w"> </span><span class="c1"># make FD x write to file</span>
<a id="__codelineno-4-2" name="__codelineno-4-2"></a><span class="o">[</span>x<span class="o">]</span>&lt;file<span class="w"> </span><span class="c1"># make FD x read from file</span>
<a id="__codelineno-4-3" name="__codelineno-4-3"></a>
<a id="__codelineno-4-4" name="__codelineno-4-4"></a><span class="o">[</span>x<span class="o">]</span>&gt;<span class="p">&amp;</span>y<span class="w"> </span><span class="c1"># make FD x write to FD y&#39;s stream</span>
<a id="__codelineno-4-5" name="__codelineno-4-5"></a><span class="o">[</span>x<span class="o">]</span>&lt;<span class="p">&amp;</span>y<span class="w"> </span><span class="c1"># make FD x read from FD y&#39;s stream</span>
<a id="__codelineno-4-6" name="__codelineno-4-6"></a><span class="p">&amp;</span>&gt;file<span class="w"> </span><span class="c1"># make both FD 1 (standard output) &amp; FD 2 (standard error) write to file</span>
<a id="__codelineno-4-7" name="__codelineno-4-7"></a>
<a id="__codelineno-4-8" name="__codelineno-4-8"></a><span class="o">[</span>x<span class="o">]</span>&gt;&gt;file<span class="w"> </span><span class="c1"># make FD x append to end of file</span>
<a id="__codelineno-4-9" name="__codelineno-4-9"></a>x&gt;<span class="p">&amp;</span>-,<span class="w"> </span>x&lt;<span class="p">&amp;</span>-<span class="w"> </span><span class="c1"># close FD x (stream disconnected from FD x)</span>
<a id="__codelineno-4-10" name="__codelineno-4-10"></a><span class="o">[</span>x<span class="o">]</span>&gt;<span class="p">&amp;</span>y-,<span class="w"> </span><span class="o">[</span>x<span class="o">]</span>&lt;<span class="p">&amp;</span>y-<span class="w"> </span><span class="c1"># replace FD x with FD y</span>
<a id="__codelineno-4-11" name="__codelineno-4-11"></a><span class="o">[</span>x<span class="o">]</span>&lt;&gt;file<span class="w"> </span><span class="c1"># open FD x for both reading and writing to file</span>
</code></pre></div></td></tr></table></div>
<h2 id="pathname-expansion-filname-pattern-glob-matching">Pathname Expansion (filname pattern [glob] matching)</h2>
<p><code>*</code> matches any kind of text (even no text).<br />
<code>?</code> matches any single character.<br />
<code>[characters]</code> matches any single character in the given set.<br />
<code>[[:classname:]]</code> specify class of characters to match.
<code>{}</code> expand list of arguments (applies command to each one)</p>
<p><code>shopt -s extglob</code> enables extended globs (patterns)</p>
<p><code>+(pattern [| pattern ...])</code> matches when any of the patterns in the list appears, once or many times over. ("at least one of ...").<br />
<code>*(pattern [| pattern ...])</code> matches when any of the patterns in the list appears, once, not at all, or many times over. ("however many of ...").<br />
<code>?(pattern [| pattern ...])</code> matches when any of the patterns in the list appears, once, not at all, or many times over. ("however many of ...").<br />
<code>@(pattern [| pattern ...])</code> matches when any of the patterns in the list appears just once. ("one of ...").<br />
<code>!(pattern [| pattern ...])</code> matches only when none of the patterns in the list appear. ("none of ...").</p>
<h2 id="command-substitution">Command Substitution</h2>
<p>With Command Substitution, we effectively write a command within a command, and we ask bash to expand the inner command into its output and use that output as argument data for the main command. </p>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-5-1">1</a></span>
<span class="normal"><a href="#__codelineno-5-2">2</a></span>
<span class="normal"><a href="#__codelineno-5-3">3</a></span>
<span class="normal"><a href="#__codelineno-5-4">4</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-5-1" name="__codelineno-5-1"></a><span class="k">$(</span>inner_command<span class="k">)</span><span class="w"> </span><span class="c1"># $ --&gt; value-expansion prefix</span>
<a id="__codelineno-5-2" name="__codelineno-5-2"></a><span class="nb">command</span><span class="w"> </span>!*<span class="w"> </span><span class="c1"># !* expands to everything except the first argument in the previous line</span>
<a id="__codelineno-5-3" name="__codelineno-5-3"></a><span class="nb">command</span><span class="w"> </span>!$<span class="w"> </span><span class="c1"># refers to the last argument of the previous command</span>
<a id="__codelineno-5-4" name="__codelineno-5-4"></a>sudo<span class="w"> </span>!!<span class="w"> </span><span class="c1"># !! expands to the entire previous command</span>
</code></pre></div></td></tr></table></div>
<h2 id="shell-variables">Shell Variables</h2>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-6-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-6-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-6-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-6-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-6-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-6-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-6-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-6-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-6-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-6-10">10</a></span>
<span class="normal"><a href="#__codelineno-6-11">11</a></span>
<span class="normal"><a href="#__codelineno-6-12">12</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-6-1" name="__codelineno-6-1"></a><span class="nv">varname</span><span class="o">=</span>value<span class="w"> </span><span class="c1"># variable assignment</span>
<a id="__codelineno-6-2" name="__codelineno-6-2"></a><span class="nv">varname</span><span class="o">=</span><span class="s2">&quot;</span><span class="k">$(</span><span class="nb">command</span><span class="k">)</span><span class="s2">&quot;</span><span class="w"> </span><span class="c1"># command substitution, MUST be double-quoted</span>
<a id="__codelineno-6-3" name="__codelineno-6-3"></a><span class="s2">&quot;</span><span class="nv">$varname</span><span class="s2">&quot;</span>,<span class="w"> </span><span class="s2">&quot;</span><span class="si">${</span><span class="nv">varname</span><span class="si">}</span><span class="s2">&quot;</span><span class="w"> </span><span class="c1"># variable expansion, MUST be double-quoted (name substituted w/ variable content)</span>
<a id="__codelineno-6-4" name="__codelineno-6-4"></a>
<a id="__codelineno-6-5" name="__codelineno-6-5"></a><span class="nv">$$</span><span class="w"> </span><span class="c1"># pid</span>
<a id="__codelineno-6-6" name="__codelineno-6-6"></a><span class="nv">$#</span><span class="w"> </span><span class="c1"># number of arguments passed</span>
<a id="__codelineno-6-7" name="__codelineno-6-7"></a><span class="nv">$@</span><span class="w"> </span><span class="c1"># all arguments passed</span>
<a id="__codelineno-6-8" name="__codelineno-6-8"></a><span class="si">${</span><span class="nv">n</span><span class="si">}</span><span class="w"> </span><span class="c1"># n-th argument passed to the command</span>
<a id="__codelineno-6-9" name="__codelineno-6-9"></a><span class="nv">$0</span><span class="w"> </span><span class="c1"># name of the script</span>
<a id="__codelineno-6-10" name="__codelineno-6-10"></a><span class="nv">$_</span><span class="w"> </span><span class="c1"># last argument passed to the command</span>
<a id="__codelineno-6-11" name="__codelineno-6-11"></a><span class="nv">$?</span><span class="w"> </span><span class="c1"># error message of the last (previous) command</span>
<a id="__codelineno-6-12" name="__codelineno-6-12"></a>!!<span class="w"> </span><span class="c1"># executes last command used (echo !! prints the last command)</span>
</code></pre></div></td></tr></table></div>
<h2 id="parameter-expansion-modifiers-in-double-quotes">Parameter Expansion Modifiers (in double-quotes)</h2>
<p><code>${parameter#pattern}</code> removes the shortest string that matches the pattern if it's at the start of the value.<br />
<code>${parameter##pattern}</code> removes the longest string that matches the pattern if it's at the start of the value.<br />
<code>${parameter%pattern}</code> removes the shortest string that matches the pattern if it's at the end of the value.<br />
<code>${parameter%%pattern}</code> removes the longest string that matches the pattern if it's at the end of the value.<br />
<code>${parameter/pattern/replacement}</code> replaces the first string that matches the pattern with the replacement.<br />
<code>${parameter//pattern/replacement}</code> replaces each string that matches the pattern with the replacement.<br />
<code>${parameter/#pattern/replacement}</code> replaces the string that matches the pattern at the beginning of the value with the replacement.<br />
<code>${parameter/%pattern/replacement}</code> replaces the string that matches the pattern at the end of the value with the replacement.<br />
<code>${#parameter}</code> expands the length of the value (in bytes).<br />
<code>${parameter:start[:length]}</code> expands a part of the value, starting at start, length bytes long.<br />
Counts from the end rather than the beginning by using a (space followed by a) negative value.<br />
<code>${parameter[^|^^|,|,,][pattern]}</code> expands the transformed value, either upper-casing or lower-casing the first or all characters that match the pattern.
Omit the pattern to match any character.</p>
<h2 id="decision-statements">Decision Statements</h2>
<h3 id="if-statement">If Statement</h3>
<p>Only the final exit code after executing the entire list is relevant for the branch's evaluation.</p>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-7-1">1</a></span>
<span class="normal"><a href="#__codelineno-7-2">2</a></span>
<span class="normal"><a href="#__codelineno-7-3">3</a></span>
<span class="normal"><a href="#__codelineno-7-4">4</a></span>
<span class="normal"><a href="#__codelineno-7-5">5</a></span>
<span class="normal"><a href="#__codelineno-7-6">6</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-7-1" name="__codelineno-7-1"></a><span class="k">if</span><span class="w"> </span>command_list<span class="p">;</span><span class="w"> </span><span class="k">then</span>
<a id="__codelineno-7-2" name="__codelineno-7-2"></a><span class="w"> </span>command_list<span class="p">;</span>
<a id="__codelineno-7-3" name="__codelineno-7-3"></a><span class="k">elif</span><span class="w"> </span>command_list<span class="p">;</span><span class="w"> </span><span class="k">then</span>
<a id="__codelineno-7-4" name="__codelineno-7-4"></a><span class="w"> </span>command_list<span class="p">;</span>
<a id="__codelineno-7-5" name="__codelineno-7-5"></a><span class="k">else</span><span class="w"> </span>command_list<span class="p">;</span>
<a id="__codelineno-7-6" name="__codelineno-7-6"></a><span class="k">fi</span>
</code></pre></div></td></tr></table></div>
<h3 id="test-command">Test Command</h3>
<p><code>[[ argument_1 &lt;operator&gt; argument_2 ]]</code></p>
<h3 id="arithmetic-expansion-and-evaluation">Arithmetic expansion and evaluation</h3>
<p><code>(( expression ))</code></p>
<h3 id="comparison-operators">Comparison Operators</h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-8-1">1</a></span>
<span class="normal"><a href="#__codelineno-8-2">2</a></span>
<span class="normal"><a href="#__codelineno-8-3">3</a></span>
<span class="normal"><a href="#__codelineno-8-4">4</a></span>
<span class="normal"><a href="#__codelineno-8-5">5</a></span>
<span class="normal"><a href="#__codelineno-8-6">6</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-8-1" name="__codelineno-8-1"></a><span class="o">[[</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span>-eq<span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="w"> </span><span class="o">]]</span><span class="w"> </span><span class="c1"># is equal to</span>
<a id="__codelineno-8-2" name="__codelineno-8-2"></a><span class="o">[[</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span>-ne<span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="w"> </span><span class="o">]]</span><span class="w"> </span><span class="c1"># in not equal to</span>
<a id="__codelineno-8-3" name="__codelineno-8-3"></a><span class="o">[[</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span>-gt<span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="w"> </span><span class="o">]]</span><span class="w"> </span><span class="c1"># greater than</span>
<a id="__codelineno-8-4" name="__codelineno-8-4"></a><span class="o">[[</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span>-ge<span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="w"> </span><span class="o">]]</span><span class="w"> </span><span class="c1"># greater than or equal to</span>
<a id="__codelineno-8-5" name="__codelineno-8-5"></a><span class="o">[[</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span>-lt<span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="w"> </span><span class="o">]]</span><span class="w"> </span><span class="c1"># less than</span>
<a id="__codelineno-8-6" name="__codelineno-8-6"></a><span class="o">[[</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span>-le<span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="w"> </span><span class="o">]]</span><span class="w"> </span><span class="c1"># less than or equal to</span>
</code></pre></div></td></tr></table></div>
<h3 id="arithmetic-comparison-operators">Arithmetic Comparison Operators</h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-9-1">1</a></span>
<span class="normal"><a href="#__codelineno-9-2">2</a></span>
<span class="normal"><a href="#__codelineno-9-3">3</a></span>
<span class="normal"><a href="#__codelineno-9-4">4</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-9-1" name="__codelineno-9-1"></a><span class="o">((</span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span>&gt;<span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="o">))</span><span class="w"> </span><span class="c1"># greater than</span>
<a id="__codelineno-9-2" name="__codelineno-9-2"></a><span class="o">((</span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span>&gt;<span class="o">=</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="o">))</span><span class="w"> </span><span class="c1"># greater than or equal to</span>
<a id="__codelineno-9-3" name="__codelineno-9-3"></a><span class="o">((</span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span>&lt;<span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="o">))</span><span class="w"> </span><span class="c1"># less than</span>
<a id="__codelineno-9-4" name="__codelineno-9-4"></a><span class="o">((</span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span>&lt;<span class="o">=</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="o">))</span><span class="w"> </span><span class="c1"># less than or equal to</span>
</code></pre></div></td></tr></table></div>
<h3 id="string-comparison-operators">String Comparison Operators</h3>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-10-1"> 1</a></span>
<span class="normal"><a href="#__codelineno-10-2"> 2</a></span>
<span class="normal"><a href="#__codelineno-10-3"> 3</a></span>
<span class="normal"><a href="#__codelineno-10-4"> 4</a></span>
<span class="normal"><a href="#__codelineno-10-5"> 5</a></span>
<span class="normal"><a href="#__codelineno-10-6"> 6</a></span>
<span class="normal"><a href="#__codelineno-10-7"> 7</a></span>
<span class="normal"><a href="#__codelineno-10-8"> 8</a></span>
<span class="normal"><a href="#__codelineno-10-9"> 9</a></span>
<span class="normal"><a href="#__codelineno-10-10">10</a></span>
<span class="normal"><a href="#__codelineno-10-11">11</a></span>
<span class="normal"><a href="#__codelineno-10-12">12</a></span>
<span class="normal"><a href="#__codelineno-10-13">13</a></span>
<span class="normal"><a href="#__codelineno-10-14">14</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-10-1" name="__codelineno-10-1"></a><span class="o">[</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="w"> </span><span class="o">]</span><span class="w"> </span><span class="c1"># is equal to (whitespace around operator)</span>
<a id="__codelineno-10-2" name="__codelineno-10-2"></a>
<a id="__codelineno-10-3" name="__codelineno-10-3"></a><span class="o">[[</span><span class="w"> </span><span class="nv">$a</span><span class="w"> </span><span class="o">==</span><span class="w"> </span>z*<span class="w"> </span><span class="o">]]</span><span class="w"> </span><span class="c1"># True if $a starts with an &quot;z&quot; (pattern matching)</span>
<a id="__codelineno-10-4" name="__codelineno-10-4"></a><span class="o">[[</span><span class="w"> </span><span class="nv">$a</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="s2">&quot;z*&quot;</span><span class="w"> </span><span class="o">]]</span><span class="w"> </span><span class="c1"># True if $a is equal to z* (literal matching)</span>
<a id="__codelineno-10-5" name="__codelineno-10-5"></a><span class="o">[</span><span class="w"> </span><span class="nv">$a</span><span class="w"> </span><span class="o">==</span><span class="w"> </span>z*<span class="w"> </span><span class="o">]</span><span class="w"> </span><span class="c1"># File globbing and word splitting take place</span>
<a id="__codelineno-10-6" name="__codelineno-10-6"></a><span class="o">[</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="s2">&quot;z*&quot;</span><span class="w"> </span><span class="o">]</span><span class="w"> </span><span class="c1"># True if $a is equal to z* (literal matching)</span>
<a id="__codelineno-10-7" name="__codelineno-10-7"></a>
<a id="__codelineno-10-8" name="__codelineno-10-8"></a><span class="o">[</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span>!<span class="o">=</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="w"> </span><span class="o">]</span><span class="w"> </span><span class="c1"># is not equal to, pattern matching within a [[ ... ]] construct</span>
<a id="__codelineno-10-9" name="__codelineno-10-9"></a>
<a id="__codelineno-10-10" name="__codelineno-10-10"></a><span class="o">[[</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span>&lt;<span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="w"> </span><span class="o">]]</span><span class="w"> </span><span class="c1"># is less than, in ASCII alphabetical order</span>
<a id="__codelineno-10-11" name="__codelineno-10-11"></a><span class="o">[</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span><span class="se">\&lt;</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="w"> </span><span class="o">]</span><span class="w"> </span><span class="c1"># &quot;&lt;&quot; needs to be escaped within a [ ] construct.</span>
<a id="__codelineno-10-12" name="__codelineno-10-12"></a>
<a id="__codelineno-10-13" name="__codelineno-10-13"></a><span class="o">[[</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span>&gt;<span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="w"> </span><span class="o">]]</span><span class="w"> </span><span class="c1"># is greater than, in ASCII alphabetical order</span>
<a id="__codelineno-10-14" name="__codelineno-10-14"></a><span class="o">[</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$a</span><span class="s2">&quot;</span><span class="w"> </span><span class="se">\&gt;</span><span class="w"> </span><span class="s2">&quot;</span><span class="nv">$b</span><span class="s2">&quot;</span><span class="w"> </span><span class="o">]</span><span class="w"> </span><span class="c1"># &quot;&gt;&quot; needs to be escaped within a [ ] construct.</span>
</code></pre></div></td></tr></table></div>
<h2 id="commands-short-circuit-evaluation">Commands short circuit evaluation</h2>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-11-1">1</a></span>
<span class="normal"><a href="#__codelineno-11-2">2</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-11-1" name="__codelineno-11-1"></a>command_1<span class="w"> </span><span class="o">||</span><span class="w"> </span>command_2<span class="w"> </span><span class="c1"># if command_1 fails executes command_2</span>
<a id="__codelineno-11-2" name="__codelineno-11-2"></a>command_1<span class="w"> </span><span class="o">&amp;&amp;</span><span class="w"> </span>command_2<span class="w"> </span><span class="c1"># executes command_2 only if command_1 succeeds</span>
</code></pre></div></td></tr></table></div>
<h2 id="loops">Loops</h2>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-12-1">1</a></span>
<span class="normal"><a href="#__codelineno-12-2">2</a></span>
<span class="normal"><a href="#__codelineno-12-3">3</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-12-1" name="__codelineno-12-1"></a><span class="k">for</span><span class="w"> </span>var<span class="w"> </span><span class="k">in</span><span class="w"> </span>iterable<span class="w"> </span><span class="p">;</span><span class="w"> </span><span class="k">do</span>
<a id="__codelineno-12-2" name="__codelineno-12-2"></a><span class="w"> </span><span class="c1"># command here</span>
<a id="__codelineno-12-3" name="__codelineno-12-3"></a><span class="k">done</span>
</code></pre></div></td></tr></table></div>
<h2 id="script-hardening">Script Hardening</h2>
<div class="highlight"><table class="highlighttable"><tr><th colspan="2" class="filename"><span class="filename">Bash</span></th></tr><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-13-1">1</a></span>
<span class="normal"><a href="#__codelineno-13-2">2</a></span>
<span class="normal"><a href="#__codelineno-13-3">3</a></span>
<span class="normal"><a href="#__codelineno-13-4">4</a></span>
<span class="normal"><a href="#__codelineno-13-5">5</a></span>
<span class="normal"><a href="#__codelineno-13-6">6</a></span>
<span class="normal"><a href="#__codelineno-13-7">7</a></span>
<span class="normal"><a href="#__codelineno-13-8">8</a></span></pre></div></td><td class="code"><div><pre><span></span><code><a id="__codelineno-13-1" name="__codelineno-13-1"></a><span class="nb">set</span><span class="w"> </span>-o<span class="w"> </span>errexit<span class="w"> </span><span class="c1"># exit on error</span>
<a id="__codelineno-13-2" name="__codelineno-13-2"></a><span class="nb">set</span><span class="w"> </span>-o<span class="w"> </span>nounset<span class="w"> </span><span class="c1"># fail on unset variable (bypass with ${VAR:-})</span>
<a id="__codelineno-13-3" name="__codelineno-13-3"></a><span class="nb">set</span><span class="w"> </span>-o<span class="w"> </span>pipefail<span class="w"> </span><span class="c1"># file entire pipeline if one step fails</span>
<a id="__codelineno-13-4" name="__codelineno-13-4"></a>
<a id="__codelineno-13-5" name="__codelineno-13-5"></a><span class="c1"># hook to enable tracing</span>
<a id="__codelineno-13-6" name="__codelineno-13-6"></a><span class="k">if</span><span class="w"> </span><span class="o">[[</span><span class="w"> </span><span class="s2">&quot;</span><span class="si">${</span><span class="nv">TRACE</span><span class="p">-0</span><span class="si">}</span><span class="s2">&quot;</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="s2">&quot;1&quot;</span><span class="w"> </span><span class="o">]]</span><span class="p">;</span><span class="w"> </span><span class="k">then</span>
<a id="__codelineno-13-7" name="__codelineno-13-7"></a><span class="w"> </span><span class="nb">set</span><span class="w"> </span>-o<span class="w"> </span>xtrace
<a id="__codelineno-13-8" name="__codelineno-13-8"></a><span class="k">fi</span>
</code></pre></div></td></tr></table></div>
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