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"description" : "More info is in this tutorial\nall value expansions (ie. all syntax with a $ prefix) can only expand inside quoted arguments if the argument was double-quoted . Single quotes will turn the dollar-syntax into literal characters, causing bash to output the dollar rather than expand its value in-place!\n\u0026ldquo;Value expansions ($...) must always be double-quoted.\u0026rdquo;\nNever leave a value expansion unquoted.\nRedirection File Redirection 0 is standard input, 1 is standard output, 2 is standard error",
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<title>Mess with Bash(2)</title>
<meta property="og:title" content="Mess with Bash(2)" />
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<meta property="og:description" content="More info is in this tutorial
all value expansions (ie. all syntax with a $ prefix) can only expand inside quoted arguments if the argument was double-quoted . Single quotes will turn the dollar-syntax into literal characters, causing bash to output the dollar rather than expand its value in-place!
&ldquo;Value expansions ($...) must always be double-quoted.&rdquo;
Never leave a value expansion unquoted.
Redirection File Redirection 0 is standard input, 1 is standard output, 2 is standard error" />
<meta name="description" content="More info is in this tutorial
all value expansions (ie. all syntax with a $ prefix) can only expand inside quoted arguments if the argument was double-quoted . Single quotes will turn the dollar-syntax into literal characters, causing bash to output the dollar rather than expand its value in-place!
&ldquo;Value expansions ($...) must always be double-quoted.&rdquo;
Never leave a value expansion unquoted.
Redirection File Redirection 0 is standard input, 1 is standard output, 2 is standard error" />
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<h1 class="post-title">Mess with Bash(2)</h1>
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<time class="post-date" datetime="2022-09-29 09:43:02 CST">
29 Sep 2022
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<a target="_blank" href="https://garhve.com">@garhve</a>
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<blockquote>
<p>More info is in this <a href="https://https://guide.bash.academy">tutorial</a></p>
</blockquote>
<blockquote>
<p>all value expansions (ie. all syntax with a <code>$</code> prefix) can only expand inside quoted arguments if the argument was <em>double-quoted</em> . Single quotes will turn the dollar-syntax into literal characters, causing bash to output the dollar rather than expand its value in-place!</p>
</blockquote>
<p><em>“Value expansions (<code>$...</code>) must <strong>always</strong> be double-quoted.”</em></p>
<p><em>Never leave a value expansion unquoted.</em></p>
<h2 id="redirection">Redirection</h2>
<h3 id="file-redirection">File Redirection</h3>
<p><strong>0 is standard input, 1 is standard output, 2 is standard error</strong></p>
<p><code>[x] > file, [x] < file</code></p>
<p>Make <em><strong>File descriptor</strong></em>(FD) <code>x</code> write to / read from file.</p>
<blockquote>
<p><code>echo hello > ~/World</code></p>
<p><code>read line < ~/Word</code></p>
<p><code>rm file 2>/dev/null</code></p>
</blockquote>
<h3 id="file-descriptor-copying">File Descriptor copying</h3>
<p><code>[x] >& y, [x] <& y</code></p>
<p>make FD <code>x</code> write to / read from FD <code>y</code>’s stream</p>
<blockquote>
<p>the connection to the stream used by FD <code>y</code> is copied to FD <code>x</code></p>
<p><code>curl cip.cc > result 2>&1</code></p>
<p><code>ping localhost > result 2>&1</code></p>
<div class="highlight"><pre tabindex="0" style="background-color:#fff;-moz-tab-size:2;-o-tab-size:2;tab-size:2;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span><span style="font-style:italic"># exec can be used to change the file descriptors of bash itself,</span>
</span></span><span style="display:flex;"><span><span style="font-style:italic"># and if you use an x that doesn't yet exist,</span>
</span></span><span style="display:flex;"><span><span style="font-style:italic"># bash will create a new file descriptor ("plug") for you with that number.</span>
</span></span><span style="display:flex;"><span><span style="font-style:italic"># - in command is to close new FD 3 we'd created before.</span>
</span></span><span style="display:flex;"><span><span style="font-style:italic"># >&- is to close FD 1, <&- is to close FD0</span>
</span></span><span style="display:flex;"><span>exec 3>&1 >mylog; echo moo; exec 1>&3 3>&-
</span></span></code></pre></div></blockquote>
<h3 id="appending-file-redirection">Appending file redirection</h3>
<p><code>[x] >> file</code></p>
<p>make FD <code>x</code> append to end of the file</p>
<blockquote>
<p>A stream to <code>file</code> is opened for writing in append mode and is connected to file descriptor <code>x</code>. The regular file redirection operator <code>></code> empties the file’s contents when it opens the file so that only your bytes will be in the file.</p>
<p><code>echo hello >> ~/world</code></p>
<p><code>echo world >> ~/world</code></p>
</blockquote>
<h3 id="redirecting-standard-output-and-standard-error">Redirecting standard output and standard error</h3>
<p><code>&>file</code></p>
<p>Make both FD 1 (standard output) and FD 2 (standard error) write to file</p>
<blockquote>
<p>This is a convenience operator which does the same thing as <code>>file 2>&1</code> but is more concise. Again, you can append rather than truncate by doubling the arrow: <code>&>>file</code></p>
<p><code>ping localhost &>result</code></p>
</blockquote>
<h3 id="here-documents">Here documents</h3>
<div class="highlight"><pre tabindex="0" style="background-color:#fff;-moz-tab-size:2;-o-tab-size:2;tab-size:2;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span><span style="font-style:italic"><<delimiter
</span></span></span><span style="display:flex;"><span><span style="font-style:italic"> Here document
</span></span></span><span style="display:flex;"><span><span style="font-style:italic">delimiter</span>
</span></span></code></pre></div><p>Make FD 0 read from the string between <code>delimiter</code>s</p>
<blockquote>
<p>Here-Documents are great for reading blocks of text to command line.</p>
<div class="highlight"><pre tabindex="0" style="background-color:#fff;-moz-tab-size:2;-o-tab-size:2;tab-size:2;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span>cat <span style="font-style:italic"><< EOF
</span></span></span><span style="display:flex;"><span><span style="font-style:italic">this is within here document
</span></span></span><span style="display:flex;"><span><span style="font-style:italic">I can write as many lines as I like
</span></span></span><span style="display:flex;"><span><span style="font-style:italic">and terminate with line of demiliter only
</span></span></span><span style="display:flex;"><span><span style="font-style:italic">EOF</span> //end of heredoc
</span></span></code></pre></div></blockquote>
<h3 id="here-strings">Here strings</h3>
<p><code><<< string</code></p>
<p>Make FD 0 read from the <code>string</code></p>
<blockquote>
<p>Here strings are very similar to here documents but more concise. They are generally preferred over here documents.</p>
<div class="highlight"><pre tabindex="0" style="background-color:#fff;-moz-tab-size:2;-o-tab-size:2;tab-size:2;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span>cat <<< <span style="font-style:italic">"This,
</span></span></span><span style="display:flex;"><span><span style="font-style:italic">is the here strings. tab will also be read."</span>
</span></span></code></pre></div></blockquote>
<h3 id="moving-file-decipher">Moving file decipher</h3>
<p><code>[x]>&y-, [x]<&y-</code></p>
<p>Replace FD <code>x</code> with FD <code>y</code> and close FD <code>y</code></p>
<blockquote>
<p>Easy way of <code>[x]>&y, y>&-</code></p>
<div class="highlight"><pre tabindex="0" style="background-color:#fff;-moz-tab-size:2;-o-tab-size:2;tab-size:2;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span><span style="font-style:italic"># 3>&1-: copy FD 1 to FD 3 and close FD 1.</span>
</span></span><span style="display:flex;"><span><span style="font-style:italic"># >&3-: copy FD 3 to FD 1 and close FD 3.</span>
</span></span><span style="display:flex;"><span>exec 3>&1- >mylog; echo moo; exec >&3-
</span></span></code></pre></div></blockquote>
<h3 id="reading-and-writing-with-file-descriptor">Reading and writing with file descriptor</h3>
<p><code>[x] <> file</code></p>
<p>Open FD <code>x</code> for both reading and writing to <code>file</code></p>
<blockquote>
<p>The file descriptor at x is opened with a stream to the file that can be used for writing as well as reading bytes. Usually you’ll use two file descriptors for this. <strong>One of the rare cases where this is useful is when setting up a stream with a read/write device such as a network socket</strong>.</p>
<div class="highlight"><pre tabindex="0" style="background-color:#fff;-moz-tab-size:2;-o-tab-size:2;tab-size:2;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span>exec 5<>aFile
</span></span><span style="display:flex;"><span>cat >&5 <span style="font-style:italic">"Hello world"</span> <span style="font-style:italic"># make FD 1 write to where FD 5 currently writing, copy file descriptor FD 5 to FD 1</span>
</span></span><span style="display:flex;"><span>cat <&5 <span style="font-style:italic"># make FD 0 read from where FD 5 currently reading, copy file descriptor FD 5 to FD 0, then cat will send content to FD 1</span>
</span></span></code></pre></div></blockquote>
<p>Exercise</p>
<blockquote>
<p>Q: fix <code>exec 3>&2 2>log; echo 'Hello!'; exec 2>&3</code> so that the message is properly saved into the <code>log</code> file and such that FD 3 is properly closed afterwards:</p>
<p>A:</p>
<ol>
<li><code>exec 3>&1- 3>log; echo 'Hello!'; exec 1>&3-</code></li>
<li><code>exec 3>&1 1>&- 3>log; echo 'Hello!'; exec 1>&3 3>&-</code></li>
</ol>
</blockquote>
<h2 id="expansion">Expansion</h2>
<h3 id="pathname-expansion">pathname expansion</h3>
<ol>
<li>
<p>pattern expansion is performed by bash before command even execute</p>
<blockquote>
<p><code>file *</code> will show info about all file in current directory. <code>*</code> will expand to content before <code>file</code> execute.</p>
</blockquote>
</li>
<li>
<p>A <em>glob</em> is the name of the type of pattern supported by the bash shell.</p>
<blockquote>
<p>basic glob name supported by bash<img src="https://assets.garhve.com/pictures/screenshots/2022/10/1665123982.png" alt="explanation"></p>
<p><img src="https://assets.garhve.com/pictures/screenshots/2022/10/1665133964.png" alt="example"></p>
<p><strong>Those glob will only affect current directory, explicit expression is required to working on other directory.</strong> <code>ls /sub/*</code></p>
</blockquote>
</li>
<li>
<p>extended glob can be enable to get more powerful but also easy confusing feature of bash</p>
<blockquote>
<p>bash: <code>shopt -s extglob</code>
zsh. : <code>setopt extendedglob</code>
<img src="https://assets.garhve.com/pictures/screenshots/2022/10/1665134156.png" alt="explanation"></p>
<p><img src="https://assets.garhve.com/pictures/screenshots/2022/10/1665134205.png" alt="!(my)* get expand because of * is outside !(), which makes it expland another whole pathname"></p>
</blockquote>
</li>
</ol>
<h3 id="command-substitution">Command Substitution</h3>
<p>we can expansion commands within commands, but must use double-quote <code>""</code> instead of <code>''</code></p>
<div class="highlight"><pre tabindex="0" style="background-color:#fff;-moz-tab-size:2;-o-tab-size:2;tab-size:2;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span><span style="font-style:italic"># this will output contents in hello.h to screen</span>
</span></span><span style="display:flex;"><span>cat hello.h
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="font-style:italic"># this will expand `cat hello.h` to real contents in </span>
</span></span><span style="display:flex;"><span><span style="font-style:italic"># file hello.h and concatenate to previous sentence</span>
</span></span><span style="display:flex;"><span>echo <span style="font-style:italic">"file hello.h contains contents of </span><span style="font-weight:bold">$(</span>cat hello.h<span style="font-weight:bold">)</span><span style="font-style:italic">"</span>
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="font-style:italic"># this will output 'file hello.h contains contents of $(cat hello.h)'</span>
</span></span><span style="display:flex;"><span><span style="font-style:italic"># without expand command in $()</span>
</span></span><span style="display:flex;"><span>echo <span style="font-style:italic">'file hello.h contains contents of $(cat hello.h)'</span>
</span></span></code></pre></div><p>In command, <code>$()</code> is called <em>value expansion</em>, it consists of value-expansion prefix <code>$</code> and subshell <code>(...)</code>. A subshell is essentially a small new bash process that is used to run a command while the main bash shell waits for the result.</p>
<h2 id="parameters">Parameters</h2>
<p>There are three kind of parameters:</p>
<ol>
<li>Environment Parameter</li>
<li>Positional Parameter</li>
<li>Variables</li>
</ol>
<h3 id="environment-parameter">Environment Parameter</h3>
<p>environment variables exist at the process level. That means they are not a feature of the bash shell, but rather a feature of any program process on your system. They can inherit by children, but children’s EV can’t be given to parent.</p>
<h3 id="positional-parameter">Positional Parameter</h3>
<p>Just as name indicates, these kind of parameters indicate arguments’ position, and always starting from 0.</p>
<p>for example, imaging we have a script <code>rename</code>, arguments could be passed to it to extend its usage:</p>
<p><code>rename dir name</code> there, we passed <code>dir</code> and <code>name</code> as argument, so that positional parameters in script would be <code>$1</code> and <code>$2</code>, representing arguments respectively. after <code>$2</code>, such as <code>$3</code> is unset since there has no more argument.</p>
<blockquote>
<p>Positional Parameter is read-only</p>
</blockquote>
<p>a new usage: <code>bash -c 'ls "$1"' -- '/home'</code>. This will working like <code>ls /home</code>, dash is necessary since it is first variable in shell we ran commands and it makes positional value of arguments populated after it stand as we expect in shell single-quoted command gonna run in.</p>
<h3 id="special-parameter">Special Parameter</h3>
<p>Special parameters are parameters whose name is a single symbolic character, they are used to request certain state information from the bash shell. Like positional parameter, they are <em>read-only</em>.</p>
<p><img src="https://assets.garhve.com/pictures/screenshots/2022/10/1666054171.png" alt="different kinds of special parameters and the information they hold"></p>
<h3 id="variables">Variables</h3>
<p>definition: <code>name=value</code> <em>//no space around <code>=</code> like other programming language support</em></p>
<p>call: like <em>command expansion</em>, using variable is to expand it with prefix <code>$</code>, e.g. <code>$name</code></p>
<ul>
<li>Keep in mind, Expansion should always be double-quoted *</li>
</ul>
<h3 id="parameter-expansion">Parameter expansion</h3>
<blockquote>
<p><a href="https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html">GNU material</a></p>
</blockquote>
<p>we expand parameters by prefixing their name with a <code>$</code> symbol</p>
<blockquote>
<p>e.g. <code>name=me; echo hello "$name"</code>. <code>hello me</code>.</p>
</blockquote>
<p>In addition, we can put braces (<code>{</code> and <code>}</code>) around our parameter, which indicates where variable is about to begin and end.</p>
<blockquote>
<p>e.g. <code>name=orange; echo there are 4 "${name}s"</code> <code>there are 4 oranges</code>.</p>
<div class="highlight"><pre tabindex="0" style="background-color:#fff;-moz-tab-size:2;-o-tab-size:2;tab-size:2;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span>name=orange
</span></span><span style="display:flex;"><span>echo <span style="font-style:italic">"there are 4 </span><span style="font-weight:bold;font-style:italic">${</span>name<span style="font-weight:bold;font-style:italic">}</span><span style="font-style:italic">s."</span> <span style="font-style:italic"># there are 4 oranges.</span>
</span></span><span style="display:flex;"><span>echo <span style="font-style:italic">"there are 4 </span>$names<span style="font-style:italic">."</span> <span style="font-style:italic"># there are 4 .</span>
</span></span></code></pre></div><p>here, we put <code>{}</code>around<code>name</code>so that bash can be told that suffix <code>s</code> is not a part of variable. otherwise, it will treat <code>names</code> as parameter and looking for its value, which is none in our example.</p>
</blockquote>
<p>parameter expansion brings up a powerful feature: <em>parameter expansion oerators</em></p>
<blockquote>
<p>While expanding a parameter, it is possible to apply an operator to the expanding value without alternate original value.</p>
<p>I use these mostly</p>
<div class="highlight"><pre tabindex="0" style="background-color:#fff;-moz-tab-size:2;-o-tab-size:2;tab-size:2;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span><span style="font-style:italic"># remove string before pattern ${name#pattern} shortest ${name##pattern} longest</span>
</span></span><span style="display:flex;"><span><span style="font-style:italic"># remove string after pattern ${name%pattern} longest ${name%%pattern} shortest</span>
</span></span><span style="display:flex;"><span><span style="font-style:italic"># delete first matching pattern ${name/pattern}</span>
</span></span><span style="display:flex;"><span><span style="font-style:italic"># delete all matching pattern ${name//pattern}</span>
</span></span><span style="display:flex;"><span><span style="font-style:italic"># substitute pattern with string ${name//pattern/string}</span>
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span>foo=<span style="font-style:italic">"foo-bar-foobar"</span>
</span></span><span style="display:flex;"><span>echo <span style="font-weight:bold;font-style:italic">${</span>foo#*-<span style="font-weight:bold;font-style:italic">}</span> <span style="font-style:italic"># echoes 'bar-foobar' (Removes 'foo-' because that matches '*-')</span>
</span></span><span style="display:flex;"><span>echo <span style="font-weight:bold;font-style:italic">${</span>foo##*-<span style="font-weight:bold;font-style:italic">}</span> <span style="font-style:italic"># echoes 'foobar' (Removes 'foo-bar-')</span>
</span></span><span style="display:flex;"><span>echo <span style="font-weight:bold;font-style:italic">${</span>foo%-*<span style="font-weight:bold;font-style:italic">}</span> <span style="font-style:italic"># echoes 'foo-bar'</span>
</span></span><span style="display:flex;"><span>echo <span style="font-weight:bold;font-style:italic">${</span>foo%%-*<span style="font-weight:bold;font-style:italic">}</span> <span style="font-style:italic"># echoes 'foo'</span>
</span></span><span style="display:flex;"><span>echo <span style="font-weight:bold;font-style:italic">${</span>foo/-<span style="font-weight:bold;font-style:italic">}</span> <span style="font-style:italic"># echoes 'foobar-foobar'</span>
</span></span><span style="display:flex;"><span>echo <span style="font-weight:bold;font-style:italic">${</span>foo//-<span style="font-weight:bold;font-style:italic">}</span> <span style="font-style:italic"># echoes 'foobarfoobar'</span>
</span></span><span style="display:flex;"><span>echo <span style="font-weight:bold;font-style:italic">${</span>foo//-/_<span style="font-weight:bold;font-style:italic">}</span> <span style="font-style:italic"># echoes 'foo_bar_foobar'</span>
</span></span></code></pre></div></blockquote>
<p><img src="https://assets.garhve.com/pictures/screenshots/2022/10/1665660923.png" alt="part of operators as shown picture"></p>
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