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                        <h1><a class="header" href="#bash1" id="bash1">bash(1)</a></h1>
<h2><a class="header" href="#expansion" id="expansion">Expansion</a></h2>
<h3><a class="header" href="#generator" id="generator">Generator</a></h3>
<pre><code class="language-bash"># generate sequence from n to m
{n..m}
# generate sequence from n to m step by s
{n..m..s}

# expand cartesian product
{a,b}{c,d}
</code></pre>
<h3><a class="header" href="#parameter" id="parameter">Parameter</a></h3>
<pre><code class="language-bash"># default value
bar=${foo:-some_val}  # if $foo set, then bar=$foo else bar=some_val

# alternate value
bar=${foo:+bla $foo}  # if $foo set, then bar=&quot;bla $foo&quot; else bar=&quot;&quot;

# check param set
bar=${foo:?msg}  # if $foo set, then bar=$foo else exit and print msg

# indirect
FOO=foo
BAR=FOO
bar=${!BAR}  # deref value of BAR -&gt; bar=$FOO

# prefix
${foo#prefix}  # remove prefix when expanding $foo
# suffix
${foo%suffix}  # remove suffix when expanding $foo

# substitute
${foo/pattern/string}  # replace pattern with string when expanding foo
# pattern starts with
# '/'   replace all occurences of pattern
# '#'   pattern match at beginning
# '%'   pattern match at end
</code></pre>
<blockquote>
<p>Note: <code>prefix</code>/<code>suffix</code>/<code>pattern</code> are expanded as <a href="#pathname">pathnames</a>.</p>
</blockquote>
<h3><a class="header" href="#pathname" id="pathname">Pathname</a></h3>
<pre><code class="language-bash">*           match any string
?           match any single char
\\          match backslash
[abc]       match any char of 'a' 'b' 'c'
[a-z]       match any char between 'a' - 'z'
[^ab]       negate, match all not 'a' 'b'
[:class:]   match any char in class, available:
              alnum,alpha,ascii,blank,cntrl,digit,graph,lower,
              print,punct,space,upper,word,xdigit
</code></pre>
<p>With <code>extglob</code> shell option enabled it is possible to have more powerful
patterns. In the following <code>pattern-list</code> is one ore more patterns separated
by <code>|</code> char.</p>
<pre><code class="language-bash">?(pattern-list)   matches zero or one occurrence of the given patterns
*(pattern-list)   matches zero or more occurrences of the given patterns
+(pattern-list)   matches one or more occurrences of the given patterns
@(pattern-list)   matches one of the given patterns
!(pattern-list)   matches anything except one of the given patterns
</code></pre>
<blockquote>
<p>Note: <code>shopt -s extglob</code>/<code>shopt -u extglob</code> to enable/disable <code>extglob</code>
option.</p>
</blockquote>
<h2><a class="header" href="#io-redirection" id="io-redirection">I/O redirection</a></h2>
<blockquote>
<p>Note: The trick with bash I/O redirection is to interpret from left-to-right.</p>
</blockquote>
<pre><code class="language-bash"># stdout &amp; stderr to file
command &gt;file 2&gt;&amp;1
# equivalent
command &amp;&gt;file

# stderr to stdout &amp; stdout to file
command 2&gt;&amp;1 &gt;file
</code></pre>
<blockquote>
<p>The article <a href="https://catonmat.net/bash-one-liners-explained-part-three">Bash One-Liners Explained, Part III: All about
redirections</a>
contains some nice visualization to explain bash redirections.</p>
</blockquote>
<h3><a class="header" href="#explanation" id="explanation">Explanation</a></h3>
<pre><code class="language-bash">j&gt;&amp;i
</code></pre>
<p>Duplicate <code>fd i</code> to <code>fd j</code>, making <code>j</code> a copy of <code>i</code>. See <a href="http://man7.org/linux/man-pages/man2/dup.2.html">dup2(2)</a>.</p>
<p>Example:</p>
<pre><code class="language-bash">command 2&gt;&amp;1 &gt;file
</code></pre>
<ol>
<li>duplicate <code>fd 1</code> to <code>fd 2</code>, effectively redirecting <code>stderr</code> to <code>stdout</code></li>
<li>redirect <code>stdout</code> to <code>file</code></li>
</ol>
<h2><a class="header" href="#argument-parsing-with-getopts" id="argument-parsing-with-getopts">Argument parsing with <code>getopts</code></a></h2>
<p>The <code>getopts</code> builtin uses following global variables:</p>
<ul>
<li><code>OPTARG</code>, value of last option argument</li>
<li><code>OPTIND</code>, index of the next argument to process (user must reset)</li>
<li><code>OPTERR</code>, display errors if set to <code>1</code></li>
</ul>
<pre><code class="language-bash">getopts &lt;optstring&gt; &lt;param&gt; [&lt;args&gt;]
</code></pre>
<ul>
<li><code>&lt;optstring&gt;</code> specifies the names of supported options, eg <code>f:c</code>
<ul>
<li><code>f:</code> means <code>-f</code> option with an argument</li>
<li><code>c</code> means <code>-c</code> option without an argument</li>
</ul>
</li>
<li><code>&lt;param&gt;</code> specifies a variable name which <code>getopts</code> fills with the last parsed option argument</li>
<li><code>&lt;args&gt;</code> optionally specify argument string to parse, by default <code>getopts</code> parses <code>$@</code></li>
</ul>
<h3><a class="header" href="#example" id="example">Example</a></h3>
<pre><code class="language-bash">#!/bin/bash
function parse_args() {
    while getopts &quot;f:c&quot; PARAM; do
        case $PARAM in
            f) echo &quot;GOT -f $OPTARG&quot;;;
            c) echo &quot;GOT -c&quot;;;
            *) echo &quot;ERR: print usage&quot;; exit 1;;
        esac
    done
    # users responsibility to reset OPTIND
    OPTIND=1
}

parse_args -f xxx -c
parse_args -f yyy
</code></pre>
<h2><a class="header" href="#regular-expressions" id="regular-expressions">Regular Expressions</a></h2>
<p>Bash supports regular expression matching with the binary operator <code>=~</code>.
The match results can be accessed via the <code>$BASH_REMATCH</code> variable:</p>
<ul>
<li><code>${BASH_REMATCH[0]}</code> contains the full match</li>
<li><code>${BASH_REMATCH[1]}</code> contains match of the first capture group</li>
</ul>
<pre><code class="language-bash">INPUT='title foo : 1234'
REGEX='^title (.+) : ([0-9]+)$'
if [[ $INPUT =~ $REGEX ]]; then
    echo &quot;${BASH_REMATCH[0]}&quot;    # title foo : 1234
    echo &quot;${BASH_REMATCH[1]}&quot;    # foo
    echo &quot;${BASH_REMATCH[2]}&quot;    # 1234
fi
</code></pre>
<blockquote>
<p><strong>Caution</strong>: When specifying a <code>regex</code> in the <code>[[ ]]</code> block directly, quotes will be treated as part of the pattern.
<code>[[ $INPUT =~ &quot;foo&quot; ]]</code> will match against <code>&quot;foo&quot;</code> not <code>foo</code>!</p>
</blockquote>
<h2><a class="header" href="#completion" id="completion">Completion</a></h2>
<p>The <code>complete</code> builtin is used to interact with the completion system.</p>
<pre><code class="language-bash">complete                    # print currently installed completion handler
complete -F &lt;func&gt; &lt;cmd&gt;    # install &lt;func&gt; as completion handler for &lt;cmd&gt;
complete -r &lt;cmd&gt;           # uninstall completion handler for &lt;cmd&gt;
</code></pre>
<p>Variables available in completion functions:</p>
<pre><code class="language-bash"># in
$1              # &lt;cmd&gt;
$2              # current word
$3              # privous word

COMP_WORDS      # array with current command line words
COMP_CWORD      # index into COMP_WORDS with current cursor position

# out
COMPREPLY       # array with possible completions
</code></pre>
<p>The <code>compgen</code> builtin is used to generate possible matches by comparing <code>word</code>
against words generated by <code>option</code>.</p>
<pre><code class="language-bash">compgen &lt;option&gt; &lt;word&gt;

# usefule options:
# -W &lt;list&gt;    specify list of possible completions
# -d           generate list with dirs
# -f           generate list with files
# -u           generate list with users
# -e           generate list with exported variables

# compare &quot;f&quot; against words &quot;foo&quot; &quot;foobar&quot; &quot;bar&quot; and generate matches
compgen -W &quot;foo foobar bar&quot; &quot;f&quot;

# compare &quot;hom&quot; against file/dir names and generate matches
compgen -d -f &quot;hom&quot;
</code></pre>
<h3><a class="header" href="#example-1" id="example-1">Example</a></h3>
<p>Skeleton to copy/paste for writing simple completions.</p>
<p>Assume a program <code>foo</code> with the following interface:</p>
<pre><code class="language-bash">foo -c green|red|blue -s low|high -f &lt;file&gt; -h
</code></pre>
<p>The completion handler could be implemented as follows:</p>
<pre><code class="language-bash">function _foo() {
    local curr=$2
    local prev=$3

    local opts=&quot;-c -s -f -h&quot;
    case $prev in
        -c) COMPREPLY=( $(compgen -W &quot;green red blue&quot; -- $curr) );;
        -s) COMPREPLY=( $(compgen -W &quot;low high&quot; -- $curr) );;
        -f) COMPREPLY=( $(compgen -f -- $curr) );;
        *)  COMPREPLY=( $(compgen -W &quot;$opts&quot; -- $curr) );;
    esac
}

complete -F _foo foo
</code></pre>

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                        <h1><a class="header" href="#bash1" id="bash1">bash(1)</a></h1>
<h2><a class="header" href="#expansion" id="expansion">Expansion</a></h2>
<h3><a class="header" href="#generator" id="generator">Generator</a></h3>
<pre><code class="language-bash"># generate sequence from n to m
{n..m}
# generate sequence from n to m step by s
{n..m..s}

# expand cartesian product
{a,b}{c,d}
</code></pre>
<h3><a class="header" href="#parameter" id="parameter">Parameter</a></h3>
<pre><code class="language-bash"># default value
bar=${foo:-some_val}  # if $foo set, then bar=$foo else bar=some_val

# alternate value
bar=${foo:+bla $foo}  # if $foo set, then bar=&quot;bla $foo&quot; else bar=&quot;&quot;

# check param set
bar=${foo:?msg}  # if $foo set, then bar=$foo else exit and print msg

# indirect
FOO=foo
BAR=FOO
bar=${!BAR}  # deref value of BAR -&gt; bar=$FOO

# prefix
${foo#prefix}  # remove prefix when expanding $foo
# suffix
${foo%suffix}  # remove suffix when expanding $foo

# substitute
${foo/pattern/string}  # replace pattern with string when expanding foo
# pattern starts with
# '/'   replace all occurences of pattern
# '#'   pattern match at beginning
# '%'   pattern match at end
</code></pre>
<blockquote>
<p>Note: <code>prefix</code>/<code>suffix</code>/<code>pattern</code> are expanded as <a href="#pathname">pathnames</a>.</p>
</blockquote>
<h3><a class="header" href="#pathname" id="pathname">Pathname</a></h3>
<pre><code class="language-bash">*           match any string
?           match any single char
\\          match backslash
[abc]       match any char of 'a' 'b' 'c'
[a-z]       match any char between 'a' - 'z'
[^ab]       negate, match all not 'a' 'b'
[:class:]   match any char in class, available:
              alnum,alpha,ascii,blank,cntrl,digit,graph,lower,
              print,punct,space,upper,word,xdigit
</code></pre>
<p>With <code>extglob</code> shell option enabled it is possible to have more powerful
patterns. In the following <code>pattern-list</code> is one ore more patterns separated
by <code>|</code> char.</p>
<pre><code class="language-bash">?(pattern-list)   matches zero or one occurrence of the given patterns
*(pattern-list)   matches zero or more occurrences of the given patterns
+(pattern-list)   matches one or more occurrences of the given patterns
@(pattern-list)   matches one of the given patterns
!(pattern-list)   matches anything except one of the given patterns
</code></pre>
<blockquote>
<p>Note: <code>shopt -s extglob</code>/<code>shopt -u extglob</code> to enable/disable <code>extglob</code>
option.</p>
</blockquote>
<h2><a class="header" href="#io-redirection" id="io-redirection">I/O redirection</a></h2>
<blockquote>
<p>Note: The trick with bash I/O redirection is to interpret from left-to-right.</p>
</blockquote>
<pre><code class="language-bash"># stdout &amp; stderr to file
command &gt;file 2&gt;&amp;1
# equivalent
command &amp;&gt;file

# stderr to stdout &amp; stdout to file
command 2&gt;&amp;1 &gt;file
</code></pre>
<blockquote>
<p>The article <a href="https://catonmat.net/bash-one-liners-explained-part-three">Bash One-Liners Explained, Part III: All about
redirections</a>
contains some nice visualization to explain bash redirections.</p>
</blockquote>
<h3><a class="header" href="#explanation" id="explanation">Explanation</a></h3>
<pre><code class="language-bash">j&gt;&amp;i
</code></pre>
<p>Duplicate <code>fd i</code> to <code>fd j</code>, making <code>j</code> a copy of <code>i</code>. See <a href="http://man7.org/linux/man-pages/man2/dup.2.html">dup2(2)</a>.</p>
<p>Example:</p>
<pre><code class="language-bash">command 2&gt;&amp;1 &gt;file
</code></pre>
<ol>
<li>duplicate <code>fd 1</code> to <code>fd 2</code>, effectively redirecting <code>stderr</code> to <code>stdout</code></li>
<li>redirect <code>stdout</code> to <code>file</code></li>
</ol>
<h2><a class="header" href="#argument-parsing-with-getopts" id="argument-parsing-with-getopts">Argument parsing with <code>getopts</code></a></h2>
<p>The <code>getopts</code> builtin uses following global variables:</p>
<ul>
<li><code>OPTARG</code>, value of last option argument</li>
<li><code>OPTIND</code>, index of the next argument to process (user must reset)</li>
<li><code>OPTERR</code>, display errors if set to <code>1</code></li>
</ul>
<pre><code class="language-bash">getopts &lt;optstring&gt; &lt;param&gt; [&lt;args&gt;]
</code></pre>
<ul>
<li><code>&lt;optstring&gt;</code> specifies the names of supported options, eg <code>f:c</code>
<ul>
<li><code>f:</code> means <code>-f</code> option with an argument</li>
<li><code>c</code> means <code>-c</code> option without an argument</li>
</ul>
</li>
<li><code>&lt;param&gt;</code> specifies a variable name which <code>getopts</code> fills with the last parsed option argument</li>
<li><code>&lt;args&gt;</code> optionally specify argument string to parse, by default <code>getopts</code> parses <code>$@</code></li>
</ul>
<h3><a class="header" href="#example" id="example">Example</a></h3>
<pre><code class="language-bash">#!/bin/bash
function parse_args() {
    while getopts &quot;f:c&quot; PARAM; do
        case $PARAM in
            f) echo &quot;GOT -f $OPTARG&quot;;;
            c) echo &quot;GOT -c&quot;;;
            *) echo &quot;ERR: print usage&quot;; exit 1;;
        esac
    done
    # users responsibility to reset OPTIND
    OPTIND=1
}

parse_args -f xxx -c
parse_args -f yyy
</code></pre>
<h2><a class="header" href="#regular-expressions" id="regular-expressions">Regular Expressions</a></h2>
<p>Bash supports regular expression matching with the binary operator <code>=~</code>.
The match results can be accessed via the <code>$BASH_REMATCH</code> variable:</p>
<ul>
<li><code>${BASH_REMATCH[0]}</code> contains the full match</li>
<li><code>${BASH_REMATCH[1]}</code> contains match of the first capture group</li>
</ul>
<pre><code class="language-bash">INPUT='title foo : 1234'
REGEX='^title (.+) : ([0-9]+)$'
if [[ $INPUT =~ $REGEX ]]; then
    echo &quot;${BASH_REMATCH[0]}&quot;    # title foo : 1234
    echo &quot;${BASH_REMATCH[1]}&quot;    # foo
    echo &quot;${BASH_REMATCH[2]}&quot;    # 1234
fi
</code></pre>
<blockquote>
<p><strong>Caution</strong>: When specifying a <code>regex</code> in the <code>[[ ]]</code> block directly, quotes will be treated as part of the pattern.
<code>[[ $INPUT =~ &quot;foo&quot; ]]</code> will match against <code>&quot;foo&quot;</code> not <code>foo</code>!</p>
</blockquote>
<h2><a class="header" href="#completion" id="completion">Completion</a></h2>
<p>The <code>complete</code> builtin is used to interact with the completion system.</p>
<pre><code class="language-bash">complete                    # print currently installed completion handler
complete -F &lt;func&gt; &lt;cmd&gt;    # install &lt;func&gt; as completion handler for &lt;cmd&gt;
complete -r &lt;cmd&gt;           # uninstall completion handler for &lt;cmd&gt;
</code></pre>
<p>Variables available in completion functions:</p>
<pre><code class="language-bash"># in
$1              # &lt;cmd&gt;
$2              # current word
$3              # privous word

COMP_WORDS      # array with current command line words
COMP_CWORD      # index into COMP_WORDS with current cursor position

# out
COMPREPLY       # array with possible completions
</code></pre>
<p>The <code>compgen</code> builtin is used to generate possible matches by comparing <code>word</code>
against words generated by <code>option</code>.</p>
<pre><code class="language-bash">compgen &lt;option&gt; &lt;word&gt;

# usefule options:
# -W &lt;list&gt;    specify list of possible completions
# -d           generate list with dirs
# -f           generate list with files
# -u           generate list with users
# -e           generate list with exported variables

# compare &quot;f&quot; against words &quot;foo&quot; &quot;foobar&quot; &quot;bar&quot; and generate matches
compgen -W &quot;foo foobar bar&quot; &quot;f&quot;

# compare &quot;hom&quot; against file/dir names and generate matches
compgen -d -f &quot;hom&quot;
</code></pre>
<h3><a class="header" href="#example-1" id="example-1">Example</a></h3>
<p>Skeleton to copy/paste for writing simple completions.</p>
<p>Assume a program <code>foo</code> with the following interface:</p>
<pre><code class="language-bash">foo -c green|red|blue -s low|high -f &lt;file&gt; -h
</code></pre>
<p>The completion handler could be implemented as follows:</p>
<pre><code class="language-bash">function _foo() {
    local curr=$2
    local prev=$3

    local opts=&quot;-c -s -f -h&quot;
    case $prev in
        -c) COMPREPLY=( $(compgen -W &quot;green red blue&quot; -- $curr) );;
        -s) COMPREPLY=( $(compgen -W &quot;low high&quot; -- $curr) );;
        -f) COMPREPLY=( $(compgen -f -- $curr) );;
        *)  COMPREPLY=( $(compgen -W &quot;$opts&quot; -- $curr) );;
    esac
}

complete -F _foo foo
</code></pre>

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