96 lines
3.3 KiB
HTML
96 lines
3.3 KiB
HTML
---
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layout: page
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category-page: scripts
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category-title: Scripting
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tags: parameter expansion brace variable check condition empty exists
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author: Marco Tereh
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title: Parameter expansion
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previous-page: pages/scripts/2-special-variables.html
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next-page: pages/scripts/4-arrays.html
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---
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There are some special operations that can be performed on
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<a href="variables.html">variables</a> and strings called parameter expansions.
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The general syntax for all parameter expansions is this one:
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{% highlight bash %}
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${CODE_HERE}
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{% endhighlight %}
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Depending on what you want to do with your variable, the code
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that goes inside the braces differs.<br>
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{% highlight bash %}
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${VARIABLE:-DEFAULT_VALUE}
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{% endhighlight %}
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If the variable VARIABLE exists and has a value (i.e. it is not null), this is equal to
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the value of VARIABLE.
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Otherwise, it is equal to DEFAULT_VALUE.<br>
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Example: <code>echo "First name: ${firstname:-John}";</code><br>
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{% highlight bash %}
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${VARIABLE:=DEFAULT_VALUE}
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{% endhighlight %}
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If the variable VARIABLE exists and has a value, this is equal to the value of VARIABLE.
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Otherwise, it sets the variable to DEFAULT_VALUE and is equal to it.<br>
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Example: <code>echo "Last name: ${lastname:=Doe}";</code><br>
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{% highlight bash %}
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${VARIABLE:?ERR_VALUE}
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{% endhighlight %}
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If the variable VARIABLE exists and has a value, this is equal to the value of VARIABLE.
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Otherwise, it prints ERR_VALUE to STDERR and exits (meaning nothing else will be executed
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after this).<br>
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Example: <code>currdate=${date:?Operation failed: date unknown};</code><br>
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{% highlight bash %}
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${VARIABLE:+VALUE}
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{% endhighlight %}
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If the variable VARIABLE exists and has a value, this is equal to VALUE.
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Otherwise, this has no effect.<br>
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Example: <code>echo -e "reading from address $read.${write:+\nWARNING: read and write set
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at the same time}";</code><br>
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All of these can also be written without the colon, in which case their meaning changes to
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"If the variable VARIABLE exists at all (even if it is null), this is ..."<br>
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{% highlight bash %}
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${VARIABLE: NUMBER}
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{% endhighlight %}
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This is equal to the substring of the value of VARIABLE, starting at the character with
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(0-based) index NUMBER.<br>
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If NUMBER is negative, the substring starts NUMBER characters before the end of the string.<br>
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Example: <code>lastname=${fullname:$firstnamelength};</code><br>
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{% highlight bash %}
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${VARIABLE: FROM:LENGTH}
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{% endhighlight %}
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This is equal to the substring of the value of VARIABLE, starting at the character with (0-based)
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index FROM with length LENGTH<br>
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If FROM is negative, the substring starts FROM characters before the end of the string.<br>
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Example: <code>lastname=${middlename:$firstnamelength:$middlenamelength};</code><br>
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<!-- If/when somebody makes the page on arithmetic operations, mention that NUMBER can be an arithmetic operation as well
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and link it. -->
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{% highlight bash %}
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${#VARIABLE}
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{% endhighlight %}
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This is equal to the length of the value of VARIABLE<br>
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Example: <code>echo "your name has ${#name} characters";</code><br>
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Paremeter expansions can also be nested, like this:
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{% highlight bash %}
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${input:?${INVALID_INPUT_ERR_MSG:-An unknown error occurred}}
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{% endhighlight %}
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Further reading: <a href="http://www.gnu.org/software/bash/manual/bashref.html#Shell-Parameter-Expansion">
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the bash reference manual</a>
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