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scripting-team: add syntax highlighting to parameter_expansion.html

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git-svn-id: svn+ssh://atelier.inf.usi.ch/home/bevilj/group-1@160 a672b425-5310-4d7a-af5c-997e18724b81
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terehm 2018-11-13 14:56:12 +00:00
parent 872d2ff86d
commit 6edac45c5d
1 changed files with 18 additions and 16 deletions

34
site/pages/scripts/parameter_expansion.html
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@ -10,35 +10,35 @@ title: Parameter expansion
<!-- TODO: insert correct link once page exists -->
There are some special operations that can be performed on <a href="#">variables</a> and strings called parameter expansions.
The general syntax for all parameter expansions is this one:
<pre>
{% highlight bash %}
${CODE_HERE}
</pre>
{% endhighlight %}
Depending on what you want to do with your variable, the code that goes inside the braces differs.<br>
<br>
<pre>
{% highlight bash %}
${VARIABLE:-DEFAULT_VALUE}
</pre>
{% endhighlight %}
If the variable VARIABLE exists and has a value (i.e. it is not null), this is equal to the value of VARIABLE.
Otherwise, it is equal to DEFAULT_VALUE.<br>
Example: <code>echo "First name: ${firstname:-John}";</code><br>
<br>
<pre>
{% highlight bash %}
${VARIABLE:=DEFAULT_VALUE}
</pre>
{% endhighlight %}
If the variable VARIABLE exists and has a value, this is equal to the value of VARIABLE.
Otherwise, it sets the variable to DEFAULT_VALUE and is equal to it.<br>
Example: <code>echo "Last name: ${lastname:=Doe}";</code><br>
<br>
<pre>
{% highlight bash %}
${VARIABLE:?ERR_VALUE}
</pre>
{% endhighlight %}
If the variable VARIABLE exists and has a value, this is equal to the value of VARIABLE.
Otherwise, it prints ERR_VALUE to STDERR and exits (meaning nothing else will be executed after this).<br>
Example: <code>currdate=${date:?Operation failed: date unknown};</code><br>
<br>
<pre>
{% highlight bash %}
${VARIABLE:+VALUE}
</pre>
{% endhighlight %}
If the variable VARIABLE exists and has a value, this is equal to VALUE.
Otherwise, this has no effect.<br>
Example: <code>echo -e "reading from address $read.${write:+\nWARNING: read and write set at the same time}";</code><br>
@ -47,23 +47,25 @@ All of these can also be written without the colon, in which case their meaning
"If the variable VARIABLE exists at all (even if it is null), this is ..."<br>
<br>
<br>
<pre>
{% highlight bash %}
${VARIABLE: NUMBER}
</pre>
{% endhighlight %}
This is equal to the substring of the value of VARIABLE, starting at the character with (0-based) index NUMBER <br>
If NUMBER is negative, the substring starts NUMBER characters before the end of the string.<br>
Example: <code>lastname=${fullname:$firstnamelength};</code><br>
<br>
<pre>
{% highlight bash %}
${VARIABLE: FROM:LENGTH}
</pre>
{% endhighlight %}
This is equal to the substring of the value of VARIABLE, starting at the character with (0-based) index FROM with length LENGTH<br>
If FROM is negative, the substring starts FROM characters before the end of the string.<br>
Example: <code>lastname=${middlename:$firstnamelength:$middlenamelength};</code><br>
<br>
<pre>
{% highlight bash %}
${#VARIABLE}
</pre>
{% endhighlight %}
This is equal to the length of the value of VARIABLE<br>
Example: <code>echo "your name has ${#name} characters";</code><br>
<br>
<br>
Further reading: <a href="http://www.gnu.org/software/bash/manual/bashref.html#Shell-Parameter-Expansion">the bash reference manual</a>