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scripting-team: add page on parameter expansion

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git-svn-id: svn+ssh://atelier.inf.usi.ch/home/bevilj/group-1@159 a672b425-5310-4d7a-af5c-997e18724b81
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terehm 2018-11-13 14:32:22 +00:00
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---
layout: page
category-page: scripts
category-title: Scripting
tags: parameter expansion brace variable check condition
author: Marco Tereh
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>
${CODE_HERE}
</pre>
Depending on what you want to do with your variable, the code that goes inside the braces differs.<br>
<br>
<pre>
${VARIABLE:-DEFAULT_VALUE}
</pre>
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>
${VARIABLE:=DEFAULT_VALUE}
</pre>
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>
${VARIABLE:?ERR_VALUE}
</pre>
If the variable VARIABLE exists and has a value, this is equal to the value of VARIABLE.
Otherwise, it prints ERR_VALUE to STDERR and exits (meaning nothing else will be executed after this).<br>
Example: <code>currdate=${date:?Operation failed: date unknown};</code><br>
<br>
<pre>
${VARIABLE:+VALUE}
</pre>
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>
<br>
All of these can also be written without the colon, in which case their meaning changes to
"If the variable VARIABLE exists at all (even if it is null), this is ..."<br>
<br>
<br>
<pre>
${VARIABLE: NUMBER}
</pre>
This is equal to the substring of the value of VARIABLE, starting at the character with (0-based) index NUMBER <br>
If NUMBER is negative, the substring starts NUMBER characters before the end of the string.<br>
Example: <code>lastname=${fullname:$firstnamelength};</code><br>
<br>
<pre>
${VARIABLE: FROM:LENGTH}
</pre>
This is equal to the substring of the value of VARIABLE, starting at the character with (0-based) index FROM with length LENGTH<br>
If FROM is negative, the substring starts FROM characters before the end of the string.<br>
Example: <code>lastname=${middlename:$firstnamelength:$middlenamelength};</code><br>
<br>
<pre>
${#VARIABLE}
</pre>
This is equal to the length of the value of VARIABLE<br>
Example: <code>echo "your name has ${#name} characters";</code><br>
<br>