634 lines
9.3 KiB
HTML
634 lines
9.3 KiB
HTML
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>Types of variables</TITLE
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>Chapter 10. More on variables</TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><DIV
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CLASS="sect1"
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><H1
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CLASS="sect1"
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><A
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NAME="sect_10_01"
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></A
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>10.1. Types of variables</H1
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="sect_10_01_01"
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></A
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>10.1.1. General assignment of values</H2
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><P
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>As we already saw, Bash understands many different kinds of variables or parameters. Thus far, we haven't bothered much with what kind of variables we assigned, so our variables could hold any value that we assigned to them. A simple command line example demonstrates this:</P
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><TABLE
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> <TT
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CLASS="prompt"
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>[bob in ~]</TT
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> <B
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CLASS="command"
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><TT
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CLASS="varname"
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>VARIABLE</TT
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>=<TT
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CLASS="parameter"
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><I
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>12</I
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></TT
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></B
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>
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<TT
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CLASS="prompt"
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>[bob in ~]</TT
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> <B
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CLASS="command"
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>echo <TT
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CLASS="varname"
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>$VARIABLE</TT
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></B
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>
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12
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<TT
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CLASS="prompt"
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>[bob in ~]</TT
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> <B
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CLASS="command"
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><TT
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CLASS="varname"
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>VARIABLE</TT
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>=<TT
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CLASS="parameter"
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><I
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>string</I
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></TT
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></B
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>
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<TT
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CLASS="prompt"
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>[bob in ~]</TT
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> <B
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CLASS="command"
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>echo <TT
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CLASS="varname"
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>$VARIABLE</TT
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></B
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>
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string
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>There are cases when you want to avoid this kind of behavior, for instance when handling telephone and other numbers. Apart from integers and variables, you may also want to specify a variable that is a constant. This is often done at the beginning of a script, when the value of the constant is declared. After that, there are only references to the constant variable name, so that when the constant needs to be changed, it only has to be done once. A variable may also be a series of variables of any type, a so-called <EM
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>array</EM
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> of variables (<TT
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CLASS="varname"
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>VAR0</TT
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><TT
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CLASS="varname"
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>VAR1</TT
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>, <TT
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CLASS="varname"
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>VAR2</TT
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>, ... <TT
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CLASS="varname"
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>VARN</TT
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>).</P
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></DIV
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="sect_10_01_02"
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></A
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>10.1.2. Using the declare built-in</H2
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><P
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>Using a <B
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CLASS="command"
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>declare</B
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> statement, we can limit the value assignment to variables.</P
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><P
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>The syntax for <B
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CLASS="command"
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>declare</B
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> is the following:</P
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><P
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><B
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CLASS="command"
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>declare <TT
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CLASS="option"
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>OPTION(s)</TT
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> <TT
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CLASS="varname"
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>VARIABLE</TT
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>=value</B
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> </P
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><P
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>The following options are used to determine the type of data the variable can hold and to assign it attributes:</P
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><DIV
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CLASS="table"
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><A
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NAME="tab_10_01"
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></A
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><P
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><B
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>Table 10-1. Options to the declare built-in</B
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></P
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><TABLE
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BORDER="1"
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CLASS="CALSTABLE"
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><THEAD
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><TR
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><TH
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Option</TH
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><TH
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Meaning</TH
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></TR
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></THEAD
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><TBODY
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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><TT
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CLASS="option"
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>-a</TT
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Variable is an array.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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><TT
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CLASS="option"
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>-f</TT
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Use function names only.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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><TT
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CLASS="option"
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>-i</TT
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>The variable is to be treated as an integer; arithmetic evaluation is performed when the variable is assigned a value (see <A
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HREF="sect_03_04.html#sect_03_04_05"
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>Section 3.4.6</A
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>).</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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><TT
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CLASS="option"
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>-p</TT
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Display the attributes and values of each variable. When <TT
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CLASS="option"
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>-p</TT
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> is used, additional options are ignored.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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><TT
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CLASS="option"
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>-r</TT
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Make variables read-only. These variables cannot then be assigned values by subsequent assignment statements, nor can they be unset.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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><TT
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CLASS="option"
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>-t</TT
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Give each variable the <EM
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>trace</EM
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> attribute.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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><TT
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CLASS="option"
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>-x</TT
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Mark each variable for export to subsequent commands via the environment.</TD
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></TR
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></TBODY
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></TABLE
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></DIV
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><P
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>Using <TT
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CLASS="option"
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>+</TT
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> instead of <TT
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CLASS="option"
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>-</TT
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> turns off the attribute instead. When used in a function, <B
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CLASS="command"
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>declare</B
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> creates local variables.</P
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><P
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>The following example shows how assignment of a type to a variable influences the value.</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> <TT
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CLASS="prompt"
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>[bob in ~]</TT
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> <B
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CLASS="command"
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>declare <TT
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CLASS="option"
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>-i</TT
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> <TT
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CLASS="varname"
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>VARIABLE</TT
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>=<TT
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CLASS="parameter"
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><I
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>12</I
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></TT
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></B
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>
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<TT
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CLASS="prompt"
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>[bob in ~]</TT
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> <B
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CLASS="command"
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><TT
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CLASS="varname"
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>VARIABLE</TT
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>=<TT
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CLASS="parameter"
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><I
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>string</I
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></TT
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></B
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>
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<TT
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CLASS="prompt"
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>[bob in ~]</TT
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> <B
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CLASS="command"
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>echo <TT
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CLASS="varname"
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>$VARIABLE</TT
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></B
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>
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0
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<TT
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CLASS="prompt"
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>[bob in ~]</TT
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> <B
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CLASS="command"
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>declare <TT
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CLASS="option"
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>-p</TT
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> <TT
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CLASS="varname"
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>VARIABLE</TT
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></B
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>
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declare -i VARIABLE="0"
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>Note that Bash has an option to declare a numeric value, but none for declaring string values. This is because, by default, if no specifications are given, a variable can hold any type of data:</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> <TT
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CLASS="prompt"
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>[bob in ~]</TT
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> <B
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CLASS="command"
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><TT
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CLASS="varname"
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>OTHERVAR</TT
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>=<TT
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CLASS="parameter"
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><I
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>blah</I
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></TT
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></B
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>
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<TT
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CLASS="prompt"
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>[bob in ~]</TT
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> <B
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CLASS="command"
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>declare <TT
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CLASS="option"
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>-p</TT
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> <TT
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CLASS="varname"
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>OTHERVAR</TT
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></B
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>
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declare -- OTHERVAR="blah"
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>As soon as you restrict assignment of values to a variable, it can only hold that type of data. Possible restrictions are either integer, constant or array.</P
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><P
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>See the Bash info pages for information on return status.</P
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></DIV
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="sect_10_01_03"
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></A
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>10.1.3. Constants</H2
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><P
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>In Bash, constants are created by making a variable read-only. The <B
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CLASS="command"
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>readonly</B
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> built-in marks each specified variable as unchangeable. The syntax is:</P
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><P
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><B
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CLASS="command"
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>readonly <TT
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CLASS="option"
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>OPTION</TT
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> <TT
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CLASS="varname"
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>VARIABLE(s)</TT
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></B
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> </P
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><P
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>The values of these variables can then no longer be changed by subsequent assignment. If the <TT
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CLASS="option"
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>-f</TT
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> option is given, each variable refers to a shell function; see <A
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HREF="chap_11.html"
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>Chapter 11</A
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>. If <TT
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CLASS="option"
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>-a</TT
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> is specified, each variable refers to an array of variables. If no arguments are given, or if <TT
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CLASS="option"
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>-p</TT
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> is supplied, a list of all read-only variables is displayed. Using the <TT
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CLASS="option"
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>-p</TT
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> option, the output can be reused as input.</P
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><P
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>The return status is zero, unless an invalid option was specified, one of the variables or functions does not exist, or <TT
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CLASS="option"
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>-f</TT
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> was supplied for a variable name instead of for a function name.</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> <TT
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CLASS="prompt"
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>[bob in ~]</TT
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> <B
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CLASS="command"
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>readonly <TT
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CLASS="varname"
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>TUX</TT
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>=<TT
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CLASS="parameter"
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><I
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>penguinpower</I
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></TT
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></B
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>
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<TT
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CLASS="prompt"
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>[bob in ~]</TT
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> <B
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CLASS="command"
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><TT
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CLASS="varname"
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>TUX</TT
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>=<TT
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CLASS="parameter"
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><I
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>Mickeysoft</I
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></TT
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></B
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>
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bash: TUX: readonly variable
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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></DIV
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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HREF="chap_10.html"
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ACCESSKEY="P"
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>Prev</A
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VALIGN="top"
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HREF="sect_10_02.html"
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>Next</A
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></TD
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></TR
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>More on variables</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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>Up</A
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ALIGN="right"
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VALIGN="top"
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>Array variables</TD
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