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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML
><HEAD
><TITLE
>Operations on variables</TITLE
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>Chapter 10. More on variables</TD
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ALIGN="LEFT"
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><DIV
CLASS="sect1"
><H1
CLASS="sect1"
><A
NAME="sect_10_03"
></A
>10.3. Operations on variables</H1
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="sect_10_03_01"
></A
>10.3.1. Arithmetic on variables</H2
><P
>We discussed this already in <A
HREF="sect_03_04.html#sect_03_04_05"
>Section 3.4.6</A
>.</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="sect_10_03_02"
></A
>10.3.2. Length of a variable</H2
><P
>Using the <B
CLASS="command"
>${#<TT
CLASS="varname"
>VAR</TT
>}</B
> syntax will calculate the number of characters in a variable. If <TT
CLASS="varname"
>VAR</TT
> is <SPAN
CLASS="QUOTE"
>"*"</SPAN
> or <SPAN
CLASS="QUOTE"
>"@"</SPAN
>, this value is substituted with the number of positional parameters or number of elements in an array in general. This is demonstrated in the example below:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13;<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>$SHELL</TT
></B
>
/bin/bash
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>${#SHELL}</TT
></B
>
9
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
><TT
CLASS="varname"
>ARRAY</TT
>=<TT
CLASS="parameter"
><I
>(one two three)</I
></TT
></B
>
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>${#ARRAY}</TT
></B
>
3
</PRE
></FONT
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="sect_10_03_03"
></A
>10.3.3. Transformations of variables</H2
><DIV
CLASS="sect3"
><H3
CLASS="sect3"
><A
NAME="sect_10_03_03_01"
></A
>10.3.3.1. Substitution</H3
><P
><B
CLASS="command"
>${<TT
CLASS="varname"
>VAR</TT
>:-<TT
CLASS="parameter"
><I
>WORD</I
></TT
>}</B
> </P
><P
>If <TT
CLASS="varname"
>VAR</TT
> is not defined or null, the expansion of <TT
CLASS="parameter"
><I
>WORD</I
></TT
> is substituted; otherwise the value of <TT
CLASS="varname"
>VAR</TT
> is substituted:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13;<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>${TEST:-test}</TT
></B
>
test
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>$TEST</TT
></B
>
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>export <TT
CLASS="varname"
>TEST</TT
>=<TT
CLASS="parameter"
><I
>a_string</I
></TT
></B
>
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>${TEST:-test}</TT
></B
>
a_string
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>${TEST2:-$TEST}</TT
></B
>
a_string
</PRE
></FONT
></TD
></TR
></TABLE
><P
>This form is often used in conditional tests, for instance in this one:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13;<B
CLASS="command"
><TT
CLASS="parameter"
><I
>[ -z "${COLUMNS:-}" ]</I
></TT
> &#38;&#38; <TT
CLASS="varname"
>COLUMNS</TT
>=<TT
CLASS="parameter"
><I
>80</I
></TT
></B
>
</PRE
></FONT
></TD
></TR
></TABLE
><P
>It is a shorter notation for</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13;<B
CLASS="command"
>if <TT
CLASS="parameter"
><I
>[ -z "${COLUMNS:-}" ]</I
></TT
>; then
<TT
CLASS="varname"
>COLUMNS</TT
>=<TT
CLASS="parameter"
><I
>80</I
></TT
>
fi</B
>
</PRE
></FONT
></TD
></TR
></TABLE
><P
>See <A
HREF="sect_07_01.html#sect_07_01_02_03"
>Section 7.1.2.3</A
> for more information about this type of condition testing.</P
><P
>If the hyphen (-) is replaced with the equal sign (=), the value is assigned to the parameter if it does not exist:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13;<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>$TEST2</TT
></B
>
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>${TEST2:=$TEST}</TT
></B
>
a_string
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>$TEST2</TT
></B
>
a_string
</PRE
></FONT
></TD
></TR
></TABLE
><P
>The following syntax tests the existence of a variable. If it is not set, the expansion of <TT
CLASS="parameter"
><I
>WORD</I
></TT
> is printed to standard out and non-interactive shells quit. A demonstration:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13;<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>cat <TT
CLASS="filename"
>vartest.sh</TT
></B
>
#!/bin/bash
# This script tests whether a variable is set. If not,
# it exits printing a message.
echo ${TESTVAR:?"There's so much I still wanted to do..."}
echo "TESTVAR is set, we can proceed."
<TT
CLASS="prompt"
>[bob in testdir]</TT
> <B
CLASS="command"
>./vartest.sh</B
>
./vartest.sh: line 6: TESTVAR: There's so much I still wanted to do...
<TT
CLASS="prompt"
>[bob in testdir]</TT
> <B
CLASS="command"
>export <TT
CLASS="varname"
>TESTVAR</TT
>=<TT
CLASS="parameter"
><I
>present</I
></TT
></B
>
<TT
CLASS="prompt"
>[bob in testdir]</TT
> <B
CLASS="command"
>./vartest.sh</B
>
present
TESTVAR is set, we can proceed.
</PRE
></FONT
></TD
></TR
></TABLE
><P
>Using <SPAN
CLASS="QUOTE"
>"+"</SPAN
> instead of the exclamation mark sets the variable to the expansion of <TT
CLASS="parameter"
><I
>WORD</I
></TT
>; if it does not exist, nothing happens.</P
></DIV
><DIV
CLASS="sect3"
><H3
CLASS="sect3"
><A
NAME="sect_10_03_03_02"
></A
>10.3.3.2. Removing substrings</H3
><P
>To strip a number of characters, equal to <TT
CLASS="parameter"
><I
>OFFSET</I
></TT
>, from a variable, use this syntax:</P
><P
><B
CLASS="command"
>${<TT
CLASS="varname"
>VAR</TT
>:<TT
CLASS="parameter"
><I
>OFFSET</I
></TT
>:<TT
CLASS="parameter"
><I
>LENGTH</I
></TT
>}</B
> </P
><P
>The <TT
CLASS="parameter"
><I
>LENGTH</I
></TT
> parameter defines how many characters to keep, starting from the first character after the offset point. If <TT
CLASS="parameter"
><I
>LENGTH</I
></TT
> is omitted, the remainder of the variable content is taken:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13;<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>export <TT
CLASS="varname"
>STRING</TT
>=<TT
CLASS="parameter"
><I
>"thisisaverylongname"</I
></TT
></B
>
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>${STRING:4}</TT
></B
>
isaverylongname
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>${STRING:6:5}</TT
></B
>
avery
</PRE
></FONT
></TD
></TR
></TABLE
><P
><B
CLASS="command"
>${<TT
CLASS="varname"
>VAR</TT
>#<TT
CLASS="parameter"
><I
>WORD</I
></TT
>}</B
> </P
><P
>and</P
><P
><B
CLASS="command"
>${<TT
CLASS="varname"
>VAR</TT
>##<TT
CLASS="parameter"
><I
>WORD</I
></TT
>}</B
> </P
><P
>These constructs are used for deleting the pattern matching the expansion of <TT
CLASS="parameter"
><I
>WORD</I
></TT
> in <TT
CLASS="varname"
>VAR</TT
>. <TT
CLASS="parameter"
><I
>WORD</I
></TT
> is expanded to produce a pattern just as in file name expansion. If the pattern matches the beginning of the expanded value of <TT
CLASS="varname"
>VAR</TT
>, then the result of the expansion is the expanded value of <TT
CLASS="varname"
>VAR</TT
> with the shortest matching pattern (<SPAN
CLASS="QUOTE"
>"#"</SPAN
>) or the longest matching pattern (indicated with <SPAN
CLASS="QUOTE"
>"##"</SPAN
>).</P
><P
>If <TT
CLASS="varname"
>VAR</TT
> is <TT
CLASS="varname"
>*</TT
> or <TT
CLASS="varname"
>@</TT
>, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list.</P
><P
>If <TT
CLASS="varname"
>VAR</TT
> is an array variable subscribed with <SPAN
CLASS="QUOTE"
>"*"</SPAN
> or <SPAN
CLASS="QUOTE"
>"@"</SPAN
>, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list. This is shown in the examples below:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13;<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>${ARRAY[*]}</TT
></B
>
one two one three one four
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>${ARRAY[*]#one}</TT
></B
>
two three four
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>${ARRAY[*]#t}</TT
></B
>
one wo one hree one four
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>${ARRAY[*]#t*}</TT
></B
>
one wo one hree one four
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>${ARRAY[*]##t*}</TT
></B
>
one one one four
</PRE
></FONT
></TD
></TR
></TABLE
><P
>The opposite effect is obtained using <SPAN
CLASS="QUOTE"
>"%"</SPAN
> and <SPAN
CLASS="QUOTE"
>"%%"</SPAN
>, as in this example below. <TT
CLASS="parameter"
><I
>WORD</I
></TT
> should match a trailing portion of string:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13;<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>$STRING</TT
></B
>
thisisaverylongname
<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>${STRING%name}</TT
></B
>
thisisaverylong
</PRE
></FONT
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="sect3"
><H3
CLASS="sect3"
><A
NAME="sect_10_03_03_03"
></A
>10.3.3.3. Replacing parts of variable names</H3
><P
>This is done using the</P
><P
><B
CLASS="command"
>${<TT
CLASS="varname"
>VAR</TT
>/<TT
CLASS="parameter"
><I
>PATTERN</I
></TT
>/<TT
CLASS="parameter"
><I
>STRING</I
></TT
>}</B
> </P
><P
>or</P
><P
><B
CLASS="command"
>${<TT
CLASS="varname"
>VAR</TT
>//<TT
CLASS="parameter"
><I
>PATTERN</I
></TT
>/<TT
CLASS="parameter"
><I
>STRING</I
></TT
>}</B
> </P
><P
>syntax. The first form replaces only the first match, the second replaces all matches of <TT
CLASS="parameter"
><I
>PATTERN</I
></TT
> with <TT
CLASS="parameter"
><I
>STRING</I
></TT
>:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
>&#13;<TT
CLASS="prompt"
>[bob in ~]</TT
> <B
CLASS="command"
>echo <TT
CLASS="varname"
>${STRING/name/string}</TT
></B
>
thisisaverylongstring
</PRE
></FONT
></TD
></TR
></TABLE
><P
>More information can be found in the Bash info pages.</P
></DIV
></DIV
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