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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><HTML><HEAD><TITLE>Typing variables: declare or typeset</TITLE><METANAME="GENERATOR"CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+"><LINKREL="HOME"TITLE="Advanced Bash-Scripting Guide"HREF="index.html"><LINKREL="UP"TITLE="Variables Revisited"HREF="variables2.html"><LINKREL="PREVIOUS"TITLE="Parameter Substitution"HREF="parameter-substitution.html"><LINKREL="NEXT"TITLE="Indirect References"HREF="ivr.html"><METAHTTP-EQUIV="Content-Style-Type"CONTENT="text/css"><LINKREL="stylesheet"HREF="common/kde-common.css"TYPE="text/css"><METAHTTP-EQUIV="Content-Type"CONTENT="text/html; charset=iso-8859-1"><METAHTTP-EQUIV="Content-Language"CONTENT="en"><LINKREL="stylesheet"HREF="common/kde-localised.css"TYPE="text/css"TITLE="KDE-English"><LINKREL="stylesheet"HREF="common/kde-default.css"TYPE="text/css"TITLE="KDE-Default"></HEAD><BODYCLASS="SECT1"BGCOLOR="#FFFFFF"TEXT="#000000"LINK="#AA0000"VLINK="#AA0055"ALINK="#AA0000"STYLE="font-family: sans-serif;"><DIVCLASS="NAVHEADER"><TABLESUMMARY="Header navigation table"WIDTH="100%"BORDER="0"CELLPADDING="0"CELLSPACING="0"><TR><THCOLSPAN="3"ALIGN="center">Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH></TR><TR><TDWIDTH="10%"ALIGN="left"VALIGN="bottom"><AHREF="parameter-substitution.html"ACCESSKEY="P">Prev</A></TD><TDWIDTH="80%"ALIGN="center"VALIGN="bottom">Chapter 9. Variables Revisited</TD><TDWIDTH="10%"ALIGN="right"VALIGN="bottom"><AHREF="ivr.html"ACCESSKEY="N">Next</A></TD></TR></TABLE><HRALIGN="LEFT"WIDTH="100%"></DIV><DIVCLASS="SECT1"><H1CLASS="SECT1"><ANAME="DECLAREREF"></A>9.4. Typing variables: <BCLASS="COMMAND">declare</B> or <BCLASS="COMMAND">typeset</B></H1><P><ANAME="DECLARE1REF"></A></P><P>The <ICLASS="FIRSTTERM">declare</I> or <ICLASS="FIRSTTERM">typeset</I> <AHREF="internal.html#BUILTINREF">builtins</A>, which are exact synonyms, permit modifying the properties of variables. This is a very weak form of the <ICLASS="FIRSTTERM">typing</I> <ANAME="AEN5568"HREF="#FTN.AEN5568">[1]</A> available in certain programming languages. The <ICLASS="FIRSTTERM">declare</I> command is specific to version 2 or later of Bash. The <ICLASS="FIRSTTERM">typeset</I> command also works in ksh scripts.</P><DIVCLASS="VARIABLELIST"><P><B><ANAME="DECLAREOPSREF1"></A>declare/typeset options</B></P><DL><DT><SPANCLASS="TOKEN">-r</SPAN> <TTCLASS="REPLACEABLE"><I>readonly</I></TT></DT><DD><P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 declare -r var1</PRE></TD></TR></TABLE></P><P>(<TTCLASS="USERINPUT"><B>declare -r var1</B></TT> works the same as <TTCLASS="USERINPUT"><B>readonly var1</B></TT>)</P><P>This is the rough equivalent of the C <BCLASS="COMMAND">const</B> type qualifier. An attempt to change the value of a readonly variable fails with an error message.</P></DD><DT><SPANCLASS="TOKEN">-i</SPAN> <TTCLASS="REPLACEABLE"><I>integer</I></TT></DT><DD><P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 declare -i number 2 # The script will treat subsequent occurrences of "number" as an integer. 3 4 number=3 5 echo "Number = $number" # Number = 3 6 7 number=three 8 echo "Number = $number" # Number = 0 9 # Tries to evaluate the string "three" as an integer.</PRE></TD></TR></TABLE></P><P>Certain arithmetic operations are permitted for declared integer variables without the need for <AHREF="moreadv.html#EXPRREF">expr</A> or <AHREF="internal.html#LETREF">let</A>.</P><P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 n=6/3 2 echo "n = $n" # n = 6/3 3 4 declare -i n 5 n=6/3 6 echo "n = $n" # n = 2</PRE></TD></TR></TABLE></P></DD><DT><ANAME="ARRAYDECLARE"></A><SPANCLASS="TOKEN">-a</SPAN> <TTCLASS="REPLACEABLE"><I>array</I></TT></DT><DD><P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 declare -a indices</PRE></TD></TR></TABLE></P><P>The variable <TTCLASS="PARAMETER"><I>indices</I></TT> will be treated as an <AHREF="arrays.html#ARRAYREF">array</A>.</P></DD><DT><SPANCLASS="TOKEN">-f</SPAN> <TTCLASS="REPLACEABLE"><I>function(s)</I></TT></DT><DD><P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 declare -f</PRE></TD></TR></TABLE></P><P>A <TTCLASS="USERINPUT"><B>declare -f</B></TT> line with no arguments in a script causes a listing of all the <AHREF="functions.html#FUNCTIONREF">functions</A> previously defined in that script.</P><P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 declare -f function_name</PRE></TD></TR></TABLE></P><P>A <TTCLASS="USERINPUT"><B>declare -f function_name</B></TT> in a script lists just the function named.</P></DD><DT><SPANCLASS="TOKEN">-x</SPAN> <AHREF="internal.html#EXPORTREF">export</A></DT><DD><P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 declare -x var3</PRE></TD></TR></TABLE></P><P>This declares a variable as available for exporting outside the environment of the script itself.</P></DD><DT>-x var=$value</DT><DD><P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="90%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 declare -x var3=373</PRE></TD></TR></TABLE></P><P>The <BCLASS="COMMAND">declare</B> command permits assigning a value to a variable in the same statement as setting its properties.</P></DD></DL></DIV><DIVCLASS="EXAMPLE"><HR><ANAME="EX20"></A><P><B>Example 9-23. Using <ICLASS="FIRSTTERM">declare</I> to type variables</B></P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 #!/bin/bash 2 3 func1 () 4 { 5 echo This is a function. 6 } 7 8 declare -f # Lists the function above. 9 10 echo 11 12 declare -i var1 # var1 is an integer. 13 var1=2367 14 echo "var1 declared as $var1" 15 var1=var1+1 # Integer declaration eliminates the need for 'let'. 16 echo "var1 incremented by 1 is $var1." 17 # Attempt to change variable declared as integer. 18 echo "Attempting to change var1 to floating point value, 2367.1." 19 var1=2367.1 # Results in error message, with no change to variable. 20 echo "var1 is still $var1" 21 22 echo 23 24 declare -r var2=13.36 # 'declare' permits setting a variable property 25 #+ and simultaneously assigning it a value. 26 echo "var2 declared as $var2" # Attempt to change readonly variable. 27 var2=13.37 # Generates error message, and exit from script. 28 29 echo "var2 is still $var2" # This line will not execute. 30 31 exit 0 # Script will not exit here.</PRE></TD></TR></TABLE><HR></DIV><DIVCLASS="CAUTION"><TABLECLASS="CAUTION"WIDTH="100%"BORDER="0"><TR><TDWIDTH="25"ALIGN="CENTER"VALIGN="TOP"><IMGSRC="common/caution.png"HSPACE="5"ALT="Caution"></TD><TDALIGN="LEFT"VALIGN="TOP"><P>Using the <ICLASS="FIRSTTERM">declare</I> builtin restricts the <AHREF="subshells.html#SCOPEREF">scope</A> of a variable. <TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 foo () 2 { 3 FOO="bar" 4 } 5 6 bar () 7 { 8 foo 9 echo $FOO 10 } 11 12 bar # Prints bar.</PRE></TD></TR></TABLE></P><P>However . . . <TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 foo (){ 2 declare FOO="bar" 3 } 4 5 bar () 6 { 7 foo 8 echo $FOO 9 } 10 11 bar # Prints nothing. 12 13 14 # Thank you, Michael Iatrou, for pointing this out.</PRE></TD></TR></TABLE></P></TD></TR></TABLE></DIV></DIV><H3CLASS="FOOTNOTES">Notes</H3><TABLEBORDER="0"CLASS="FOOTNOTES"WIDTH="100%"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="5%"><ANAME="FTN.AEN5568"HREF="declareref.html#AEN5568">[1]</A></TD><TDALIGN="LEFT"VALIGN="TOP"WIDTH="95%"><P><ANAME="TYPINGREF"></A>In this context, <ICLASS="FIRSTTERM">typing</I> a variable means to classify it and restrict its properties. For example, a variable <ICLASS="FIRSTTERM">declared</I> or <ICLASS="FIRSTTERM">typed</I> as an integer is no longer available for <AHREF="refcards.html#STRINGOPSTAB">string operations</A>.</P></TD></TR></TABLE><DIVCLASS="NAVFOOTER"><HRALIGN="LEFT"WIDTH="100%"><TABLESUMMARY="Footer navigation table"WIDTH="100%"BORDER="0"CELLPADDING="0"CELLSPACING="0"><TR><TDWIDTH="33%"ALIGN="left"VALIGN="top"><AHREF="parameter-substitution.html"ACCESSKEY="P">Prev</A></TD><TDWIDTH="34%"ALIGN="center"VALIGN="top"><AHREF="index.html"ACCESSKEY="H">Home</A></TD><TDWIDTH="33%"ALIGN="right"VALIGN="top"><AHREF="ivr.html"ACCESSKEY="N">Next</A></TD></TR><TR><TDWIDTH="33%"ALIGN="left"VALIGN="top">Parameter Substitution</TD><TDWIDTH="34%"ALIGN="center"VALIGN="top"><AHREF="variables2.html"ACCESSKEY="U">Up</A></TD><TDWIDTH="33%"ALIGN="right"VALIGN="top">Indirect References</TD></TR></TABLE></DIV></BODY></HTML>⌨️ 快捷键说明
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