ch08_14.htm

the unix power tools

>6.8</A
>, <A
CLASS="linkend"
HREF="ch06_01.htm#UPT-ART-1170"
TITLE="What Environment Variables Are Good For "
>6.1</A
>)</SPAN
>
and
<SPAN
CLASS="link"
>command substitution (<A
CLASS="linkend"
HREF="ch09_16.htm"
TITLE="Command Substitution "
>9.16</A
>)</SPAN
>
inside double quotes&nbsp;- and also to stop that substitution where you need to.</P
><P
CLASS="para"
>&#13;For now, let's repeat the example above.
This time, put double quotes around the single quotes (actually, around
the whole string):</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>$ <CODE
CLASS="userinput"
><B
>echo &quot;Hey!       What's next?  Mike's #1 friend has $$.&quot;</B
></CODE
>
Hey!       What's next?  Mike's #1 friend has 18437.</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>The opening double quote isn't matched until the end of the string.
So, all the spaces between the double quotes lose their special meaning&nbsp;- and the
shell passes the whole string to <EM
CLASS="emphasis"
>echo</EM
> as one argument.
The single quotes also lose their special meaning&nbsp;- because double quotes
turn off the special meaning of single quotes!
So, the single quotes aren't stripped off as they were in the previous
example; <EM
CLASS="emphasis"
>echo</EM
> prints them.</P
><P
CLASS="para"
>What else lost its special meaning?
The hash mark (<CODE
CLASS="literal"
>#</CODE
>) did; notice that the rest of the string was
passed to <EM
CLASS="emphasis"
>echo</EM
> this time&nbsp;- because it wasn't &quot;commented out.&quot;<A
CLASS="indexterm"
NAME="AUTOID-8544"
></A
>
But the dollar sign (<CODE
CLASS="literal"
>$</CODE
>) didn't lose its meaning;
the <CODE
CLASS="literal"
>$$</CODE
> was expanded
into the shell's
<SPAN
CLASS="link"
>process ID number (<A
CLASS="linkend"
HREF="ch38_03.htm"
TITLE="Managing Processes: Overall Concepts "
>38.3</A
>)</SPAN
>
(in this shell, <CODE
CLASS="literal"
>18437</CODE
>).</P
></LI
></UL
><P
CLASS="para"
>In the previous example, what would happen if you put the <CODE
CLASS="literal"
>$</CODE
> inside
the single quotes?
(Single quotes turn off the meaning of <CODE
CLASS="literal"
>$</CODE
>, remember.)
Would the shell still expand <CODE
CLASS="literal"
>$$</CODE
> to its value?
Yes, it would: the single quotes have lost their special meaning,
so they don't affect any characters between themselves:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>$ <CODE
CLASS="userinput"
><B
>echo &quot;What's next?  How many $$ did Mike's friend bring?&quot;</B
></CODE
>
What's next?  How many 18437 did Mike's friend bring?</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>How can you make both the <CODE
CLASS="literal"
>$$</CODE
> and the single quotes print literally?
The easiest way is with a backslash, which still works inside double
quotes:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>$ <CODE
CLASS="userinput"
><B
>echo &quot;What's next?  How many \$\$ did Mike's friend bring?&quot;</B
></CODE
>
What's next?  How many $$ did Mike's friend bring?</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Here's another way to solve the problem.
A careful look at this will show a lot about shell quoting:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>$ <CODE
CLASS="userinput"
><B
>echo &quot;What's next?  How many &quot;'$$'&quot; did Mike's friend bring?&quot;</B
></CODE
>
What's next?  How many $$ did Mike's friend bring?</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>To read that example, remember that a double quote quotes characters
until the next double quote is found.
The same is true for single quotes.
So, the string <CODE
CLASS="literal"
>What's next?&nbsp;&nbsp;How many&nbsp;</CODE
>
(including the space at the end) is inside a pair of double quotes.
The <CODE
CLASS="literal"
>$$</CODE
> is inside a pair of single quotes.
The rest of the line is inside another pair of double quotes.
Both of the double-quoted strings contain a single quote; the double
quotes turn off its special meaning and the single quote is printed literally.</P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-12-SECT-1.3"
>8.14.3 Single Quotes Inside Single Quotes? </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-8573"
></A
>You can't put single quotes inside single quotes.
A single quote turns off <EM
CLASS="emphasis"
>all</EM
> special meaning until the next
single quote.
Use double quotes and backslashes.<A
CLASS="indexterm"
NAME="AUTOID-8576"
></A
></P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-12-SECT-1.4"
>8.14.4 Multiline Quoting </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-8580"
></A
>Once you type a single quote or double quote, everything is quoted.
The quoting can stretch across many lines.
(The C shell doesn't work this way.)</P
><P
CLASS="para"
>For example, in the short script shown in
<A
CLASS="xref"
HREF="ch08_14.htm#UPT-ART-12-FIG-0"
TITLE="Matching Quotes"
>Figure 8.1</A
>,
you might think that the <CODE
CLASS="literal"
>$1</CODE
> is
inside quotes... but it isn't.</P
><H4
CLASS="figure"
><A
CLASS="title"
NAME="UPT-ART-12-FIG-0"
>Figure 8.1: Matching Quotes</A
></H4
><IMG
CLASS="graphic"
SRC="figs/0012.gif"
ALT="Figure 8.1"><P
CLASS="para"
>Actually, everything <EM
CLASS="emphasis"
>but</EM
> <CODE
CLASS="literal"
>$1</CODE
> is in quotes.
The gray shaded area shows the quoted parts.
So <CODE
CLASS="literal"
>$1</CODE
> 
is expanded by the Bourne shell, and not by <EM
CLASS="emphasis"
>awk</EM
>.</P
><P
CLASS="para"
>Here's another example.
Let's store a
<SPAN
CLASS="link"
>shell variable (<A
CLASS="linkend"
HREF="ch06_08.htm"
TITLE="Shell Variables "
>6.8</A
>)</SPAN
>
with a multiline message, the kind that might be used in a shell program.<A
CLASS="indexterm"
NAME="AUTOID-8596"
></A
>
A shell variable must be stored as a single argument; any argument
separators (spaces, etc.) must be quoted.
Inside double quotes, <CODE
CLASS="literal"
>$</CODE
> and <CODE
CLASS="literal"
>`</CODE
> are interpreted
(<EM
CLASS="emphasis"
>before</EM
> the variable is stored, by the way).
The opening double quote isn't closed by the end of the first line;
the Bourne shell prints
<SPAN
CLASS="link"
>secondary prompts (<A
CLASS="linkend"
HREF="ch09_13.htm"
TITLE="Multiline Commands, Secondary Prompts "
>9.13</A
>)</SPAN
>
(<CODE
CLASS="literal"
>&gt;</CODE
>) until all quotes
are closed:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>$ <CODE
CLASS="userinput"
><B
>greeting=&quot;Hi, $USER.
&gt; The date and time now
&gt; are:  `date`.&quot;</B
></CODE
>
$ <CODE
CLASS="userinput"
><B
>echo &quot;$greeting&quot;</B
></CODE
>
Hi, jerry.
The date and time now
are:  Tue Sep  1 13:48:12 EDT 1992.
$ <CODE
CLASS="userinput"
><B
>echo $greeting</B
></CODE
>
Hi, jerry. The date and time now are: Tue Sep 1 13:48:12 EDT 1992.
$</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>The first <EM
CLASS="emphasis"
>echo</EM
> command line uses double quotes.
So, the shell variable is expanded,
but the shell doesn't use the spaces and newlines in the variable
as argument separators.
(Look at the extra spaces after the word <CODE
CLASS="literal"
>are:</CODE
>.)
The second <EM
CLASS="emphasis"
>echo</EM
> doesn't use double quotes.
The spaces and newlines are treated as argument separators; the shell
passes 14 arguments to <EM
CLASS="emphasis"
>echo</EM
>, which prints them with single
spaces between.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-8615"
></A
>A backslash has a quirk you should know about.
If you use it outside quotes, at the end of a line (just before the
newline), the newline will be <EM
CLASS="emphasis"
>deleted</EM
>.
Inside single quotes, though, a backslash at the end of a line is
copied

as is.
Here are examples.
I've numbered the prompts (<CODE
CLASS="literal"
>1$</CODE
>, <CODE
CLASS="literal"
>2$</CODE
>, and so on):</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>1$ <CODE
CLASS="userinput"
><B
>echo &quot;a long long long long long long</B
></CODE
>
&gt; <CODE
CLASS="userinput"
><B
>line or two&quot;</B
></CODE
>
a long long long long long long
line or two
2$ <CODE
CLASS="userinput"
><B
>echo a long long long long long long\</B
></CODE
>
&gt; <CODE
CLASS="userinput"
><B
>line</B
></CODE
>
a long long long long long longline
3$ <CODE
CLASS="userinput"
><B
>echo a long long long long long long \</B
></CODE
>
&gt; <CODE
CLASS="userinput"
><B
>line</B
></CODE
>
a long long long long long long line
4$ <CODE
CLASS="userinput"
><B
>echo &quot;a long long long long long long\</B
></CODE
>
&gt; <CODE
CLASS="userinput"
><B
>line&quot;</B
></CODE
>
a long long long long long longline
5$ <CODE
CLASS="userinput"
><B
>echo 'a long long long long long long\</B
></CODE
>
&gt; <CODE
CLASS="userinput"
><B
>line'</B
></CODE
>
a long long long long long long\
line</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>You've seen an example like <B
CLASS="emphasis.bold"
>example 1</B
> before.
The newline is in quotes, so it isn't an argument separator;
<EM
CLASS="emphasis"
>echo</EM
> prints it with the rest of the (single two-line) argument.
In <B
CLASS="emphasis.bold"
>example 2</B
>, the backslash before the newline tells the shell to delete
the newline; the words <CODE
CLASS="literal"
>long</CODE
> and <CODE
CLASS="literal"
>line</CODE
> are passed to
<EM
CLASS="emphasis"
>echo</EM
> as one argument.
<B
CLASS="emphasis.bold"
>Example 3</B
> is usually what you want when you're typing long lists of
command-line arguments: Type a space (an argument separator) before the
backslash and newline.
In <B
CLASS="emphasis.bold"
>example 4</B
>, the backslash inside the double quotes is ignored
(compare to example 1).
Inside single quotes, as in <B
CLASS="emphasis.bold"
>example 5</B
>, the backslash has no
special meaning; it's passed on to <EM
CLASS="emphasis"
>echo</EM
>.</P
><A
CLASS="indexterm"
NAME="AUTOID-8644"
></A
><A
CLASS="indexterm"
NAME="AUTOID-8645"
></A
></DIV
><DIV
CLASS="sect1info"
><P
CLASS="SECT1INFO"
>- <SPAN
CLASS="authorinitials"
>JP</SPAN
></P
></DIV
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