ch37_04.htm

the unix power tools

CLASS="literal"
>\b</CODE
>'s or whatever.
With Perl, it's all the same&nbsp;- and, as far as I can tell, a proper
superset of all the others.</P
><P
CLASS="para"
>Perl has a fully functional symbolic debugger (written, of course, in
Perl) that is an indispensable aid in debugging complex programs.
Neither the shell nor <EM
CLASS="emphasis"
>sed</EM
>/<EM
CLASS="emphasis"
>awk</EM
>/<EM
CLASS="emphasis"
>sort</EM
>/<EM
CLASS="emphasis"
>tr</EM
>/... have
such a thing.</P
><P
CLASS="para"
>Perl has a loop control mechanism that's more powerful even than C's.
You can do the equivalent of a <EM
CLASS="emphasis"
>break</EM
> or <EM
CLASS="emphasis"
>continue</EM
>
(<EM
CLASS="emphasis"
>last</EM
> and <EM
CLASS="emphasis"
>next</EM
> in Perl) of any arbitrary loop, not merely
the nearest enclosing one.  You can even do a kind of <EM
CLASS="emphasis"
>continue</EM
>
that doesn't trigger the re-initialization part of a loop, something
you may, from time to time, want to do.</P
><P
CLASS="para"
>Perl's data types and operators are richer than the shells' or
<EM
CLASS="emphasis"
>awk</EM
>'s, because you have scalars, numerically-indexed arrays
(lists), and string-indexed (hashed) arrays.  Each of these holds
arbitrary data values, including floating-point numbers, for which
mathematic built-in subroutines and power operators are available.  It
can handle binary data of arbitrary size.</P
><P
CLASS="para"
>Speaking of LISP, you can generate strings, perhaps with
<EM
CLASS="emphasis"
>sprintf</EM
>&nbsp;(&nbsp;), and then <EM
CLASS="emphasis"
>eval</EM
> them.  That way you can
generate code on the fly.  You can even do lambda-type functions that
return newly created functions that you can call later. The scoping of
variables is dynamic; fully recursive subroutines are supported; and
you can pass or return any type of data into or out of your
subroutines.</P
><P
CLASS="para"
>You have a built-in automatic formatter for generating pretty printed
forms with automatic pagination and headers and center-justified and
text-filled fields like <CODE
CLASS="literal"
>%(|fmt)s</CODE
>, if you can imagine what that
would actually be were it legal.</P
><P
CLASS="para"
>There's a mechanism for writing
<SPAN
CLASS="link"
>SUID (<A
CLASS="linkend"
HREF="ch01_23.htm"
TITLE="File Access Permissions "
>1.23</A
>)</SPAN
>
programs that can be made more secure than even C programs, thanks to
an elaborate data-tracing mechanism that understands the &quot;taintedness&quot;
of data derived from external sources.  It won't let you do anything
really stupid that you might not have thought of.</P
><P
CLASS="para"
>You have access to just about any system-related function or system
call, like <EM
CLASS="emphasis"
>ioctl</EM
>s, <EM
CLASS="emphasis"
>fcntl</EM
>, <EM
CLASS="emphasis"
>select</EM
>, <EM
CLASS="emphasis"
>pipe</EM
> and
<EM
CLASS="emphasis"
>fork</EM
>, <EM
CLASS="emphasis"
>getc</EM
>, <EM
CLASS="emphasis"
>socket</EM
> and <EM
CLASS="emphasis"
>bind</EM
>, and <EM
CLASS="emphasis"
>connect</EM
>
and <EM
CLASS="emphasis"
>attach</EM
>, and indirect <EM
CLASS="emphasis"
>syscall</EM
> invocation, as well as
things like <EM
CLASS="emphasis"
>getpwuid</EM
>, <EM
CLASS="emphasis"
>gethostbyname</EM
>, etc.  You can
read in binary data laid out by a C program or system call using
structure-conversion templates.</P
><P
CLASS="para"
>At the same time you can get at the high-level shell-type operations
like the
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>-r</EM
> or <EM
CLASS="emphasis"
>-w</EM
> tests (<A
CLASS="linkend"
HREF="ch44_20.htm"
TITLE="test: Testing Files and Strings "
>44.20</A
>)</SPAN
>
on files or
<SPAN
CLASS="link"
><CODE
CLASS="literal"
>`backquote`</CODE
> (<A
CLASS="linkend"
HREF="ch09_16.htm"
TITLE="Command Substitution "
>9.16</A
>)</SPAN
>
command interpolation.  You can do file-globbing with the
<SPAN
CLASS="link"
><CODE
CLASS="literal"
>&lt;*.[</CODE
><CODE
CLASS="replaceable"
><I
>ch</I
></CODE
><CODE
CLASS="literal"
>]&gt;</CODE
> (<A
CLASS="linkend"
HREF="ch15_01.htm#UPT-ART-1480"
TITLE="File Naming Wildcards "
>15.1</A
>)</SPAN
>
notation or do low-level <EM
CLASS="emphasis"
>readdir</EM
>s as suits your fancy.</P
><P
CLASS="para"
>DBM files can be accessed using simple array notation.  This is really
nice for dealing with system databases (aliases, news, ...), efficient
access mechanisms over large data sets, and for keeping persistent
data.</P
><P
CLASS="para"
>Don't be dismayed by the apparent complexity of what I've just
discussed.  Perl is actually very easy to learn because so much of it
derives from existing tools.  It's like interpreter C with <EM
CLASS="emphasis"
>sh</EM
>,
<EM
CLASS="emphasis"
>sed</EM
>, <EM
CLASS="emphasis"
>awk</EM
>, and a lot more built into it.  And, finally,
there's a lot of code out there already written in Perl, including
libraries to handle things you don't feel like re-implementing.<A
CLASS="indexterm"
NAME="AUTOID-42056"
></A
></P
><DIV
CLASS="sect1info"
><P
CLASS="SECT1INFO"
>- <SPAN
CLASS="authorinitials"
>TC</SPAN
></P
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