ch45_01.htm

the unix power tools

explains why you should care.
UNIX keeps a file on-disk once it's been opened; as article
<A
CLASS="xref"
HREF="ch45_10.htm"
TITLE="Removing a File Once It's Opened&nbsp;- for Security and Easy Cleanup"
>45.10</A
>
explains, this has its ups and downs.
The <EM
CLASS="emphasis"
>jot</EM
> command, article
<A
CLASS="xref"
HREF="ch45_11.htm"
TITLE="The Multipurpose jot Command "
>45.11</A
>,
is useful for all kinds of operations with lists of numbers and characters.</P
></LI
><LI
CLASS="listitem"
><P
CLASS="para"
>Next are techniques for handling variables and parameters.
Parameter substitution, explained in article
<A
CLASS="xref"
HREF="ch45_12.htm"
TITLE="Parameter Substitution "
>45.12</A
>,
is a compact way to test, set, and give default values for variables.
You can use the <CODE
CLASS="literal"
>$0</CODE
> parameter 
and UNIX links to make the same script
have multiple names and do multiple things; see article
<A
CLASS="xref"
HREF="ch45_13.htm"
TITLE="Save Disk Space and Programming: Multiple Names for a Program"
>45.13</A
>.
Article
<A
CLASS="xref"
HREF="ch45_14.htm"
TITLE="Finding the Last Command-Line Argument "
>45.14</A
>
shows the easy way to get the last command-line argument.
Article
<A
CLASS="xref"
HREF="ch45_15.htm"
TITLE="How to Unset all Command-Line Parameters "
>45.15</A
>
has an easy way to remove all the command-line arguments.</P
></LI
><LI
CLASS="listitem"
><P
CLASS="para"
>Four articles cover <EM
CLASS="emphasis"
>sh</EM
> loops.
A <EM
CLASS="emphasis"
>for</EM
> loop usually reads a list of single arguments into a single
shell variable.
Article
<A
CLASS="xref"
HREF="ch45_16.htm"
TITLE="Standard Input to a for Loop "
>45.16</A
>
shows how to make the <EM
CLASS="emphasis"
>for</EM
> loop read from standard input.
Article
<A
CLASS="xref"
HREF="ch45_17.htm"
TITLE="Making a for Loop with Multiple Variables "
>45.17</A
>
has techniques for making a <EM
CLASS="emphasis"
>for</EM
> loop set more than one variable.
The <EM
CLASS="emphasis"
>dirname</EM
> and <EM
CLASS="emphasis"
>basename</EM
> commands can be used to split pathnames
with a loop; see article
<A
CLASS="xref"
HREF="ch45_18.htm"
TITLE="Using basename and dirname "
>45.18</A
>.
A <EM
CLASS="emphasis"
>while</EM
> loop can have more than one command line at the start;
see article
<A
CLASS="xref"
HREF="ch45_19.htm"
TITLE="A while Loop with Several Loop Control Commands "
>45.19</A
>.</P
></LI
><LI
CLASS="listitem"
><P
CLASS="para"
>Next is an assortment of articles about input/output.
Article
<A
CLASS="xref"
HREF="ch45_20.htm"
TITLE="Overview: Open Files and File Descriptors "
>45.20</A
>
introduces open files and file descriptors&nbsp;- there's more to know about
standard input/output/error than you might have realized!
Article
<A
CLASS="xref"
HREF="ch45_21.htm"
TITLE="n&gt;&amp;m: Swap Standard Output and Standard Error "
>45.21</A
>
has a look at file descriptor handling in the Bourne shell, swapping
standard output and standard error.
The shell can redirect the I/O from all commands in a loop at once;
article
<A
CLASS="xref"
HREF="ch45_22.htm"
TITLE="Handling Files Line-by-Line "
>45.22</A
>
explains one use for this technique
and article
<A
CLASS="xref"
HREF="ch45_23.htm"
TITLE="The Ins and Outs of Redirected I/O Loops "
>45.23</A
>
explains good and bad points of doing this.</P
></LI
><LI
CLASS="listitem"
><P
CLASS="para"
>The shell can read commands directly from a shell script file.
As article
<A
CLASS="xref"
HREF="ch45_24.htm"
TITLE="A Shell Can Read a Script from its Standard Input, But..."
>45.24</A
>
points out, a shell can also read commands from its standard input, but
that can cause some problems.
Article
<A
CLASS="xref"
HREF="ch45_25.htm"
TITLE="Shell Scripts On-the-Fly from Standard Input "
>45.25</A
>
shows one place scripts from <EM
CLASS="emphasis"
>stdin</EM
> are useful:
writing a script that creates
another script as it goes.</P
><P
CLASS="para"
>Next are two articles about miscellaneous I/O.
One gotcha with the here-document operator (for redirecting input from a
script file) is that the terminators are different in the Bourne and C shells;
article
<A
CLASS="xref"
HREF="ch45_26.htm"
TITLE="Quoted hereis Document Terminators: sh vs. csh "
>45.26</A
>
explains.
Article
<A
CLASS="xref"
HREF="ch45_27.htm"
TITLE='Turn Off echo for "Secret&quot; Answers'
>45.27</A
>
shows how to turn off echoing while your script reads a &quot;secret&quot;
answer such as a password.</P
></LI
><LI
CLASS="listitem"
><P
CLASS="para"
>Three articles-
<A
CLASS="xref"
HREF="ch45_28.htm"
TITLE="Quick Reference: expr "
>45.28</A
>,
<A
CLASS="xref"
HREF="ch45_29.htm"
TITLE="Testing Characters in a String with expr "
>45.29</A
>,
and
<A
CLASS="xref"
HREF="ch45_30.htm"
TITLE="Grabbing Parts of a String "
>45.30</A
>-
show uses for the versatile <EM
CLASS="emphasis"
>expr</EM
> expression-handling command.
Article
<A
CLASS="xref"
HREF="ch45_31.htm"
TITLE="Nested Command Substitution "
>45.31</A
>.
covers multiple 
<SPAN
CLASS="link"
>command substitution (<A
CLASS="linkend"
HREF="ch09_16.htm"
TITLE="Command Substitution "
>9.16</A
>)</SPAN
>.
The <EM
CLASS="emphasis"
>grabchars</EM
> program
in article
<A
CLASS="xref"
HREF="ch45_32.htm"
TITLE="A Better read Command: grabchars "
>45.32</A
>
is similar to
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>read</EM
> (<A
CLASS="linkend"
HREF="ch44_13.htm"
TITLE="read: Reading from the Keyboard "
>44.13</A
>)</SPAN
>-
but <EM
CLASS="emphasis"
>grabchars</EM
> doesn't need a RETURN after the answer;
<EM
CLASS="emphasis"
>grabchars</EM
> also can prompt and do basic tests on the answer.</P
><P
CLASS="para"
>Article <A
CLASS="xref"
HREF="ch45_33.htm"
TITLE="Testing Two Strings with One case Statement "
>45.33</A
>
shows a trick for making one
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>case</EM
> statement (<A
CLASS="linkend"
HREF="ch44_05.htm"
TITLE="Test String Values with Bourne Shell case "
>44.5</A
>)</SPAN
>
test two things at once.
Article
<A
CLASS="xref"
HREF="ch45_34.htm"
TITLE="Arrays in the Bourne Shell "
>45.34</A
>
has a trick for simulating arrays in the Bourne Shell.
Article
<A
CLASS="xref"
HREF="ch45_35.htm"
TITLE="Using a Control Character in a Script "
>45.35</A
>
uses <EM
CLASS="emphasis"
>echo</EM
> and <EM
CLASS="emphasis"
>tr</EM
> to get a control character in a script
without typing the literal character into the file. Finally, article
<A
CLASS="xref"
HREF="ch45_36.htm"
TITLE="Shell Lockfile "
>45.36</A
>
has a simple technique for getting exclusive access to a file or other
system resource.</P
></LI
></UL
><DIV
CLASS="sect1info"
><P
CLASS="SECT1INFO"
>- <SPAN
CLASS="authorinitials"
>JP</SPAN
></P
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