ch10_02.htm

the unix power tools

></A
>Note that we put this alias in quotes.
Why? Because it's a compound
command (<EM
CLASS="emphasis"
>setenv</EM
>, then <EM
CLASS="emphasis"
>vi</EM
>).
We want the alias to
include both stages of the command. Think about what this means if we
don't put quotes around the alias definition when defining the alias:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>alias vtext setenv EXINIT &quot;source $HOME/.exrc.text&quot; ; vi    <EM
CLASS="emphasis"
>Wrong!</EM
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>The shell sees the
<SPAN
CLASS="link"
>semicolon (<A
CLASS="linkend"
HREF="ch08_05.htm"
TITLE="Command-Line Evaluation "
>8.5</A
>)</SPAN
>
(a command separator) outside of quotes,
so it separates the command line into two commands.
The first command defines the <EM
CLASS="emphasis"
>vtext</EM
> alias to run <EM
CLASS="emphasis"
>setenv</EM
>,
not <EM
CLASS="emphasis"
>vi</EM
>.
After the alias is defined, the shell runs the second command:
<EM
CLASS="emphasis"
>vi</EM
> with no filename.
In any case, the results have nothing to do with what you want.</P
><P
CLASS="para"
>The way we originally defined the <EM
CLASS="emphasis"
>vtext</EM
> alias, with quotes
around the whole definition, is what we want.
The outer quotes tell the shell to put everything into the alias
definition.
The semicolon in the alias will be interpreted, and separate the two commands,
any time you use the alias.</P
><P
CLASS="para"
>Next, look at the <EM
CLASS="emphasis"
>clean</EM
> alias.
As with the <EM
CLASS="emphasis"
>vtext</EM
> alias, this one needs to be quoted.
The
reason now is a bit different; the quotes prevent the shell
from expanding the <CODE
CLASS="literal"
>*</CODE
> wildcard immediately. That is, if you just typed:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>% <CODE
CLASS="userinput"
><B
>alias clean rm *~</B
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>the shell would expand the wildcard immediately.
So if a file named <EM
CLASS="emphasis"
>foo~</EM
> is in your current directory, the
<EM
CLASS="emphasis"
>clean</EM
> alias will be <CODE
CLASS="literal"
>rm</CODE
>&nbsp;<CODE
CLASS="literal"
>foo~</CODE
>.
That (most likely) isn't what you want; a
<EM
CLASS="emphasis"
>clean</EM
> alias that will only delete one particular file isn't very
interesting.</P
><P
CLASS="para"
>So you need a way to prevent the shell from interpreting the <CODE
CLASS="literal"
>*</CODE
> right
now (when you define the alias); you want the shell to interpret <CODE
CLASS="literal"
>*</CODE
>
later, when you use the alias.
There are plenty of articles in this book about 
<SPAN
CLASS="link"
>quoting (<A
CLASS="linkend"
HREF="ch08_14.htm"
TITLE="Bourne Shell Quoting "
>8.14</A
>, <A
CLASS="linkend"
HREF="ch08_15.htm"
TITLE="Differences Between Bourne and C Shell Quoting "
>8.15</A
>)</SPAN
>,
but the
simplest way to write an alias that uses wildcards (or other special
characters) is to put it inside of quotation marks.</P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-2340-SECT-1.3"
>10.2.3 Setting Aliases Automatically, Unsetting Aliases </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-11739"
></A
><A
CLASS="indexterm"
NAME="AUTOID-11742"
></A
>Any aliases you define can be placed in your <EM
CLASS="emphasis"
>.cshrc</EM
> file, so
that they'll be available whenever you're using the C shell.
(Note: aliases are
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>not</EM
> passed to subprocesses (<A
CLASS="linkend"
HREF="ch02_02.htm"
TITLE="Shell Setup Files-Which, Where, and Why "
>2.2</A
>)</SPAN
>,
so putting them
in your <EM
CLASS="emphasis"
>.login</EM
> file probably isn't wise.)</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-11750"
></A
>Some people like to use aliases to redefine UNIX commands.
For instance, you could make an alias named <EM
CLASS="emphasis"
>rm</EM
> that actually
runs <EM
CLASS="emphasis"
>mv</EM
>, moving a file to a &quot;trashcan&quot; directory instead of
actually removing it.
[1]
Redefining commands can be
<SPAN
CLASS="link"
>confusing or dangerous (<A
CLASS="linkend"
HREF="ch10_06.htm"
TITLE="Avoiding C Shell Alias Loops "
>10.6</A
>)</SPAN
>.
Still, in some cases, aliases that redefine commands can be useful.</P
><BLOCKQUOTE
CLASS="footnote"
><P
CLASS="para"
>[1] Article
<A
CLASS="xref"
HREF="ch23_09.htm"
TITLE="delete: Protecting Files from Accidental Deletion "
>23.9</A
>
shows the <EM
CLASS="emphasis"
>delete</EM
> programs, a better way to do this.</P
></BLOCKQUOTE
><P
CLASS="para"
>To temporarily use the default <EM
CLASS="emphasis"
>rm</EM
> (not your alias named <EM
CLASS="emphasis"
>rm</EM
>),
type 
<SPAN
CLASS="link"
>a backslash (<CODE
CLASS="literal"
>\</CODE
>) before the name (<A
CLASS="linkend"
HREF="ch08_12.htm"
TITLE="Which One Will the C Shell Use? "
>8.12</A
>)</SPAN
>:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>% <CODE
CLASS="userinput"
><B
>\rm</B
></CODE
> <CODE
CLASS="replaceable"
><I
>filename</I
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>To use the default <EM
CLASS="emphasis"
>rm</EM
> for the rest of your login session:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>% <CODE
CLASS="userinput"
><B
>unalias rm</B
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Unless you remove the definition from <EM
CLASS="emphasis"
>.cshrc</EM
>, 
the alias is restored the next time you log in
(or the next time you create any new C shell).</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-11777"
></A
>A final piece of trivia: the C shell manual page tells us that aliases
can be nested; that is, they can refer to other aliases. Personally,
I think this would get too complicated too quickly to be very useful,
so I don't do it and can't recommend it. But you can try.</P
></DIV
><DIV
CLASS="sect1info"
><P
CLASS="SECT1INFO"
>- <SPAN
CLASS="authorinitials"
>ML</SPAN
>, <SPAN
CLASS="authorinitials"
>JP</SPAN
>, <SPAN
CLASS="authorinitials"
>DG</SPAN
></P
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