ch38_03.htm

the unix power tools

></A
>When you're typing commands at the shell, it is the <B
CLASS="emphasis.bold"
>controlling
process</B
> of your terminal, meaning that it (the shell) is the process that
gets the input you type.
See article
<A
CLASS="xref"
HREF="ch38_06.htm"
TITLE="The Controlling Terminal "
>38.6</A
>.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-42329"
></A
><A
CLASS="indexterm"
NAME="AUTOID-42332"
></A
><A
CLASS="indexterm"
NAME="AUTOID-42335"
></A
><A
CLASS="indexterm"
NAME="AUTOID-42338"
></A
><A
CLASS="indexterm"
NAME="AUTOID-42340"
></A
>Normally, when you type a command at the shell prompt, that command runs and
is allowed by the shell to take over the terminal for its lifetime.
For example, if you type <CODE
CLASS="literal"
>more</CODE
> <CODE
CLASS="literal"
>.login</CODE
> to view your <EM
CLASS="emphasis"
>.login</EM
>
file, the shell starts up the
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>more</EM
> (<A
CLASS="linkend"
HREF="ch25_03.htm"
TITLE="Using more to Page Through Files "
>25.3</A
>)</SPAN
>
program and then sits around waiting
for it to finish; while <EM
CLASS="emphasis"
>more</EM
> is running, you can type commands to
page through the file and <EM
CLASS="emphasis"
>more</EM
> (not the shell) will see them.
The command you run is called a <B
CLASS="emphasis.bold"
>child</B
> or <B
CLASS="emphasis.bold"
>subprocess</B
> of the
shell process, which is its <B
CLASS="emphasis.bold"
>parent</B
>.
All process information (user ID, group ID, etc.) is inherited by the child
from its parent, except for the process ID, since the child is assigned a
new one.
[
<SPAN
CLASS="link"
>Built-in shell commands (<A
CLASS="linkend"
HREF="ch01_10.htm"
TITLE="Internal and External Commands "
>1.10</A
>)</SPAN
>
like <EM
CLASS="emphasis"
>cd</EM
> don't start a child process. <EM
CLASS="emphasis"
>-JP</EM
> ]</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-42357"
></A
>Although the normal behavior is for the shell to wait until any
command you run has finished before it becomes active again, there are
some situations in which you don't want this to occur. For example,
if you're using a window system such as
<SPAN
CLASS="link"
>X (<A
CLASS="linkend"
HREF="ch01_31.htm"
TITLE="The X Window System "
>1.31</A
>)</SPAN
>
and
want to start up a new <EM
CLASS="emphasis"
>xterm</EM
> window from your shell, you don't
want to type just <CODE
CLASS="literal"
>xterm</CODE
>, because then your original shell will
wait until the <EM
CLASS="emphasis"
>xterm</EM
> finishes before allowing you to type any more
commands. This would mean that you still have only one shell to work
in, thus defeating the purpose of starting the new <EM
CLASS="emphasis"
>xterm</EM
>.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-42366"
></A
><A
CLASS="indexterm"
NAME="AUTOID-42368"
></A
><A
CLASS="indexterm"
NAME="AUTOID-42371"
></A
><A
CLASS="indexterm"
NAME="AUTOID-42374"
></A
><A
CLASS="indexterm"
NAME="AUTOID-42377"
></A
><A
CLASS="indexterm"
NAME="AUTOID-42379"
></A
>When you don't want a process to finish before getting back to the
shell, you can run it in the
<SPAN
CLASS="link"
><B
CLASS="emphasis.bold"
>background</B
> (<A
CLASS="linkend"
HREF="ch01_26.htm"
TITLE="What a Multiuser System Can Do for You "
>1.26</A
>)</SPAN
>.
You do this by putting
an ampersand (<CODE
CLASS="literal"
>&amp;</CODE
>) character at the end of the command, for example,
<CODE
CLASS="literal"
>xterm</CODE
> <CODE
CLASS="literal"
>&amp;</CODE
>. The shell will start the child process and then
immediately
prompt you for another command. Note that in this situation, the
shell retains control of the terminal and the newly created background
process cannot read input. Some shells have additional
<SPAN
CLASS="link"
><B
CLASS="emphasis.bold"
>job control</B
> (<A
CLASS="linkend"
HREF="ch12_08.htm"
TITLE="Job Control in a Nutshell "
>12.8</A
>)</SPAN
>
features (processes that are running in the background are
often described as <B
CLASS="emphasis.bold"
>background jobs</B
> or just jobs) that enable
you to do things such as kill jobs or bring a job from the background
into the <B
CLASS="emphasis.bold"
>foreground</B
> so that it becomes the controlling process of
the terminal and you can type input at it.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-42391"
></A
>An important thing to remember is that although process information is
inherited by children <EM
CLASS="emphasis"
>when they are started</EM
>, it is impossible for
the parent to affect its child's process information (or vice versa)
after that point. For example, if you
<SPAN
CLASS="link"
>start up the editor <EM
CLASS="emphasis"
>vi</EM
>, suspend it (<A
CLASS="linkend"
HREF="ch12_04.htm"
TITLE="Job Control and autowrite: Real Time Savers! "
>12.4</A
>)</SPAN
>,
and then use the <EM
CLASS="emphasis"
>cd</EM
> command in the shell to change directories,
<EM
CLASS="emphasis"
>vi</EM
> will still have the old working directory when you bring it
back into the foreground.  Similarly, if you write a shell script that
changes some environment variables, those variables will contain their
old values in the shell when the shell script exits. This sometimes
confuses MS-DOS users, since information such as the current
directory is stored in a global area which is referenced by all
programs. If it is necessary to communicate information from a child
back to a parent shell,
<SPAN
CLASS="link"
> other methods are needed . (<A
CLASS="linkend"
HREF="ch38_08.htm"
TITLE="What Are Signals? "
>38.8</A
>, <A
CLASS="linkend"
HREF="ch44_23.htm"
TITLE="Reading Files with the . and source Commands "
>44.23</A
>)</SPAN
></P
><P
CLASS="para"
>[One more concept that I think is useful: When a process exits, it
returns a numeric
<SPAN
CLASS="link"
>exit status (<A
CLASS="linkend"
HREF="ch44_07.htm"
TITLE="Exit Status of UNIX Processes "
>44.7</A
>)</SPAN
><A
CLASS="indexterm"
NAME="AUTOID-42402"
></A
><A
CLASS="indexterm"
NAME="AUTOID-42404"
></A
><A
CLASS="indexterm"
NAME="AUTOID-42407"
></A
><A
CLASS="indexterm"
NAME="AUTOID-42410"
></A
><A
CLASS="indexterm"
NAME="AUTOID-42413"
></A
>
to its parent process.
By convention, a zero status means success; non-zero means some kind
of failure. <EM
CLASS="emphasis"
>-JP</EM
>]</P
><P
CLASS="para"
>Just as there are ways to modify the environment and the current
working directory of the shell, there are also useful ways to
<SPAN
CLASS="link"
>manipulate file descriptors (<A
CLASS="linkend"
HREF="ch45_20.htm"
TITLE="Overview: Open Files and File Descriptors "
>45.20</A
>, <A
CLASS="linkend"
HREF="ch45_21.htm"
TITLE="n&gt;&amp;m: Swap Standard Output and Standard Error "
>45.21</A
>, <A
CLASS="linkend"
HREF="ch45_22.htm"
TITLE="Handling Files Line-by-Line "
>45.22</A
>)</SPAN
>.<A
CLASS="indexterm"
NAME="AUTOID-42419"
></A
></P
><DIV
CLASS="sect1info"
><P
CLASS="SECT1INFO"
>- <SPAN
CLASS="authorinitials"
>JIK</SPAN
></P
></DIV
></DIV
><DIV
CLASS="htmlnav"
><P
></P
><HR
ALIGN="LEFT"
WIDTH="515"
TITLE="footer"><TABLE
WIDTH="515"
BORDER="0"
CELLSPACING="0"
CELLPADDING="0"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="172"
><A
CLASS="SECT1"
HREF="ch38_02.htm"
TITLE="38.2 fork and exec "
><IMG
SRC="gifs/txtpreva.gif"
SRC="gifs/txtpreva.gif"
ALT="Previous: 38.2 fork and exec "
BORDER="0"></A
></TD
><TD
ALIGN="CENTER"
VALIGN="TOP"
WIDTH="171"
><A
CLASS="book"
HREF="index.htm"
TITLE="UNIX Power Tools"
><IMG
SRC="gifs/txthome.gif"
SRC="gifs/txthome.gif"
ALT="UNIX Power Tools"
BORDER="0"></A
></TD
><TD
ALIGN="RIGHT"
VALIGN="TOP"
WIDTH="172"
><A
CLASS="SECT1"
HREF="ch38_04.htm"
TITLE="38.4 Subshells "
><IMG
SRC="gifs/txtnexta.gif"
SRC="gifs/txtnexta.gif"
ALT="Next: 38.4 Subshells "
BORDER="0"></A
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="172"
>38.2 fork and exec </TD
><TD
ALIGN="CENTER"
VALIGN="TOP"
WIDTH="171"
><A
CLASS="index"
HREF="index/idx_0.htm"
TITLE="Book Index"
><IMG
SRC="gifs/index.gif"
SRC="gifs/index.gif"
ALT="Book Index"
BORDER="0"></A
></TD
><TD
ALIGN="RIGHT"
VALIGN="TOP"
WIDTH="172"
>38.4 Subshells </TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="515"
TITLE="footer"><IMG
SRC="gifs/smnavbar.gif"
SRC="gifs/smnavbar.gif"
USEMAP="#map"
BORDER="0"
ALT="The UNIX CD Bookshelf Navigation"><MAP
NAME="map"
><AREA
SHAPE="RECT"
COORDS="0,0,73,21"
HREF="../index.htm"
ALT="The UNIX CD Bookshelf"><AREA
SHAPE="RECT"
COORDS="74,0,163,21"
HREF="index.htm"
ALT="UNIX Power Tools"><AREA
SHAPE="RECT"
COORDS="164,0,257,21"
HREF="../unixnut/index.htm"
ALT="UNIX in a Nutshell"><AREA
SHAPE="RECT"
COORDS="258,0,321,21"
HREF="../vi/index.htm"
ALT="Learning the vi Editor"><AREA
SHAPE="RECT"
COORDS="322,0,378,21"
HREF="../sedawk/index.htm"
ALT="sed &amp; awk"><AREA
SHAPE="RECT"
COORDS="379,0,438,21"
HREF="../ksh/index.htm"
ALT="Learning the Korn Shell"><AREA
SHAPE="RECT"
COORDS="439,0,514,21"
HREF="../lrnunix/index.htm"
ALT="Learning the UNIX Operating System"></MAP
></DIV
></BODY
></HTML
>