project1.htm

· Develop clear, readable, well-documented and well-designed programs in the C Programming Language.

have special meanings within the context of user input. These characters include 
&, |, <, and >. Your shell can assume that these meta-characters 
cannot occur inside strings representing programs, arguments, or files. Instead 
they are reserved for use by the shell. The purpose of meta-characters is 
discussed later in this section. </SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
<H4><SPAN lang=EN-US>Parsing User Input – Internal Commands</SPAN></H4>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US>If the command 
line matches the format of an internal command as described earlier in this 
document, it should be accepted as an internal command.<SPAN 
style="mso-spacerun: yes">&nbsp; </SPAN>If not, it should be considered to 
specify the execution of external programs, or an error, as appropriate. 
</SPAN></P>
<P class=MsoNormal><SPAN 
lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
<P class=MsoNormal><SPAN lang=EN-US><SPAN 
style="mso-tab-count: 1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 
</SPAN><B style="mso-bidi-font-weight: normal">Parsing User Input – Executing A 
Single Program<o:p></o:p></B></SPAN></P>
<P class=MsoNormal><SPAN 
lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US>The execution of 
a<SPAN style="mso-spacerun: yes">&nbsp; </SPAN>program is specified by a 
sequence of delimited strings. The first of these is the name of the executable 
file that contains the desired program (modulo a search path as explained in the 
</SPAN><SPAN class=HTMLTypewriter><SPAN lang=EN-US 
style="FONT-FAMILY: 'Courier New'; FONT-SIZE: 10pt; mso-bidi-font-family: 黑体">execvp</SPAN></SPAN><SPAN 
lang=EN-US> man page, see </SPAN><SPAN class=HTMLTypewriter><SPAN lang=EN-US 
style="FONT-FAMILY: 'Courier New'; FONT-SIZE: 10pt; mso-bidi-font-family: 黑体">man 
-s 2 execvp</SPAN></SPAN><SPAN lang=EN-US>) and the others are arguments passed 
to the program. The command is an error if the executable file named by the 
first string does not exist, or is not an executable. </SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
<H4><SPAN lang=EN-US>Paring User Input – I/O Redirection</SPAN></H4>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US>A program's 
execution specified as above may be followed by the meta-character </SPAN><SPAN 
lang=EN-US style="FONT-FAMILY: Symbol">&lt; </SPAN><SPAN lang=EN-US><SPAN 
style="mso-spacerun: yes">&nbsp;</SPAN>or &gt; which is in turn followed by a 
file name. In the case of &lt;, the input of the program will be redirected from 
the specified file name. In the case of &gt;, the output of the program will be 
redirected to the specified file name. If the output file does not exist, it 
should be created. If the input file does not exist, this is an 
error.</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
<H4><SPAN lang=EN-US>Parsing User Input – Pipes and Cooperating 
Programs</SPAN></H4>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US>Several program 
invocations can be present in a single command line, when separated by the shell 
meta-character ``</SPAN><SPAN lang=EN-US 
style="FONT-FAMILY: Symbol">|</SPAN><SPAN lang=EN-US>''. In this case, the shell 
should fork all of them, chaining their outputs and inputs using pipes 
appropriately. For instance, the command line </SPAN></P>
<P class=MsoNormal><SPAN class=HTMLTypewriter><SPAN lang=EN-US 
style="FONT-FAMILY: 'Courier New'; FONT-SIZE: 10pt; mso-bidi-font-family: 黑体"><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 36pt; TEXT-INDENT: 36pt"><SPAN 
class=HTMLTypewriter><SPAN lang=EN-US 
style="FONT-FAMILY: 'Courier New'; FONT-SIZE: 10pt; mso-bidi-font-family: 黑体">progA 
argA1 argA2 &lt; infile </SPAN></SPAN><SPAN class=HTMLTypewriter><SPAN 
lang=EN-US style="FONT-FAMILY: Symbol; FONT-SIZE: 10pt">|</SPAN></SPAN><SPAN 
class=HTMLTypewriter><SPAN lang=EN-US 
style="FONT-FAMILY: 'Courier New'; FONT-SIZE: 10pt; mso-bidi-font-family: 黑体"> 
progB argB1 &gt; outfile</SPAN></SPAN><SPAN lang=EN-US> </SPAN></P>
<P class=MsoNormal><SPAN 
lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US>should fork 
</SPAN><SPAN class=HTMLTypewriter><SPAN lang=EN-US 
style="FONT-FAMILY: 'Courier New'; FONT-SIZE: 10pt; mso-bidi-font-family: 黑体">progA</SPAN></SPAN><SPAN 
lang=EN-US> and </SPAN><SPAN class=HTMLTypewriter><SPAN lang=EN-US 
style="FONT-FAMILY: 'Courier New'; FONT-SIZE: 10pt; mso-bidi-font-family: 黑体">progB</SPAN></SPAN><SPAN 
lang=EN-US>, make the input for </SPAN><SPAN class=HTMLTypewriter><SPAN 
lang=EN-US 
style="FONT-FAMILY: 'Courier New'; FONT-SIZE: 10pt; mso-bidi-font-family: 黑体">progA</SPAN></SPAN><SPAN 
lang=EN-US> come from file </SPAN><SPAN class=HTMLTypewriter><SPAN lang=EN-US 
style="FONT-FAMILY: 'Courier New'; FONT-SIZE: 10pt; mso-bidi-font-family: 黑体">infile</SPAN></SPAN><SPAN 
lang=EN-US>, the output from </SPAN><SPAN class=HTMLTypewriter><SPAN lang=EN-US 
style="FONT-FAMILY: 'Courier New'; FONT-SIZE: 10pt; mso-bidi-font-family: 黑体">progA</SPAN></SPAN><SPAN 
lang=EN-US> go to the input of </SPAN><SPAN class=HTMLTypewriter><SPAN 
lang=EN-US 
style="FONT-FAMILY: 'Courier New'; FONT-SIZE: 10pt; mso-bidi-font-family: 黑体">progB</SPAN></SPAN><SPAN 
lang=EN-US>, and the output of </SPAN><SPAN class=HTMLTypewriter><SPAN 
lang=EN-US 
style="FONT-FAMILY: 'Courier New'; FONT-SIZE: 10pt; mso-bidi-font-family: 黑体">progB</SPAN></SPAN><SPAN 
lang=EN-US> go to the file </SPAN><SPAN class=HTMLTypewriter><SPAN lang=EN-US 
style="FONT-FAMILY: 'Courier New'; FONT-SIZE: 10pt; mso-bidi-font-family: 黑体">outfile</SPAN></SPAN><SPAN 
lang=EN-US>. This should be accomplished using a pipe IPC primative. </SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US>A command line 
with one or more ``pipes'' is an error if any of its component program 
invocations is an error. A command line with ``pipes'' is an error if the input 
of any but the first command is redirected, or if the output of any but the last 
command is redirected. A job consisting of<SPAN style="mso-spacerun: yes">&nbsp; 
</SPAN>piped processes is not considered to have completed until all of its 
component processes have completed.</SPAN></P>
<P class=MsoNormal><SPAN 
lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
<P class=MsoNormal><SPAN lang=EN-US><SPAN 
style="mso-tab-count: 1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 
</SPAN><B style="mso-bidi-font-weight: normal">Parsing User Input – Background 
Jobs<o:p></o:p></B></SPAN></P>
<P class=MsoNormal><SPAN 
lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US>The user can 
specify that a job should be executed in the background by ending the command 
line with the meta-character &amp;. If this is the case, all program invocations 
required by the command line are to be carried out in the background. 
</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US><SPAN 
style="mso-spacerun: yes">&nbsp;</SPAN></SPAN></P>
<H5><A name=tth_sEc3.1></A><SPAN lang=EN-US>Parsing User Input – A 
Grammar</SPAN></H5>
<P class=MsoNormal><SPAN 
lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US>The grammar below 
provides a more formal description of the syntax governing user input. If you 
are using lex/yacc to implement your parser, you’ll have to massage the grammar 
slightly to make it LALR(1). This grammar doesn’t include the special 
keystrokes, because they won’t show up in stdin as user input and should be 
handled separately.</SPAN></P>
<P class=MsoNormal style="TEXT-INDENT: 36pt"><SPAN lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 36pt"><SPAN lang=EN-US>A </SPAN><SPAN 
class=HTMLTypewriter><SPAN lang=EN-US 
style="FONT-FAMILY: 'Courier New'; FONT-SIZE: 10pt; mso-bidi-font-family: 黑体">CommandLine</SPAN></SPAN><SPAN 
lang=EN-US> is legal input provided by the user, as a direction to the shell, in 
response to the prompt. The grammar assumes that the existence of a lexical 
analyzer that considers blank-space to be a delimiter, recognizes the 
meta-characters as tokens, &amp;c.</SPAN></P>
<P class=MsoNormal><SPAN 
lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
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<TABLE cellPadding=0 cellSpacing=0 width="100%">
  <TBODY>
  <TR>
    <TD><![endif]>
      <DIV>
      <P class=MsoNormal><SPAN lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
      <P class=MsoNormal style="TEXT-INDENT: 36pt"><SPAN 
      lang=EN-US>CommandLine&nbsp;&nbsp;&nbsp; :=&nbsp; <SPAN 
      style="mso-tab-count: 1">&nbsp;&nbsp;&nbsp;&nbsp; </SPAN>NULL</SPAN></P>
      <P class=MsoNormal style="MARGIN-LEFT: 108pt; TEXT-INDENT: 36pt"><SPAN 
      lang=EN-US>FgCommandLine</SPAN></P>
      <P class=MsoNormal style="MARGIN-LEFT: 108pt; TEXT-INDENT: 36pt"><SPAN 
      lang=EN-US>FgCommandLine &amp;</SPAN></P>
      <P class=MsoNormal><SPAN lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
      <P class=MsoNormal style="TEXT-INDENT: 36pt"><SPAN 
      lang=EN-US>FgCommandLine&nbsp; :=&nbsp; <SPAN 
      style="mso-tab-count: 1">&nbsp;&nbsp; </SPAN>SimpleCommand</SPAN></P>
      <P class=MsoNormal style="MARGIN-LEFT: 108pt; TEXT-INDENT: 36pt"><SPAN 
      lang=EN-US>FirstCommand MidCommand LastCommand</SPAN></P>
      <P class=MsoNormal><SPAN lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
      <P class=MsoNormal style="TEXT-INDENT: 36pt"><SPAN 
      lang=EN-US>SimpleCommand&nbsp; :=&nbsp; <SPAN 
      style="mso-tab-count: 1">&nbsp;&nbsp; </SPAN>ProgInvocation InputRedirect 
      OutputRedirect</SPAN></P>
      <P class=MsoNormal><SPAN lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
      <P class=MsoNormal style="TEXT-INDENT: 36pt"><SPAN 
      lang=EN-US>FirstCommand&nbsp;&nbsp; :=&nbsp; <SPAN 
      style="mso-tab-count: 1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 
      </SPAN>ProgInvocation InputRedirect</SPAN></P>
      <P class=MsoNormal><SPAN lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
      <P class=MsoNormal style="TEXT-INDENT: 36pt"><SPAN 
      lang=EN-US>MidCommand&nbsp;&nbsp;&nbsp;&nbsp; :=<SPAN 
      style="mso-tab-count: 1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 
      </SPAN>NULL</SPAN></P>
      <P class=MsoNormal><SPAN 
      lang=EN-US>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 
      <SPAN 
      style="mso-tab-count: 3">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 
      </SPAN>| ProgInvocation MidCommand</SPAN></P>
      <P class=MsoNormal><SPAN lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
      <P class=MsoNormal style="TEXT-INDENT: 36pt"><SPAN 
      lang=EN-US>LastCommand&nbsp;&nbsp;&nbsp; :=&nbsp; <SPAN 
      style="mso-tab-count: 1">&nbsp;&nbsp;&nbsp;&nbsp; </SPAN>| ProgInvocation 
      OutputRedirect</SPAN></P>
      <P class=MsoNormal><SPAN 
      lang=EN-US>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</SPAN></P>
      <P class=MsoNormal style="TEXT-INDENT: 36pt"><SPAN 
      lang=EN-US>ProgInvocation :=&nbsp; <SPAN 
      style="mso-tab-count: 1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 
      </SPAN>ExecFile Args</SPAN></P>
      <P class=MsoNormal><SPAN lang=EN-US><![if !supportEmptyParas]>&nbsp;<![endif]><o:p></o:p></SPAN></P>
      <P class=MsoNormal style="TEXT-INDENT: 36pt"><SPAN