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<title>Gri: Using System Tools With Gri</title>
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<br>
Chapters:
<br>
<a href="Introduction.html">1: Introduction</a><br>
<a href="SimpleExample.html">2: Simple example</a><br>
<a href="InvokingGri.html">3: Invocation</a><br>
<a href="GettingMoreControl.html">4: Finer Control</a><br>
<a href="X-Y.html">5: X-Y Plots</a><br>
<a href="ContourPlots.html">6: Contour Plots</a><br>
<a href="Images.html">7: Image Plots</a><br>
<a href="Examples.html">8: Examples</a><br>
<a href="Commands.html">9: Gri Commands</a><br>
<a href="Programming.html">10: Programming</a><br>
<a href="Environment.html">11: Environment</a><br>
<a href="Emacs.html">12: Emacs Mode</a><br>
<a href="History.html">13: History</a><br>
<a href="Installation.html">14: Installation</a><br>
<a href="Bugs.html">15: Gri Bugs</a><br>
<a href="TestSuite.html">16: Test Suite</a><br>
<a href="GriInThePress.html">17: Gri in Press</a><br>
<a href="Acknowledgments.html">18: Acknowledgments</a><br>
<a href="License.html">19: License</a><br>
<br>
Indices:<br>
<a href="ConceptIndex.html"><i>Concepts</i></a><br>
<a href="CommandIndex.html"><i>Commands</i></a><br>
<a href="BuiltinIndex.html"><i>Variables</i></a><br>
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<h2>11.2: Using System Tools With Gri</h2>
Using system tools to manipulate your data makes sense for several
reasons. First, you may be familiar with those tools already. Second,
learning these tools will help you in all your work.
<p>
<UL>
<LI><a href="SystemTools.html#WhyUseTheEnvironment">Why Use The Environment</a>: Introduction
<LI><a href="SystemTools.html#Grep">Grep</a>: Search files for patterns
<LI><a href="SystemTools.html#Sed">Sed</a>: Serial editor
<LI><a href="SystemTools.html#Awk">Awk</a>: Search and manipulate data
<LI><a href="SystemTools.html#Perl">Perl</a>: Search and manipulate data
</UL>
<!-- @node Why Use The Environment, Grep, Using System Tools, Using System Tools -->
<a name="WhyUseTheEnvironment" ></a>
<h3>11.2.1: Introduction</h3>
Each of the programs listed in the sections below is available for Unix.
Some (e.g. Perl and the Awk variant called Gawk) are available on other
operating systems as well. Each of these tools is fully documented in
Unix manpages, so here I'll just give an indication of a couple of
useful techniques you might want to use.
<p>
Bear in mind that these tools can do very similar jobs. For example,
Awk can do much of what Sed and Grep can do, but also a whole lot more.
If you don't know Sed or Grep, I suggest you learn Awk instead. Then
again, Perl can also do anything Gawk can do, and a whole lot more!
(For one thing, it is easier to work with multiple files in Perl.) In
fact, Perl is the most powerful of this list. If you know none of these
tools, you might want to learn Perl instead of the others. But Perl is
more complicated for simple work than Awk is, so the most reasonable
path might be to learn both Awk and Perl.
<p>
For simple applications, you will probably want to use these tools in a
piped open command, e.g.
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "awk '{print $1, $3/$2}' MyFile |"
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
which creates a temporary file (automatically erased when Gri finishes)
which contains the output from running the system command that preceeds
the pipe symbol (`<font color="82140F"><code>|</code></font>') (see <a href="Open.html#Open">Open</a>).
<p>
(Here and in all the examples of this chapter, it is assumed that the
user's input file is named `<font color="#82140F"><samp>MyFile</samp></font>'.)
<p>
For more complicated appplications, you may use the Gri `<font color="#82140F"><code>system</code></font>'
command as follows.
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
system perl >tmp <<"EOF"
open(IN, "MyFile");
while(<IN>) {
($x, $y) = split;
print "$x", " ", cos($y), "\n";
}
EOF
open tmp
read columns x y
draw curve
system rm -f tmp
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
Here a temporary file, named `<font color="#82140F"><samp>tmp</samp></font>', has been used to store the
results of the calculation. Note that this file was specifically
cleaned up by the second `<font color="#82140F"><code>system</code></font>' command. (Many folks, including
the author, would prefer to take the perl script out of the above and
make it a standalone executable script, calling it from Gri with the
one-line form. But it is just a matter of style.)
<p>
<!-- @node Grep, Sed, Why Use The Environment, Using System Tools -->
<a name="Grep" ></a>
<h3>11.2.2: Grep</h3>
The most common application of Grep is to select lines matching a
pattern, or not matching a pattern. Here is how to skip all lines with
the word "HEADER" in them:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "grep -v 'HEADER' MyFile |"
...
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
Note that Gawk and Perl do this just as easily.
<p>
<!-- @node Sed, Awk, Grep, Using System Tools -->
<a name="Sed" ></a>
<h3>11.2.3: Sed</h3>
Sed is normally used for simple changes to files. For example, if you
have columnar data which are separated with comma characters instead of
whitespace, you could make it Gri compatible by
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "sed -e 's/,/ /g' MyFile |"
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
Where the `<font color="#82140F"><code>-e</code></font>' flag indicates that the next item is a command to
Sed, in this case a command to switch ("s") the comma character with the
blank character, globally ("g") across each line of the file. See also
the overview of Perl.
<p>
<!-- @node Awk, Perl, Sed, Using System Tools -->
<a name="Awk" ></a>
<h3>11.2.4: Awk</h3>
Awk is great for one-liners. If your system lacks Awk, you can procure
the GNU version, called Gawk, from the web for free. For better or
worse, Gawk is not fully compatible with Awk. The good thing is that
Gawk is pretty much the same on all operating systems, whereas the
installed Awk may not be. I use Gawk instead of Awk, for this reason
and because it is normally faster.
<p>
The main concept in Awk is of "patterns" and "actions." In the Awk
syntax, actions are written in braces following patterns written with no
braces. (This will become clear presently.)
<p>
Whenever a line in the data file matches the pattern, the action is
done.
<p>
The default pattern is to match to every line in the file. This is done
if no pattern is supplied.
<p>
The default action is to print the line, and this is done if no action
is supplied.
<p>
Here are a few examples. To skip first 10 lines of a file:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "awk 'NR>10' MyFile |"
read ...
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
Here the pattern was that `<font color="#82140F"><code>NR</code></font>' (a special Awk variable for the
number of the record, starting with 1 for the first line of the file)
exceeded 10. And the action was taken by default since nothing was
supplied between braces, to print this line.
<p>
To plot the cosine of the second column against the first column:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "awk '{print ($1, cos($2))}' MyFile |"
read columns x y
draw curve
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
Here no pattern was supplied, so the action was done for every line.
<p>
Combining these two forms, then, and supplying both a pattern and an
action, here is how one might print the first and eighth columns of the
file `<font color="#82140F"><samp>MyFile</samp></font>', but only for the first 10 lines of the file:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "awk NR<=10 {print ($1, $8)} MyFile |"
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
<!-- @node Perl, Discussion Group, Awk, Using System Tools -->
<a name="Perl" ></a>
<h3>11.2.5: Perl</h3>
Perl can do almost <b>anything</b> with your data, since it is a full
programming language designed to also emulate several Unix utilities.
<p>
In perl, as in other commands, the commandline switch `<font color="#82140F"><code>-e</code></font>'
indicates that a Perl command is given in the next word of the command
line. The commandline switch `<font color="#82140F"><code>-p</code></font>' indicates to print the line,
after any indicated Perl actions have been done on it. Here, for
example, is how one would emulate a Sed replacement of comma by blank:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "perl -pe 's/,/ /g' MyFile |"
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
Perl also has a commandline switch `<font color="#82140F"><code>-a</code></font>' indicating that lines
should be "autosplit" into an array called `<font color="#82140F"><code>$F</code></font>'. The first element
of this array is `<font color="#82140F"><code>$F[0]</code></font>'. Splitting is normally done on
white-space character(s), although this may be changed if desired.
Here, for example, is how to take the cosine of the second column of a
file, and print this after the first column:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "perl -pea 'print($F[0], cos($F[1]))' MyFile |"
</font></PRE>
</TD>
</TR>
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<p>
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