<html> <head> <title>Gri: `open' command</title> </head> <body bgcolor="#FFFFFF" text="#000000" link="#0000EE" vlink="#551A8B" alink="FF0000"> <!-- newfile Open.html "Gri: `open' command" "Gri Commands" --> <!-- @node Open, Opening Simple Files, New Postscript File, List Of Gri Commands --> <a name="Open" ></a> <img src="./resources/top_banner.gif" usemap="#navigate_top" border="0"> <table summary="top banner" border="0" cellspacing="0" cellpadding="0"> <tr> <td width="150" valign="top"> <font size=-1> <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="Acknowledgments.html">17: Acknowledgments</a><br> <a href="License.html">18: 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> </font> <td width="500" valign="top"> <map name="navigate_top"> <area alt="index.html#Top" shape="rect" coords="5,2,218,24" href="index.html#Top"> <area alt="ListOfGriCommands.html#ListOfGriCommands" shape="rect" coords="516,2,532,24" href="ListOfGriCommands.html#ListOfGriCommands"> <area alt="Gri: `newpage' command" shape="rect" coords="557,2,573,24" href="Newpage.html"> <area alt="Gri: `postscript' command" shape="rect" coords="581,2,599,24" href="PostScript.html"> </map> <map name="navigate_bottom"> <area alt="index.html#Top" shape="rect" coords="5,2,218,24" href="index.html#Top"> <area alt="Gri: `postscript' command" shape="rect" coords="581,2,599,24" href="PostScript.html"></map> <h3>9.3.27: `<font color="#82140F"><code>open</code></font>'</h3> <!-- latex: \index{open} --> There are two styles of `<font color="#82140F"><code>open</code></font>' command. In the first style, a simple file is to be opened. In the second style a unix-like "pipe" is opened, i.e. Gri will read the output of a system command instead of a file. <UL> <LI><a href="Open.html#OpeningSimpleFiles">Opening Simple Files</a>: <LI><a href="Open.html#OpeningPipes">Opening Pipes</a>: </UL> <p> <!-- @node Opening Simple Files, Ascii Files, Open, Open --> <a name="OpeningSimpleFiles" ></a> <h4>9.3.27.1: Opening simple files</h4> <UL> <LI><a href="Open.html#AsciiFiles">Ascii Files</a>: <LI><a href="Open.html#BinaryFiles">Binary Files</a>: <LI><a href="Open.html#NetCDFFiles">NetCDF Files</a>: </UL> <!-- @node Ascii Files, Binary Files, Opening Simple Files, Opening Simple Files --> <a name="AsciiFiles" ></a> <b>Ascii Files</b> Most applications involve ascii files, and these are very easy to handle in Gri. For example given a data file named `<font color="#82140F"><samp>foo.dat</samp></font>', just use the command <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> open foo.dat </font></PRE> </TD> </TR> </TABLE> <p> and then you can read the data using various commands. Thus a complete program might be <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> open foo.dat read columns x y draw curve </font></PRE> </TD> </TR> </TABLE> <p> If a filename contains blanks or punctuation symbols, you must put it in double quotes (`<font color="#82140F"><code>"</code></font>'), e.g. <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> open "foo bar.dat" </font></PRE> </TD> </TR> </TABLE> <p> Indeed, Gri accepts double-quotes on any `<font color="#82140F"><code>open</code></font>' command and some folks use it on all commands, as a matter of habit. <p> Gri can handle compressed files appropriately, e.g. <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> open foo.data.gz </font></PRE> </TD> </TR> </TABLE> <p> so that there is no need to uncompress data for use with Gri. <p> Gri is quite persistant in looking for your file, and if a given file is not found, it will then check to see if a compressed version is available, and use that instead. Thus <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> open foo.dat </font></PRE> </TD> </TR> </TABLE> <p> will look for a file named `<font color="#82140F"><samp>foo.dat.gz</samp></font>' if `<font color="#82140F"><samp>foo.dat</samp></font>' is not available. (Only files compressed with the GNU `<font color="#82140F"><code>gzip</code></font>' utility are handled.) <p> If the `<font color="#82140F"><code>open</code></font>' command was successful in opening the file, it will set the value of the synonym `<font color="#82140F"><code>\.return_value.</code></font>' to the <b>full</b> pathname of the file. Thus, if `<font color="#82140F"><code>open a.dat</code></font>' is done in directory `<font color="#82140F"><code>/home/gri</code></font>', then `<font color="#82140F"><code>\.return_value.</code></font>' will equal the string `<font color="#82140F"><code>/home/gri/a.dat</code></font>'. <p> <!-- @node Binary Files, NetCDF Files, Ascii Files, Opening Simple Files --> <a name="BinaryFiles" ></a> <b>Binary Files</b> Like most computer programs, Gri has some trouble with binary files. One big issue is the so-called "endian" character of the computer. Some computers store multi-byte values with the most significant bytes first, while others store them with the most significant bytes last. The problem is that nothing is stored in data files to indicate which convention was employed. For this reason, a version of Gri compiled on a so-called "big-endian" computer will misinterpret multi-byte values that were created on a so-called "little-endian" computer. Many folks in the scientific community have converted to using the NetCDF format (see <a href="Open.html#NetCDFFiles">NetCDF Files</a>) for precisely this reason, since this format is independent of the endian character of the computer. <p> Presuming an appropriate endian character, however, reading is straightforward. A command of the form <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> open foo.dat binary </font></PRE> </TD> </TR> </TABLE> <p> tells Gri that the data are stored in a binary format. With the above syntax, Gri expects images to be in `<font color="#82140F"><code>unsigned char</code></font>' (8 bits), while other data, such as columns and grids, are expected to be in 32-bit format (suitable for reading into a so-called "float" variable in the C programming language). <p> You may also specify the format directly, as in the following examples; Gri then interprets all data as being in the indicated format and then converts to the internal format before using the data. <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> open \filename binary uchar open \filename binary 8bit open \filename binary int open \filename binary float open \filename binary double open \filename binary 16bit </font></PRE> </TD> </TR> </TABLE> <p> As with ascii files, Gri will automatically uncompress any files that are compressed, and if it fails to find a given filename, it will try to open a compressed version of it (i.e. one with a `<font color="#82140F"><samp>.gz</samp></font>' suffix). <p> <!-- @node NetCDF Files, Opening Pipes, Binary Files, Opening Simple Files --> <a name="NetCDFFiles" ></a> <b>NetCDF Files</b> The NetCDF format provides the best of both worlds. It is binary, so that data are relatively compact, and may be read very quickly. (Reading ascii data is time-consuming in C++, the language in which Gri is written.) But it does not suffer the endian problem problem of normal binary files (see <a href="Open.html#BinaryFiles">Binary Files</a>), since information about the endian character is stored in the file itself, and Gri uses this information to decode the data without difficulty, regardless of the endian characteristics of the computer on which Gri is running and of the computer that created the data. <p> For more information on netCDF format, see <p> `<font color="#82140F"><code>http://www.unidata.ucar.edu/packages/netcdf/index.html</code></font>' <a href="http://www.unidata.ucar.edu/packages/netcdf/index.html"> here </a>. <p> The syntax of opening NetCDF files is as below <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> open foo.nc netCDF </font></PRE> </TD> </TR> </TABLE> <p> and the syntax for reading such files is described in sections on the various `<font color="82140F"><code>read</code></font>' commands (see e.g. see <a href="Read.html#ReadColumns">Read Columns</a>). <p> <!-- @node Opening Pipes, Postscript, NetCDF Files, Open --> <a name="OpeningPipes" ></a> <h4>9.3.27.2: Opening pipes</h4> Sometimes it makes sense to get Gri to work with the results of another command in the OS. Gri handles this by creating a so-called "pipe", thus reading the output from the other command. (Readers familiar with the unix OS will know what pipes are all about, and especially why they are a good thing. Other readers might wish to skip this section.) <p> Suppose we wish to plot an x-y plot using just the first few lines of a datafile named `<font color="#82140F"><samp>foo.dat</samp></font>'. Unix users will know that a good way to see the first few lines of such a file would be to type the command `<font color="#82140F"><code>head foo.dat</code></font>'. They also know that these lines could be provided to a second unix command, named `<font color="#82140F"><samp>do_foo</samp></font>' say, by the command `<font color="#82140F"><code>head foo.dat | do_foo</code></font>'. This uses a so-called "pipe", designated by the vertical line (called a pipe symbol below). <p> Gri can read the output from system commands by using a syntax in which the (quoted) system command ends in a pipe symbol, e.g. <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> open "head foo.dat |" </font></PRE> </TD> </TR> </TABLE> <p> as in the example above. <p> <b>Aside</b>: When pipe-open commands are used, Gri creates a temporary file (often located in `<font color="#82140F"><samp>/usr/tmp</samp></font>', but that varies with machine). This is automatically cleaned up when Gri completes executation, but if Gri dies (or is interrupted) before it finishes, you'll be left with an extra file in this temporary-storage directory. It's up to you to clean that directory up from time to time. <p> Some common examples of pipe-open commands are given below. <p> <ol> <p> <li> <b>Comma-separated values</b> are common in files created by, or intended for, spreadsheets. Since Gri expects data elements to be separated by blanks (or tabs), you'll have to convert the commas into blanks. There are many ways to do that using pipes, e.g. `<font color="#82140F"><samp>sed</samp></font>' system utility, e.g. <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> open "sed -e 's/,/ /g' foo.dat |" </font></PRE> </TD> </TR> </TABLE> <p> Other unix facilities, such as `<font color="#82140F"><code>tr</code></font>' will also work, of course. If the file has headers, you'll want to remove them also. This can be done with the `<font color="82140F"><code>skip</code></font>' command (see <a href="Skip.html#Skip">Skip</a>) but you could also do it at the open stage, e.g. to remove the first two lines, use <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> open "sed -e 's/,/ /g' foo.dat | tail +2 |" </font></PRE> </TD> </TR> </TABLE> <p> <li> <b>Manipulating column data</b> is done by e.g. <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> open "cat foo.dat | awk '{$1, $2 * 22}' |" </font></PRE> </TD> </TR> </TABLE> <p> where `<font color="#82140F"><samp>awk</samp></font>' has been used to multiply the second column in the file named `<font color="#82140F"><samp>foo.dat</samp></font>' by 22. <p> <li> <b>Web-based files</b> may be opened by various OS tools, e.g. with `<font color="#82140F"><code>lynx</code></font>': <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> \url = "http://gri.sourceforge.net/gridoc/html/examples/example1.dat" open "lynx -dump \url |" read columns x y draw curve </font></PRE> </TD> </TR> </TABLE> <p> or with `<font color="#82140F"><code>wget</code></font>': <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> \url = "http://gri.sourceforge.net/gridoc/html/examples/example1.dat" open "wget --quiet -O - \url |" read columns x y draw curve </font></PRE> </TD> </TR> </TABLE> <p> <li> <b>Time-based and geographical data</b> are sometimes encountered. For an example, suppose that longitude/latitude (i.e. x/y) data are stored in Hour.minutesecond format, e.g. 12.2133 means hour 12, minute 21, second 33. Gri doesn't read HMS format, but gawk can be told to: <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> open "cat datafile.HMS | \ awk '{ \ split($1, hms, \".\"); \ h = hms[1]; \ m = int(hms[2] / 100); \ s = hms[2] - 100 * m; \ x = h + m / 60 + s / 3600; \ split($2, hms, \".\"); \ h = hms[1]; \ m = int(hms[2] / 100); \ s = hms[2] - 100 * m; \ y = h + m / 60 + s / 3600; \ print(x,y) \ }' | " read columns x y </font></PRE> </TD> </TR> </TABLE> <p> <li> <b>Timeseries data</b> are often stored in formats that blend letters and numbers. For one thing, using letters (e.g. `<font color="#82140F"><code>aug</code></font>') removes an ambiguity in numerically-based data. (Example: 02/03/2000 means one thing to an American and another thing in the rest of the world. However, everybody agrees on what 2000-Feb-03 means.) Suppose, for example, that we have data in a format such as <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> Tue_Jul_25_11:07:51 0.62 Tue_Jul_25_11:22:51 0.59 Tue_Jul_25_11:37:51 0.56 </font></PRE> </TD> </TR> </TABLE> <p> (stored in a file called `<font color="#82140F"><samp>foo.dat</samp></font>' say) and we want a graph of the y-variable (0.62, 0.59, 0.56) versus x-variable, time expressed say as seconds in the day. Then here is how that could be done: <p> <TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%"> <TR> <TD> <PRE> <font color="#82140F"> open "cat foo.dat |\ sed -e 's/_/ /g' -e 's/:/ /g' |\ awk '{print ($4*3600+$5*60+$6, $7)}' |" read columns x y draw curve </font></PRE> </TD> </TR> </TABLE> <p> Note that the actual day information is skipped in this example; seasoned `<font color="#82140F"><code>awk</code></font>' users could easily fill in the code to handle datasets spanning several days. </ol> <p> </table> <img src="./resources/bottom_banner.gif" usemap="#navigate_bottom" border="0"> </body> </html>