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fox2mike 05/07/14 18:03:09 |
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Added: xml/htdocs/doc/en/articles bash-by-example-p2.xml |
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bash-by-example-p3.xml |
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Log: |
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#99008 - Bash by example, parts 2 and 3, initial version. Thanks to Lukasz Damentko. |
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|
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Revision Changes Path |
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1.1 xml/htdocs/doc/en/articles/bash-by-example-p2.xml |
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|
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file : http://www.gentoo.org/cgi-bin/viewcvs.cgi/xml/htdocs/doc/en/articles/bash-by-example-p2.xml?rev=1.1&content-type=text/x-cvsweb-markup&cvsroot=gentoo |
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plain: http://www.gentoo.org/cgi-bin/viewcvs.cgi/xml/htdocs/doc/en/articles/bash-by-example-p2.xml?rev=1.1&content-type=text/plain&cvsroot=gentoo |
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|
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Index: bash-by-example-p2.xml |
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=================================================================== |
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<?xml version='1.0' encoding="UTF-8"?> |
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<!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/articles/bash-by-example-p2.xml,v 1.1 2005/07/14 18:03:09 fox2mike Exp $ --> |
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<!DOCTYPE guide SYSTEM "/dtd/guide.dtd"> |
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|
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<guide link="/doc/en/articles/bash-by-example-part2.xml"> |
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<title>Bash by example, Part 2</title> |
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|
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<author title="Author"> |
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<mail link="drobbins@g.o">Daniel Robbins</mail> |
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</author> |
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<author title="Editor"> |
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<mail link="rane@××××××.pl">Łukasz Damentko</mail> |
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</author> |
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|
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<abstract> |
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In his introductory article on bash, Daniel Robbins walked you through some of |
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the scripting language's basic elements and reasons for using bash. In this, the |
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second installment, Daniel picks up where he left off and looks at bash's basic |
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constructs like conditional (if-then) statements, looping, and more. |
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</abstract> |
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|
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<!-- The original version of this article was published on IBM developerWorks, |
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and is property of Westtech Information Services. This document is an updated |
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version of the original article, and contains various improvements made by the |
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Gentoo Linux Documentation team --> |
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|
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<version>1.0</version> |
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<date>2005-07-14</date> |
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|
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<chapter> |
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<title>More bash programming fundamentals</title> |
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<section> |
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<title>Accepting arguments</title> |
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<body> |
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|
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<note> |
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The original version of this article was published on IBM developerWorks, and |
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is property of Westtech Information Services. This document is an updated |
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version of the original article, and contains various improvements made by the |
| 55 |
Gentoo Linux Documentation team. |
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</note> |
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|
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<p> |
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Let's start with a brief tip on handling command-line arguments, and then look |
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at bash's basic programming constructs. |
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</p> |
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|
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<p> |
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In the sample program in the <uri |
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link="/doc/en/articles/bash-by-example-p1.xml">introductory article</uri>, we |
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used the environment variable "$1", which referred to the first command-line |
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argument. Similarly, you can use "$2", "$3", etc. to refer to the second and |
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third arguments passed to your script. Here's an example: |
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</p> |
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|
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<pre caption="Referring to arguments passed to the script"> |
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#!/usr/bin/env bash |
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|
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echo name of script is $0 |
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echo first argument is $1 |
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echo second argument is $2 |
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echo seventeenth argument is $17 |
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echo number of arguments is $# |
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</pre> |
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|
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<p> |
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The example is self explanatory except for two small details. First, "$0" will |
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expand to the name of the script, as called from the command line, and "$#" will |
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expand to the number of arguments passed to the script. Play around with the |
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above script, passing different kinds of command-line arguments to get the hang |
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of how it works. |
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</p> |
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|
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<p> |
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Sometimes, it's helpful to refer to all command-line arguments at once. For this |
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purpose, bash features the "$@" variable, which expands to all command-line |
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parameters separated by spaces. We'll see an example of its use when we take a |
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look at "for" loops, a bit later in this article. |
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</p> |
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|
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</body> |
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</section> |
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<section> |
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<title>Bash programming constructs</title> |
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<body> |
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|
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<p> |
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If you've programmed in a procedural language like C, Pascal, Python, or Perl, |
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then you're familiar with standard programming constructs like "if" statements, |
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"for" loops, and the like. Bash has its own versions of most of these standard |
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constructs. In the next several sections, I will introduce several bash |
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constructs and demonstrate the differences between these constructs and others |
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you are already familiar with from other programming languages. If you haven't |
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programmed much before, don't worry. I include enough information and examples |
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so that you can follow the text. |
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</p> |
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|
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</body> |
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</section> |
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<section> |
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<title>Conditional love</title> |
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<body> |
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|
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<p> |
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If you've ever programmed any file-related code in C, you know that it requires |
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a significant amount of effort to see if a particular file is newer than |
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another. That's because C doesn't have any built-in syntax for performing such a |
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comparison; instead, two stat() calls and two stat structures must be used to |
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perform the comparison by hand. In contrast, bash has standard file comparison |
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operators built in, so determining if "<path>/tmp/myfile</path> is readable" is |
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as easy as checking to see if "<c>$myvar</c> is greater than 4". |
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</p> |
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|
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<p> |
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The following table lists the most frequently used bash comparison operators. |
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You'll also find an example of how to use every option correctly. The example is |
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meant to be placed immediately after the "if". For example: |
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</p> |
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|
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<pre caption="Bash comparison operator"> |
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if [ -z "$myvar" ] |
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then |
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echo "myvar is not defined" |
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fi |
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</pre> |
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|
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<p> |
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Sometimes, there are several different ways that a particular comparison can be |
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made. For example, the following two snippets of code function identically: |
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</p> |
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|
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<pre caption="Two ways of making comparison"> |
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if [ "$myvar" -eq 3 ] |
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then |
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echo "myvar equals 3" |
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fi |
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|
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if [ "$myvar" = "3" ] |
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then |
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echo "myvar equals 3" |
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fi |
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</pre> |
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|
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<p> |
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In the above two comparisons do exactly the same thing, but the first uses |
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arithmetic comparison operators, while the second uses string comparison |
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operators. |
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</p> |
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|
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</body> |
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</section> |
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<section> |
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<title>String comparison caveats</title> |
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<body> |
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|
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<p> |
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Most of the time, while you can omit the use of double quotes surrounding |
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strings and string variables, it's not a good idea. Why? Because your code will |
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work perfectly, unless an environment variable happens to have a space or a tab |
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in it, in which case bash will get confused. Here's an example of a fouled-up |
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comparison: |
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</p> |
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|
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<pre caption="Fouled-up comparison example"> |
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if [ $myvar = "foo bar oni" ] |
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then |
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echo "yes" |
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fi |
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</pre> |
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|
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<p> |
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In the above example, if myvar equals "foo", the code will work as expected and |
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not print anything. However, if myvar equals "foo bar oni", the code will fail |
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with the following error: |
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</p> |
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|
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<pre caption="Error when variable contains spaces"> |
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[: too many arguments |
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</pre> |
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|
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<p> |
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In this case, the spaces in "$myvar" (which equals "foo bar oni") end up |
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confusing bash. After bash expands "$myvar", it ends up with the following |
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comparison: |
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</p> |
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|
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<pre caption="Ending comparison"> |
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[ foo bar oni = "foo bar oni" ] |
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</pre> |
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|
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<p> |
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Because the environment variable wasn't placed inside double quotes, bash thinks |
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that you stuffed too many arguments in-between the square brackets. You can |
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easily eliminate this problem by surrounding the string arguments with |
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double-quotes. Remember, if you get into the habit of surrounding all string |
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arguments and environment variables with double-quotes, you'll eliminate many |
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similar programming errors. Here's how the "foo bar oni" comparison should have |
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been written: |
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</p> |
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|
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|
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|
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1.1 xml/htdocs/doc/en/articles/bash-by-example-p3.xml |
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|
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file : http://www.gentoo.org/cgi-bin/viewcvs.cgi/xml/htdocs/doc/en/articles/bash-by-example-p3.xml?rev=1.1&content-type=text/x-cvsweb-markup&cvsroot=gentoo |
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plain: http://www.gentoo.org/cgi-bin/viewcvs.cgi/xml/htdocs/doc/en/articles/bash-by-example-p3.xml?rev=1.1&content-type=text/plain&cvsroot=gentoo |
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|
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Index: bash-by-example-p3.xml |
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=================================================================== |
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<?xml version='1.0' encoding="UTF-8"?> |
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<!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/articles/bash-by-example-p3.xml,v 1.1 2005/07/14 18:03:09 fox2mike Exp $ --> |
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<!DOCTYPE guide SYSTEM "/dtd/guide.dtd"> |
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|
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<guide link="/doc/en/articles/bash-by-example-part3.xml"> |
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<title>Bash by example, Part 3</title> |
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|
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<author title="Author"> |
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<mail link="drobbins@g.o">Daniel Robbins</mail> |
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</author> |
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<author title="Editor"> |
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<mail link="rane@××××××.pl">Łukasz Damentko</mail> |
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</author> |
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|
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<abstract> |
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In his final Bash by example article, Daniel Robbins takes a good look at the |
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Gentoo Linux ebuild system, an excellent example of the power of bash. Step by |
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step, he shows you how the ebuild system was implemented, and touches on many |
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handy bash techniques and design strategies. By the end of the article, you'll |
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have a good grasp of what's involved in producing a full-blown bash-based |
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application, as well as a start at coding your own auto-build system. |
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</abstract> |
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|
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<!-- The original version of this article was published on IBM developerWorks, |
| 249 |
and is property of Westtech Information Services. This document is an updated |
| 250 |
version of the original article, and contains various improvements made by the |
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Gentoo Linux Documentation team --> |
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|
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<version>1.0</version> |
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<date>2005-07-14</date> |
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|
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<chapter> |
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<title>Exploring the ebuild system</title> |
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<section> |
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<title>Enter the ebuild system</title> |
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<body> |
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|
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<note> |
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The original version of this article was published on IBM developerWorks, and |
| 264 |
is property of Westtech Information Services. This document is an updated |
| 265 |
version of the original article, and contains various improvements made by the |
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Gentoo Linux Documentation team. |
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</note> |
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|
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<p> |
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I've really been looking forward to this third and final <e>Bash by example</e> |
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article, because now that we've already covered bash programming fundamentals in |
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<uri link="/doc/en/articles/bash-by-example-p1.xml">Part 1</uri> and <uri |
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link="/doc/en/articles/bash-by-example-p2.xml">Part 2</uri>, we can focus on |
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more advanced topics, like bash application development and program design. For |
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this article, I will give you a good dose of practical, real-world bash |
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development experience by presenting a project that I've spent many hours coding |
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and refining: The Gentoo Linux ebuild system. |
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</p> |
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|
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<p> |
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I'm the chief architect of Gentoo Linux, a next-generation Linux OS currently in |
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beta. One of my primary responsibilities is to make sure that all of the binary |
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packages (similar to RPM packages) are created properly and work together. As |
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you probably know, a standard Linux system is not composed of a single unified |
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source tree (like BSD), but is actually made up of about 25+ core packages that |
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work together. Some of the packages include: |
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</p> |
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|
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<table> |
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<tr> |
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<th>Package</th> |
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<th>Description</th> |
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</tr> |
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<tr> |
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<ti>linux</ti> |
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<ti>The actual kernel</ti> |
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</tr> |
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<tr> |
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<ti>util-linux</ti> |
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<ti>A collection of miscellaneous Linux-related programs</ti> |
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</tr> |
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<tr> |
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<ti>e2fsprogs</ti> |
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<ti>A collection of ext2 filesystem-related utilities</ti> |
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</tr> |
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<tr> |
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<ti>glibc</ti> |
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<ti>The GNU C library</ti> |
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</tr> |
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</table> |
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|
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<p> |
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Each package is in its own tarball and is maintained by separate independent |
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developers, or teams of developers. To create a distribution, each package has |
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to be separately downloaded, compiled, and packaged. Every time a package must |
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be fixed, upgraded, or improved, the compilation and packaging steps must be |
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repeated (and this gets old really fast). To help eliminate the repetitive steps |
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involved in creating and updating packages, I created the ebuild system, written |
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almost entirely in bash. To enhance your bash knowledge, I'll show you how I |
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implemented the unpack and compile portions of the ebuild system, step by step. |
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As I explain each step, I'll also discuss why certain design decisions were |
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made. By the end of this article, not only will you have an excellent grasp of |
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larger-scale bash programming projects, but you'll also have implemented a good |
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portion of a complete auto-build system. |
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</p> |
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|
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</body> |
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</section> |
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<section> |
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<title>Why bash?</title> |
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<body> |
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|
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<p> |
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Bash is an essential component of the Gentoo Linux ebuild system. It was chosen |
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as ebuild's primary language for a number of reasons. First, it has an |
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uncomplicated and familiar syntax that is especially well suited for calling |
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external programs. An auto-build system is "glue code" that automates the |
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calling of external programs, and bash is very well suited to this type of |
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application. Second, Bash's support for functions allowed the ebuild system to |
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have modular, easy-to-understand code. Third, the ebuild system takes advantage |
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of bash's support for environment variables, allowing package maintainers and |
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developers to configure it easily, on-the-fly. |
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</p> |
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|
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</body> |
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</section> |
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<section> |
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<title>Build process review</title> |
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<body> |
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|
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<p> |
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Before we look at the ebuild system, let's review what's involved in getting a |
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package compiled and installed. For our example, we will look at the "sed" |
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package, a standard GNU text stream editing utility that is part of all Linux |
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distributions. First, download the source tarball (<path>sed-3.02.tar.gz</path>) |
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(see <uri link="#resources">Resources</uri>). We will store this archive in |
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<path>/usr/src/distfiles</path>, a directory we will refer to using the |
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environment variable <c>$DISTDIR</c>. <c>$DISTDIR</c> is the directory where all |
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of our original source tarballs live; it's a big vault of source code. |
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</p> |
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|
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<p> |
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Our next step is to create a temporary directory called <path>work</path>, which |
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houses the uncompressed sources. We'll refer to this directory later using the |
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<c>$WORKDIR</c> environment variable. To do this, change to a directory where we |
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have write permission and type the following: |
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</p> |
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|
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<pre caption="Uncompressing sed into a temporary directory"> |
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$ <i>mkdir work</i> |
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$ <i>cd work</i> |
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$ <i>tar xzf /usr/src/distfiles/sed-3.02.tar.gz</i> |
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</pre> |
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|
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<p> |
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The tarball is then decompressed, creating a directory called |
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<path>sed-3.02</path> that contains all of the sources. We'll refer to the |
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<path>sed-3.02</path> directory later using the environment variable |
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<c>$SRCDIR</c>. To compile the program, type the following: |
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</p> |
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|
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<pre caption="Uncompressing sed into a temporary directory"> |
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$ <i>cd sed-3.02</i> |
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$ <i>./configure --prefix=/usr</i> |
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<comment>(autoconf generates appropriate makefiles, this can take a while)</comment> |
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|
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$ <i>make</i> |
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|
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<comment>(the package is compiled from sources, also takes a bit of time)</comment> |
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</pre> |
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|
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<p> |
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We're going to skip the "make install" step, since we are just covering the |
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unpack and compile steps in this article. If we wanted to write a bash script to |
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perform all these steps for us, it could look something like this: |
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</p> |
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|
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<pre caption="Sample bash script to perform the unpack/compile process"> |
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#!/usr/bin/env bash |
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|
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if [ -d work ] |
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then |
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<comment># remove old work directory if it exists</comment> |
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rm -rf work |
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fi |
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mkdir work |
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cd work |
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tar xzf /usr/src/distfiles/sed-3.02.tar.gz |
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cd sed-3.02 |
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./configure --prefix=/usr |
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make |
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</pre> |
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|
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</body> |
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</section> |
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<section> |
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<title>Generalizing the code</title> |
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<body> |
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|
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<p> |
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Although this autocompile script works, it's not very flexible. Basically, the |
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bash script just contains the listing of all the commands that were typed at the |
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command line. While this solution works, it would be nice to make a generic |
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|
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|
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|
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-- |
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