第 12 章 编程

The "echo" command must be used with following cares since its implementation differs among shell builtin and external commands.

	注意
	Although "-n" option is not really POSIX syntax, it is generally accepted.

	提示
	Use the "printf" command instead of the "echo" command if you need to embed escape sequences in the output string.

特殊的 shell 参数经常在 shell 脚本里面被用到。

Basic parameter expansions to remember are as follows.

Here, the colon ":" in all of these operators is actually optional.

Each command returns an exit status which can be used for conditional expressions.

	注意
	"0" in the shell conditional context means "True", while "0" in the C conditional context means "False".

	注意
	"[" is the equivalent of the test command, which evaluates its arguments up to "]" as a conditional expression.

Basic conditional idioms to remember are the following.

if [ <conditional_expression> ]; then
 <if_success_run_this_command>
else
 <if_not_success_run_this_command>
fi

Here trailing "|| true" was needed to ensure this shell script does not exit at this line accidentally when shell is invoked with "-e" flag.

算术整数的比较在条件表达式中为 "-eq"，"-ne"，"-lt"，"-le"，"-gt" 和 "-ge"。

这里有几种可用于 POSIX shell 的循环形式。

	提示
	The C-language like numeric iteration can be realized by using seq(1) as the "foo1 foo2 …" generator.

	提示
	参见 第 9.3.9 节 “使用文件循环来重复一个命令”。

shell 大致以下列的顺序来处理一个脚本。

双引号中的单引号是没有效果的。

在 shell 中执行 “set -x” 或使用 “-x” 选项启动 shell 可以让 shell 显示出所有执行的命令。这对调试来说是非常方便的。

为了使你的 shell 程序在 Debian 系统上尽可能地具有可移植性，你应该只使用 必要的 软件包所提供的应用程序。

一个简单的 shell 程序的用户界面中，echo 和 read 命令的交互性较为一般，你可以使用对话程序等来提升交互性。

下面是一个简单的脚本，它通过 dvdisaster(1) 创建了带有 RS02 补充数据的 ISO 映像。

一个简单的例子 “example.c” 可以通过如下方式和 “libm” 库一起编译为可执行程序 “run_example”。

$ cat > example.c << EOF
#include <stdio.h>
#include <math.h>
#include <string.h>
int main(int argc, char **argv, char **envp){
        double x;
        char y[11];
        x=sqrt(argc+7.5);
        strncpy(y, argv[0], 10); /* prevent buffer overflow */
        y[10] = '\0'; /* fill to make sure string ends with '\0' */
        printf("%5i, %5.3f, %10s, %10s\n", argc, x, y, argv[1]);
        return 0;
}
EOF
$ gcc -Wall -g -o run_example example.c -lm
$ ./run_example
        1, 2.915, ./run_exam,     (null)
$ ./run_example 1234567890qwerty
        2, 3.082, ./run_exam, 1234567890qwerty

为了使用 sqrt(3)，必须使用 “-lm” 链接来自 libc6 软件包的库 “/usr/lib/libm.so”。实际的库文件位于 “/lib/”，文件名为 “libm.so.6”，它是指向 “libm-2.7.so” 的一个链接。

请看一下输出文本的最后一段。即使指定了 “%10s”，它依旧超出了 10 个字符。

The use of pointer memory operation functions without boundary checks, such as sprintf(3) and strcpy(3), is deprecated to prevent buffer overflow exploits that leverage the above overrun effects. Instead, use snprintf(3) and strncpy(3).

Since all installed binaries should be stripped on the Debian system by default, most debugging symbols are removed in the normal package. In order to debug Debian packages with gdb(1), corresponding *-dbg packages need to be installed (e.g. libc6-dbg in the case of libc6).

If a package to be debugged does not provide its *-dbg package, you need to install it after rebuilding it by the following.

$ mkdir /path/new ; cd /path/new
$ sudo apt-get update
$ sudo apt-get dist-upgrade
$ sudo apt-get install fakeroot devscripts build-essential
$ sudo apt-get build-dep source_package_name
$ apt-get source package_name
$ cd package_name*

Fix bugs if needed.

Bump package version to one which does not collide with official Debian versions, e.g. one appended with "+debug1" when recompiling existing package version, or one appended with "~pre1" when compiling unreleased package version by the following.

$ dch -i

Compile and install packages with debug symbols by the following.

$ export DEB_BUILD_OPTIONS=nostrip,noopt
$ debuild
$ cd ..
$ sudo debi package_name*.changes

You need to check build scripts of the package and ensure to use "CFLAGS=-g -Wall" for compiling binaries.

When you encounter program crash, reporting bug report with cut-and-pasted backtrace information is a good idea.

The backtrace can be obtained by the following steps.

In case of program freeze, you can crash the program by pressing Ctrl-C in the terminal running gdb to obtain gdb prompt.

	提示
	Often, you see a backtrace where one or more of the top lines are in "malloc()" or "g_malloc()". When this happens, chances are your backtrace isn't very useful. The easiest way to find some useful information is to set the environment variable "$MALLOC_CHECK_" to a value of 2 (malloc(3)). You can do this while running gdb by doing the following.

 $ MALLOC_CHECK_=2 gdb hello

If a GNOME program preview1 has received an X error, you should see a message as follows.

The program 'preview1' received an X Window System error.

If this is the case, you can try running the program with "--sync", and break on the "gdk_x_error" function in order to obtain a backtrace.

Use ldd(1) to find out a program's dependency on libraries by the followings.

$ ldd /bin/ls
        librt.so.1 => /lib/librt.so.1 (0x4001e000)
        libc.so.6 => /lib/libc.so.6 (0x40030000)
        libpthread.so.0 => /lib/libpthread.so.0 (0x40153000)
        /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)

For ls(1) to work in a `chroot`ed environment, the above libraries must be available in your `chroot`ed environment.

Debian 上有一些可用的内存泄漏检测工具。

You can disassemble binary code with objdump(1) by the following.

$  objdump -m i386 -b binary -D /usr/lib/grub/x86_64-pc/stage1

	注意
	gdb(1) may be used to disassemble code interactively.

	警告
	当你安装编译好的程序的时候，注意不要覆盖系统文件。

Debian 不会在 "/usr/local" 或 "/opt" 目录下创建文件。如果你想要源码编译程序，把它安装到 "/usr/local/" 目录下，因为这并不会影响到 Debian。

$ cd src
$ ./configure --prefix=/usr/local
$ make
$ make install # 这一步是把文件安装到系统上

如果你有源码并且它使用 autoconf(1)/automake(1)，如果你能记得你是怎样配置它的话，执行如下的命令来卸载程序。

$ ./configure "all-of-the-options-you-gave-it"
# make uninstall

或者，如果你十分确信安装进程把文件都放在了 "/usr/local/" 下并且这里没什么重要的东西，你可以通过如下的命令来清除它所有的内容。

# find /usr/local -type f -print0 | xargs -0 rm -f

If you are not sure where files are installed, you should consider using checkinstall(8) from the checkinstall package, which provides a clean path for the uninstall. It now supports to create a Debian package with "-D" option.