Chapter 12. Programming

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

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

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

Special shell parameters are frequently used in the shell script.

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.

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

	Note
	"[" 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.

Arithmetic integer comparison operators in the conditional expression are "-eq", "-ne", "-lt", "-le", "-gt", and "-ge".

There are several loop idioms to use in POSIX shell.

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

	Tip
	See Section 9.3.9, “Repeating a command looping over files”.

The shell processes a script roughly as the following sequence.

Single quotes within double quotes have no effect.

Executing "set -x" in the shell or invoking the shell with "-x" option make the shell to print all of commands executed. This is quite handy for debugging.

In order to make your shell program as portable as possible across Debian systems, it is a good idea to limit utility programs to ones provided by essential packages.

The user interface of a simple shell program can be improved from dull interaction by echo and read commands to more interactive one by using one of the so-called dialog program etc.

Here is a simple script which creates ISO image with RS02 data supplemented by dvdisaster(1).

A simple example "example.c" can compiled with a library "libm" into an executable "run_example" by the following.

$ 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

Here, "-lm" is needed to link library "/usr/lib/libm.so" from the libc6 package for sqrt(3). The actual library is in "/lib/" with filename "libm.so.6", which is a symlink to "libm-2.7.so".

Look at the last parameter in the output text. There are more than 10 characters even though "%10s" is specified.

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.

	Tip
	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.

There are several memory leak detection tools available in Debian.

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

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

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

	Warning
	Do not overwrite system files with your compiled programs when installing them.

Debian does not touch files in "/usr/local/" or "/opt". So if you compile a program from source, install it into "/usr/local/" so it does not interfere with Debian.

$ cd src
$ ./configure --prefix=/usr/local
$ make
$ make install # this puts the files in the system

If you have the original source and if it uses autoconf(1)/automake(1) and if you can remember how you configured it, execute as follows to uninstall the program.

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

Alternatively, if you are absolutely sure that the install process puts files only under "/usr/local/" and there is nothing important there, you can erase all its contents by the following.

# 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.