Bab 12. Pemrograman

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

	Catatan
	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.

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

	Catatan
	"[" adalah setara dengan perintah test, yang mengevaluasi argumennya sampai "]" sebagai ekspresi bersyarat.

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".

Ada beberapa idiom pengulangan untuk digunakan dalam shell POSIX.

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

	Tip
	Lihat Bagian 9.4.9, “Mengulangi perintah atas berkas”.

Some popular environment variables for the normal shell command prompt may not be available under the execution environment of your script.

Shell memproses skrip kira-kira sebagai urutan berikut.

Kutip tunggal dalam kutip ganda tidak berpengaruh.

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.

Most interpreters offer basic syntax check and code tracing functionalities.

The shell script can be improved to create an attractive GUI program. The trick is to use one of so-called dialog programs instead of dull interaction using echo and read commands.

Berikut adalah contoh program GUI untuk menunjukkan betapa mudahnya itu hanya dengan suatu skrip shell.

Skrip ini menggunakan zenity untuk memilih berkas (baku /etc/motd) dan menampilkannya.

GUI filer programs can be extended to perform some popular actions on selected files using additional extension packages. They can also made to perform very specific custom actions by adding your specific scripts.

Untuk memproses data, sh perlu men-spawn sub-proses yang menjalankan cut, grep, sed, dll, dan lambat. Di sisi lain, perl memiliki kemampuan internal untuk memroses data, dan cepat. Begitu banyak skrip pemeliharaan sistem pada Debian menggunakan perl.

Mari kita pikirkan untuk mengikuti cuplikan skrip AWK satu-baris dan yang setara di Perl.

awk '($2=="1957") { print $3 }' |

Ini setara dengan salah satu dari baris berikut.

perl -ne '@f=split; if ($f[1] eq "1957") { print "$f[2]\n"}' |

perl -ne 'if ((@f=split)[1] eq "1957") { print "$f[2]\n"}' |

perl -ne '@f=split; print $f[2] if ( $f[1]==1957 )' |

perl -lane 'print $F[2] if $F[1] eq "1957"' |

perl -lane 'print$F[2]if$F[1]eq+1957' |

Yang terakhir adalah teka-teki. Itu mengambil keuntungan dari mengikuti fitur Perl.

Fleksibilitas ini adalah kekuatan Perl. Pada saat yang sama, ini memungkinkan kita untuk membuat kode sulit dibaca dan kusut. Jadi berhati-hatilah.

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).

Tutorial for bison(1) can be found in "info bison".

You need to provide your own "main()" and "yyerror()". "main()" calls "yyparse()" which calls "yylex()", usually created with Flex.

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

The backtrace can be obtained by gdb(1) using one of the following approaches:

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.

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.

Ada beberapa alat deteksi kebocoran memori yang tersedia di 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

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

Jalankan "make -p -f/dev/null" untuk melihat aturan internal otomatis.

The software build system has been evolving: