精通Linux Bash编程：从基础到专业实战指南

bash

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■ CONTENTS

OPTERR..........................................................................................................................215

PATH ..............................................................................................................................215

POSIXLY_CORRECT ........................................................................................................216

PROMPT_COMMAND......................................................................................................216

PROMPT_DIRTRIM .........................................................................................................216

PS1.................................................................................................................................216

PS2.................................................................................................................................216

PS3.................................................................................................................................216

PS4.................................................................................................................................216

SHELL.............................................................................................................................217

TIMEFORMAT .................................................................................................................217

TMOUT ...........................................................................................................................217

TMPDIR ..........................................................................................................................217

auto_resume..................................................................................................................217

histchars........................................................................................................................218

■Index....................................................................................................................221

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■ INTRODUCTION

xix

Introduction

Although most users think of the shell as an interactive command interpreter, it is really a

programming language in which each statement runs a command. Because it must satisfy

both the interactive and programming aspects of command execution, it is a strange

language, shaped as much by history as by design.

Brian Kernighan and Rob Pike, The UNIX Programming Environment, Prentice-Hall, 1984

The shell is a programming language. Don’t let anyone tell you otherwise. The shell is not just glue that

sticks bits together. The shell is a lot more than a tool that runs other tools. The shell is a complete

programming language!

When a Linux user asked me about membership databases, I asked him what he really needed. He

wanted to store names and addresses for a couple of hundred members and print mailing labels for each

of them. I recommended using a text editor to store the information in a text file, and I provided a shell

script to create the labels in PostScript. (The script, ps-labels, appeared in my first book, Shell Scripting

Recipes: A Problem-Solution Approach.)

When the SWEN worm was dumping hundreds of megabytes of junk into my mailbox every few

minutes, I wrote a shell script to filter them out on the mail server and download the remaining mail to

my home computer. That script has been doing its job for several years.

I used to tell people that I did most of my programming in the shell but switched to C for anything

that needed the extra speed. It has been several years since I have needed to use C, so I no longer

mention it. I do everything in the shell.

A shell script is as much a program as anything written in C, Python, or any other language. Just

because shell scripts are easier to write doesn’t mean they should take a backseat to compiled programs

or other scripting languages. I use the terms script and program interchangeably when referring to tasks

written in the shell.

Why the Shell?

Some Linux users do all of their work in a GUI environment and never see a command line. Most,

however, use the shell at least occasionally and know something about Unix commands. It’s not a big

step from there to saving oft-repeated commands in a script file. When they need to extend the

capabilities of their system, the shell is the natural way to go.

The shell also has important advantages over other programming languages:

• It interfaces simply and seamlessly with the hundreds of Unix utilities.

• It automatically expands wildcards into a list of file names.

• Lists contained in a variable are automatically split into their constituent parts.

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精通Linux Bash编程：从基础到专业实战指南

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