利用Shell脚本解决Linux、MacOSX和UNIX系统问题

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"《101个酷酷的脚本编程》是一本专注于Linux、MacOSX和Unix系统脚本实践的学习书籍，由Dave Taylor所著，由NoStarchPress在2004年出版。这本书面向已有一定基础的读者，提供了一系列实用、可定制且有趣的脚本，旨在帮助解决常见的操作系统问题以及个性化你的计算环境。书中包含12个章节，覆盖了从用户命令改进、创建工具到系统管理、网络和互联网应用等多个领域，同时也包括了针对MacOSX系统的特定脚本和一些娱乐性的脚本游戏。" 在《101个酷酷的脚本编程》中，作者Dave Taylor首先通过介绍，引导读者进入脚本编程的世界，解释了为何shell脚本对于Linux、MacOSX和Unix系统如此重要，并阐述了这本书的目标——提供解决问题和提升用户体验的工具。第一章"The Missing Code Library"可能涵盖了作者收集的常用代码片段和函数库，这些是编写高效脚本的基础。第二章"Improving on User Commands"则可能教导读者如何增强或扩展系统内建的用户命令，提高日常工作效率。第三章"Creating Utilities"可能是关于创建自定义的实用程序，这些工具可以填补系统原有功能的空白。第四章"Tweaking Unix"涉及对Unix系统的个性化调整，包括优化系统设置、配置和性能。第五至第七章关注系统管理和网络应用。"System Administration: Managing Users"探讨用户管理，包括用户账户的创建、权限控制等。"System Administration: System Maintenance"可能涵盖定期维护任务的自动化脚本，如备份、更新和性能监控。"Web and Internet Users"和"Webmaster Hacks"章节可能涉及网页浏览和网站管理的脚本解决方案。第八至第十章进一步深入网络和互联网的脚本应用。"Web and Internet Administration"和"Internet Server Administration"讨论了网络服务的管理和自动化，包括邮件服务器、FTP服务器等的脚本管理。"MacOSX Scripts"专门针对苹果操作系统的独特特性和需求提供了特定的脚本示例。第十一章"Shell Script Fun and Games"将脚本编程带入轻松的一面，提供了用于娱乐和学习的脚本实例，这有助于保持学习的乐趣。最后，书中的附录如索引、图例列表和表格列表为读者查找和理解内容提供了便利。通过这本书，读者不仅可以提升脚本编写技能，还能学会如何利用shell脚本来优化和定制自己的计算环境。

#1 Finding Programs in the PATH

Shell scripts that use environment variables (like MAILER and PAGER) have a hidden danger: Some of their settings

may well point to nonexistent programs. For example, if you decide to be flexible by using the PAGER setting to

display script output, instead of just hard-coding a specific tool, how do you ensure that the PAGER value is set to a

valid program? After all, if it's not a valid program, your script will break. This first script addresses how to test whether

a given program can be found in the user's PATH, and it's also a good demonstration of a number of different shell

scripting techniques, including script functions and variable slicing.

The Code

#!/bin/sh

# inpath - Verifies that a specified program is either valid as is,

# or that it can be found in the PATH directory list.

in_path()

{

# Given a command and the PATH, try to find the command. Returns

# 0 if found and executable, 1 if not. Note that this temporarily modifies

# the IFS (input field separator) but restores it upon completion.

cmd=$1 path=$2 retval=1

oldIFS=$IFS IFS=":"

for directory in $path

if [ -x $directory/$cmd ] ; then

retval=0 # if we're here, we found $cmd in $directory

done

IFS=$oldIFS

return $retval

}

checkForCmdInPath()

{

var=$1

# The variable slicing notation in the following conditional

# needs some explanation: ${var#expr} returns everything after

# the match for 'expr' in the variable value (if any), and

# ${var%expr} returns everything that doesn't match (in this

# case, just the very first character. You can also do this in

# Bash with ${var:0:1}, and you could use cut too: cut -c1.

if [ "$var" != "" ] ; then

if [ "${var%${var#?}}" = "/" ] ; then

if [ ! -x $var ] ; then

return 1

elif ! in_path $var $PATH ; then

return 2

}

Where to put your scripts I recommend that you create a new directory called "scripts,"

probably as a part of your HOME directory, and then add that

fully qualified directory name to your PATH variable. (Use echo

$PATH to see your current PATH, and edit the contents of your

.login or .profile (depending on the shell) to modify your PATH

appropriately.)

Running the Script

To run this script, we first need to append a short block of commands to the very end of the file. These commands

pass a starting parameter to the validation program and check the return code, like so:

if [ $# -ne 1 ] ; then

echo "Usage: $0 command" >&2 ; exit 1

checkForCmdInPath "$1"

case $? in

0 ) echo "$1 found in PATH" ;;

1 ) echo "$1 not found or not executable" ;;

2 ) echo "$1 not found in PATH" ;;

esac

exit 0

Once you've added the additional code snippet, you can invoke this script directly, as shown in " The Results ," next.

Make sure to remove or comment out the additional code before you're done with this script, however, so its later

inclusion as a library function doesn't mess things up.

The Results

To test the script, let's invoke inpath with the names of three programs: a program that exists, a program that exists but

isn't in the PATH, and a program that does not exist but that has a fully qualified filename and path:

$ inpath echo

echo found in PATH

$ inpath MrEcho

MrEcho not found in PATH

$ inpath /usr/bin/MrEcho

/usr/bin/MrEcho not found or not executable

Hacking the Script

Perhaps the most unusual aspect of this code is that it uses the POSIX variable slicing method of ${var%${var#?}} . To

understand this notation, realize that the apparent gobbledygook is really two nested string slices. The inner call,

${var#?} , extracts everything but the first character of the variable var ( ? is a regular expression that matches one

character). Next, the call ${var%pattern} produces a substring with everything left over once the specified pattern is

applied to the inner call. In this case, what's left is the first character of the string.

This is a pretty dense explanation, admittedly, but the key to getting check- ForCmdInPath to work is for it to be able to

differentiate between variables that contain just the program name (like echo ) and variables that contain a full directory

path plus the filename (like " /bin/echo "). It does this by examining the very first character of the given value to see if it's

a " / " or not; hence the need to isolate the first character from the rest of the variable value.

If this POSIX notation is too funky for you, Bash and Ksh support another method of variable slicing. The substring

function ${varname:start:size} requests a certain number of characters from varname specified by size and beginning with

the character number in varname specified by start . For example, ${varname:1:1} would produce a substring consisting

of just the first character of varname . Of course, if you don't like either of these techniques for extracting just the first

#2 Validating Input: Alphanumeric Only

Users are constantly ignoring directions and entering data that's inconsistent or incorrectly formatted, or that uses

incorrect syntax. As a shell script developer, you need to intercept and correct these errors before they become

problems.

A typical situation you may encounter in this regard involves filenames or database keys. You prompt the user for a

string that's supposed to be made up exclusively of uppercase characters, lowercase characters, and digits. No

punctuation, no special characters, no spaces. Did they enter a valid string or not? That's what this script tests.

The Code

#!/bin/sh

# validAlphaNum - Ensures that input consists only of alphabetical

# and numeric characters.

validAlphaNum()

{

# Validate arg: returns 0 if all upper+lower+digits, 1 otherwise

# Remove all unacceptable chars

compressed="$(echo $1 | sed -e 's/[^[:alnum:]]//g')"

if [ "$compressed" != "$input" ] ; then

return 1

else

return 0

}

# Sample usage of this function in a script

echo -n "Enter input: "

read input

if ! validAlphaNum "$input" ; then

echo "Your input must consist of only letters and numbers." >&2

exit 1

else

echo "Input is valid."

exit 0

How It Works

The logic of this script is straightforward. First, it transforms the input with a sed-based transform to create a new

version of the input data, and then it compares the new version with the original. If the two versions are the same, all is

well. If not, the transform lost data that wasn't part of the acceptable alphanumeric (alphabetic plus numeric) character

set, and the input was unacceptable.

Specifically, the sed substitution is for any characters not in the set [:alnum:], the POSIX shorthand for the local

definition of all upper-and lowercase characters and digits (alnum stands for alphanumeric). If this new, compressed

value doesn't match the input entered earlier, the removal of all the alphanumeric values reveals nonalphanumeric

values in the input string (which is illegal) and the function returns a nonzero result, indicating a problem.

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Gikor

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