02 介绍

#Cleanup
# Run as root, of course

cd /var/log
cat /dev/null > messages
cat /dev/null > wtmp

echo "Log cleaned up."

#!/bin/bash
# Proper header for a Bash script.

# Cleanup, version 2

# Run as root, of course.
# Insert code here to print error message and exit if not root.

LOG_DIR=/var/log
# Variables are better than hard-coded values.
cd $LOG_DIR

cat /dev/null > messages
cat /dev/null > wtmp

echo "Logs cleaned up."

exit # The right and proper method of "exiting" from a script.

#!/bin/bash
# Cleanup, version 3

# Warning:
# ---------------------------------------------
#       This script uses quite a number of features that will be explained
#+      later on.
#       By the time you've finished the first halt of the book,
#+      there should be nothing mysterious about it.



LOG_DIR=/var/log
ROOT_UID=0              # only users with $UID 0 have root privileges.
LINES=50                # default number of lines saved.
E_XCD=66                # can't change directory ?
E_NOTROOT=67    # Non-root exit error.




# Run as root, of course.
if [ "$UID" -ne "$ROOT_UID" ]
then
        echo "Must be root to run this script."
        exit $E_NOTROOT
fi

if [ -n "$1" ]
# Test if command line argument present(non-empty).
then
        lines=$1
else
        lines=$LINES    # default, if not specified on command line.
fi


#       Stephane Chazelas suggests the following,
#+      as a better way of checking command line arguments,
#+      but this is still a bit advanced for this stage of the tutorial.
#
#               E_WORNGARGS=65  # Non-numerical argument (bad arg format)
#
#               case "$1" in
#               ""              ) lines=50;;
#               *[!0-9]*) echo "Usage: `basename $0` file-to-cleanup"; exit $E_WRONGARGS;;
#               *               ) lines=$1;;
#               esac
#*      Skip ahead to "Loops" chapter to decipher all this.

cd $LOG_DIR

if [ `pwd` != "$LOG_DIR" ]      # or    if ["$PWD" != "$LOG_DIR" ]
                                # Not in /var/log?
then
        echo "Can't change to $LOG_DIR."
        exit $E_XCD
fi              # Doublecheck if in right directory, before messing with log file.

# far more efficient is:
#
# cd /var/log || {
#       echo "Cannot change to necessary directory." >&2
#       exit $E_XCD;
# }

1. 注意使用 模块化的编程方法

2. 使用变量代替常量, 这样可以做到一改全改

什么时候不使用 sheill

调用一个脚本

两种方法, 使用 sh 来执行脚本, 不过不推荐这种做法, 因为这样禁用了脚本从 stdin 中读数据的功能. 更方便的方法是让脚本本身就具有可执行权限, 通过 chmod 命令可以修改.

比如: chmod 555 scriptname

chmod +rx scriptname (注意+号后边不能有空格)

chmod u+rx scriptname (只给脚本所有者增加读和执行的权限)

最后, 在脚本测试没问题之后, 你可能想把它移动到 /usr/local/bin, 来让你的脚本对所有用户都可以使用, 这样直接敲脚本名字就可以运行了

注意:

1. unix 味道的脚本, #! /bin/sh, 即在 ! 后要有个空格.

2. #! 任务是命令解释器, 另外这个要在第 1 行