直接执行命令和将命令作为字符串赋值给变量，然后再显示变量效果一样。
killppp="eval kill -9 `ps ax | awk '/ppp/ { print $1 }'`"
$killppp # 这个变量现在成为了一个命令.
cut -d ' ' -f2,3 filename等价于awk -F'[ ]' '{ print $2, $3 }' filename
output=$(sed -n /"$1"/p $file) # 命令替换.
if [[ -z $(echo "$1" | sed -n '/^[A-Z]/p') ]]
在一个脚本中, 使用后台运行命令(&)可能会使这个脚本挂起, 直到 敲ENTER, 挂起的脚本才会被恢复. 看起来只有在这个命令的结果需要输出 到stdout的时候, 这种现象才会出现. 这是个很烦人的现象. 1 #!/bin/bash 2 # test.sh 3 4 ls -l & 5 echo "Done." bash$ ./test.sh Done. [bozo@localhost test-scripts]$ total 1 -rwxr-xr-x 1 bozo bozo 34 Oct 11 15:09 test.sh _ 看起来只要在后台运行命令的后边加上一个wait命令就会解决这个问题. 1 #!/bin/bash 2 # test.sh 3 4 ls -l & 5 echo "Done." 6 wait bash$ ./test.sh Done. [bozo@localhost test-scripts]$ total 1 -rwxr-xr-x 1 bozo bozo 34 Oct 11 15:09 test.sh 如果将后台运行命令的输出重定向到文件中或/dev/null中, 也能解决这个 问题.
测试脚本参数数量是否正确
E_WRONG_ARGS=85 script_parameters="-a -h -m -z" # -a = all, -h = help, etc. if [ $# -ne $Number_of_expected_args ] then echo "Usage: `basename $0` $script_parameters" # `basename $0` is the script's filename. exit $E_WRONG_ARGS fi
Example 2-1. cleanup: A script to clean up log files in /var/log
# Cleanup # Run as root, of course. cd /var/log cat /dev/null > messages cat /dev/null > wtmp echo "Log files cleaned up." |
Example 2-2. cleanup: An improved clean-up script
#!/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.
# A bare "exit" (no parameter) returns the exit status
#+ of the preceding command.
Example 2-3. cleanup: An enhanced and generalized version of above scripts.
#!/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 half of the book,
#+ there should be nothing mysterious about it.
LOG_DIR=/var/log
ROOT_UID=0 # Only users with $UID 0 have root privileges.
LINES=50 # Default number of lines saved.
E_XCD=86 # Can't change directory?
E_NOTROOT=87 # Non-root exit error.
# Run as root, of course.
if [ "$UID" -ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
exit $E_NOTROOT
fi
if [ -n "$1" ]
# Test whether command-line argument is 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_WRONGARGS=85 # Non-numerical argument (bad argument format).
#
# case "$1" in
# "" ) lines=50;;
# *[!0-9]*) echo "Usage: `basename $0` lines-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;
# }
tail -n $lines messages > mesg.temp # Save last section of message log file.
mv mesg.temp messages # Rename it as system log file.
# cat /dev/null > messages
#* No longer needed, as the above method is safer.
cat /dev/null > wtmp # ': > wtmp' and '> wtmp' have the same effect.
echo "Log files cleaned up."
# Note that there are other log files in /var/log not affected
#+ by this script.
exit 0
# A zero return value from the script upon exit indicates success
#+ to the shell.
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