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变量
将$变量表达式放在双引号中，变量汇编替换为它的值；
如果放在单引号中，就不会发生替换。
$ 前面加 \ 可取消其特殊含义。
字符串通常被放在双引号中，以防止变量被空白字符分开，同时又允许 $ 扩展。
#!/bin/sh
myvar="Hi there"
echo $myvar
echo "$myvar"
echo '$myvar'
echo \$myvar
echo Enter come text
read myvar
echo '$myvar' now equals $myvar
exit 0
结果：
yjp@yjp-VirtualBox:~/shell$ /bin/sh ./variable
Hi there
Hi there
$myvar
$myvar
Enter come text
aaa
$myvar now equals aaa
$1, $2, … 脚本程序的参数
$* 列出所有参数。如图 IFS 被修改，$* 将命令行分割为参数的方式也会改变
$@ 是 $* 的变体，不使用 IFS 环境变量。
#!/bin/sh
salutation="Hello"
echo $salutation
echo "The second parameter was $2"
echo "The first parameter was $1"
echo "The parameter list was $*"
echo "The user's home directory is $HOME"
echo "Please enter a new greetion"
read salutation
echo $salutation
echo "The script is now complete"
exit 0
结果：
yjp@yjp-VirtualBox:~/shell$ /bin/sh ./try_var foo var baz
Hello
The second parameter was var
The first parameter was foo
The parameter list was foo var baz
The user's home directory is /home/yjp
Please enter a new greetion
aaa
aaa
The script is now complete
条件
test 与 [
test -f <filename> 检查文件是否存在，等价于命令 [ -f <filename> ]
[ 和被检查的条件之间要有空格。
test -d <filename> 检查文件是否为目录
#!/bin/sh
if [ -f /bin/bash ]
then
echo "file /bin/bash exists"
fi
if [ -d /bin/bash ]
then
echo "bin/bash is a directory"
else
echo "bin/bash is not a directory"
fi
结果：
yjp@yjp-VirtualBox:~/shell$ /bin/sh test
file /bin/bash exists
bin/bash is not a directory
控制结构
语法：
if <condition>
then
<statements>
else
<statements>
fi
#!/bin/sh
echo "Is it mornint? Please answer yes or no"
read timeofday
if [ "$timeofday" = "yes" ]; then
echo "Good morning"
elif [ "$timeofday" = "no" ]; then
echo "Good afternoon"
else
echo "Sory, $timeofday not recognized. Enter yes or no"
exit 1
fi
exit 0
结果：
yjp@yjp-VirtualBox:~/shell$ /bin/sh test
Is it mornint? Please answer yes or no
yes
Good morning
语法：
for <variable> in <values>
do
<statements>
done
#!/bin/sh
for foo in bar fud 43
do
echo $foo
done
exit 0
结果：
bar
fud
43
使用通配符扩展的 for 循环：
#!/bin/sh
for file in $(ls t*); do
echo $file
done
exit 0
结果：
test
try_var
条件为真时反复执行：
while <condition> do
statements
done
密码检查程序：
#!/bin/sh
echo "Enter password"
read trythis
while [ "$trythis" != "secret" ]; do
echo "Sorry, try again"
read trythis
done
exit 0
结果：
Enter password
aaa
Sorry, try again
secret
循环将反复执行直到条件为真：
until <condition>
do
<statememts>
done
设置一个警报，当某个特定的用户登录时，警报启动。如果用户已经登录，循环不需要执行：
#!/bin/sh
until who | grep "$1" > /dev/null
do
sleep 60
done
echo -e 'a'
echo "*** $1 has just logged in ***"
exit 0
case 语句：
case <variable> in
pattern [ | <pattern>] ...) <statements>;;
pattern [ | <pattern>] ...) <statements>;;
...
esac
双分号标记前一个模式的结束和后一个模式的开启。
用户输入：
#!/bin/sh
echo "Is it morning? Please answer yes or no"
read timeofday
case "$timeofday" in
yes) echo "Good Morning";;
no) echo "Good Afternoon";;
y) echo "Good Morning";;
n) echo "Good Afternoon";;
*) echo "Sory, answer no recognized";;
esac
exit 0
合并匹配模式：
#!/bin/sh
echo "Is it morning? Please answer yes or no"
read timeofday
case "$timeofday" in
yes | y | Yes | YES) echo "Good Morning";;
n* | N*) echo "Good Afternoon";;
*) echo "Sory, answer no recognized";;
esac
exit 0
执行多条语句：
#!/bin/sh
echo "Is it morning? Please answer yes or no"
read timeofday
case "$timeofday" in
yes | y | Yes | YES)
echo "Good Morning"
echo "Up bright and early this morning"
;;
[nN]*)
echo "Good Afternoon"
;;
*)
echo "Sory, answer no recognized"
echo "Please answer yes or no"
exit 1
;;
esac
exit 0
结果：
Is it morning? Please answer yes or no
y
Good Morning
Up bright and early this morning
AND 列表：
<statement1> && <statement2> && <statement3>
#!/bin/sh
touch file_one
rm -f file_two
if [ -f file_one ] && echo "hello" && [ -f file_two ] && echo " there"
then
echo "in if"
else
echo "in else"
fi
exit 0
结果：
hello
in else
OR 列表：
#!/bin/sh
rm -f file_one
if [ -f file_one ] || echo "hello" || echo " there"
then
echo "in if"
else
echo "in else"
fi
exit 0
结果：
hello
in if
源码：《Linux 程序设计》