ex45.sh

BASH Shell 编程 经典教程 《高级SHELL脚本编程》中文版

#!/bin/bash

# 展示一些使用'expr'的例子
# ========================

echo

# 算术 操作
# ---- ----

echo "Arithmetic Operators"
echo
a=`expr 5 + 3`
echo "5 + 3 = $a"

a=`expr $a + 1`
echo
echo "a + 1 = $a"
echo "(incrementing a variable)"

a=`expr 5 % 3`
# 取模操作
echo
echo "5 mod 3 = $a"

echo
echo

# 逻辑 操作
# ---- ----

#  true返回1, false返回0,
#+ 而Bash的使用惯例则相反.

echo "Logical Operators"
echo

x=24
y=25
b=`expr $x = $y`         # 测试相等.
echo "b = $b"            # 0  ( $x -ne $y )
echo

a=3
b=`expr $a \> 10`
echo 'b=`expr $a \> 10`, therefore...'
echo "If a > 10, b = 0 (false)"
echo "b = $b"            # 0  ( 3 ! -gt 10 )
echo

b=`expr $a \< 10`
echo "If a < 10, b = 1 (true)"
echo "b = $b"            # 1  ( 3 -lt 10 )
echo
# 注意转义操作.

b=`expr $a \<= 3`
echo "If a <= 3, b = 1 (true)"
echo "b = $b"            # 1  ( 3 -le 3 )
# 也有 "\>=" 操作 (大于等于).


echo
echo



# 字符串 操作
# ------ ----

echo "String Operators"
echo

a=1234zipper43231
echo "The string being operated upon is \"$a\"."

# 长度: 字符串长度
b=`expr length $a`
echo "Length of \"$a\" is $b."

# 索引: 从字符串的开头查找匹配的子串,
#       并取得第一个匹配子串的位置.
b=`expr index $a 23`
echo "Numerical position of first \"2\" in \"$a\" is \"$b\"."

# substr: 从指定位置提取指定长度的字串.
b=`expr substr $a 2 6`
echo "Substring of \"$a\", starting at position 2,\
and 6 chars long is \"$b\"."


#  'match' 操作的默认行为就是从字符串的开始进行搜索,
#+  并匹配第一个匹配的字符串.
#
#        使用正则表达式
b=`expr match "$a" '[0-9]*'`               #  数字的个数.
echo Number of digits at the beginning of \"$a\" is $b.
b=`expr match "$a" '\([0-9]*\)'`           #  注意, 需要转义括号
#                   ==      ==              + 这样才能触发子串的匹配.
echo "The digits at the beginning of \"$a\" are \"$b\"."

echo

exit 0