Tutorial

与流行的编程语言一样,shell也支持逻辑决策。

基本的条件决策构造为:

if [ expression ]; then

如果'expression'为true的代码

fi

NAME="John"
if [ "$NAME" = "John" ]; then
  echo "True - my name is indeed John"
fi

可以用“ else”扩展

NAME="Bill"
if [ "$NAME" = "John" ]; then
  echo "True - my name is indeed John"
else
  echo "False"
  echo "You must mistaken me for $NAME"
fi

可以使用'elif'(else-if)进行扩展

NAME="George"
if [ "$NAME" = "John" ]; then
  echo "John Lennon"
elif [ "$NAME" = "George" ]; then
  echo "George Harrison"
else
  echo "This leaves us with Paul and Ringo"
fi

条件构造使用的表达式被评估为true或false。 表达式可以是单个字符串或变量。空字符串或由空格或未定义的变量名组成的字符串将被评估为false。 该表达式可以是比较的逻辑组合:负号由!表示,逻辑与(连接)由&&表示,逻辑或(分离)由||表示。条件表达式应用双括号[[]括起来]。

数值比较的类型

comparison    Evaluated to true when
$a -lt $b    $a < $b
$a -gt $b    $a > $b
$a -le $b    $a <= $b
$a -ge $b    $a >= $b
$a -eq $b    $a is equal to $b
$a -ne $b    $a is not equal to $b

字符串比较的类型

comparison    Evaluated to true when
"$a" = "$b"     $a is the same as $b
"$a" == "$b"    $a is the same as $b
"$a" != "$b"    $a is different from $b
-z "$a"         $a is empty

-note1:=周围必须有空格

-note2:在字符串变量周围使用“”,以避免将特殊字符的shell扩展为*

逻辑组合

if [[ $VAR_A -eq 1 && ($VAR_B = "bee" || $VAR_T = "tee") ]] ; then
    command...
fi

案例结构

case "$variable" in
    "$condition1" )
        command...
    ;;
    "$condition2" )
        command...
    ;;
esac

简单案例bash结构

请注意,在这种情况下,$ case是可变的,不必命名为case-这仅是示例

mycase=1
case $mycase in
    1) echo "You selected bash";;
    2) echo "You selected perl";;
    3) echo "You selected phyton";;
    4) echo "You selected c++";;
    5) exit
esac

Exercise

在第一部分中更改变量,以便每个if语句都解析为True。

Tutorial Code

#!/bin/bash
# change these variables
NUMBER=10
APPLES=12
KING=GEORGE
# modify above variables
# to make all decisions below TRUE
if [ $NUMBER -gt 15 ] ; then
  echo 1
fi
if [ $NUMBER -eq $APPLES ] ; then
  echo 2
fi
if [[ ($APPLES -eq 12) || ("$KING" = "LUIS") ]] ; then
  echo 3
fi
if [[ $(($NUMBER + $APPLES)) -le 32 ]] ; then
  echo 4
fi

Expected Output

1
2
3
4

Solution

#!/bin/bash
# change these variables
NUMBER=16
APPLES=16
KING="LUIS"
# modify above variables
# to make all decisions below TRUE
if [ $NUMBER -gt 15 ] ; then
  echo 1
fi
if [ $NUMBER -eq $APPLES ] ; then
  echo 2
fi
if [[ ($APPLES -eq 12) || ("$KING" = "LUIS") ]] ; then
  echo 3
fi
if [[ $(($NUMBER + $APPLES)) -le 32 ]] ; then
  echo 4
fi