条件语句

• 结构化命令
• bash-handbook/translations/zh-CN at master · denysdovhan/bash-handbook

基元和组合表达式

跟文件系统相关：

跟字符串相关：

算数二元运算符：

if else

if condition
then
  statements(s)
fi

• condition 是判断条件 - 如果 condition 成立（返回 true），那么 then 后边的语句将会被执行 - 如果 condition 不成立（返回 false），那么不会执行任何语句

if else 语句

#!/bin/bash
read a
read b
if (($a == $b))
then
  echo "a 和 b 相等"
else
  echo "a 和 b 不想等，输入错误"
fi

if elif else 语句

if condition1
then
  statement1
elif condition2
then
  statement2
elif condition3
then
  statement3
...
else
  statementN
fi

嵌套 if 语句

if [ 测试条件成立 ]
then 执行相应命令
  if [ 测试条件成立 ]
  then 执行相应命令
  fi
fi 结束

case in

case expression in
  pattern1) statement1;;
  pattern2) statement2;;
  pattern3 | pattern4) statement3;;
  *) default statement
esac

$ cat case-in.sh
#!/bin/bash
# using the case command
#
case $USER in
rich | barbara)
    echo "Welcom $USER"
    echo "Please enjoy your visit";;
testing)
    echo "Special testing account";;
jessica)
    echo "Do not forget to log off when you're done";;
*)
    echo "Sorry, you are not allowed here";;
esac
$ ./case-in.sh

for

for var in list
do
  commands
done

#!/bin/bash
# basic for command
for item in Guangdong Sichuan Henan Heilongjiang Zhejiang Jiangsu
do
  echo The next province is $item
done

• for 循环假定每个值都是用空格分割的
• 当列表的值含有特殊字符，可以使用转义字符解决
• 当在某个值的两边使用双引号时，Shell 并不会将双引号当成值的一部分

从命令中读取值

# for-read-values-from-a-file.sh
# !/bin/bash
# reading values from  a file
file="provinces"
for province in $(cat $file)
do
  echo "Visit beautiful $province"
done

Guangdong
Sichuan
Henan
Heilongjiang
Zhejiang
Jiangsu

更改字段分隔符

• 空格
• 制表符
• 换行符

#/bin/bash
# reading values from a file
file="provinces"
IFS=$'\n'
for province in $(cat $file)
do
  echo "Visit beautiful $province"
done

用通配符读取目录

#!/bin/bash
# iterate through all the files in a directory
for file in ./*
do
  if [ -d "$file"]
  then
    echo "$file is a directory"
  elif [ -f "$file" ]
  then
    echo "$file is a file"
  fi
done

C 语言风格的 for 命令

for (( variable assignment ; condition ; iteration process ))

for (( i=1; i<= 10; i++ ))
do
  echo "The next number is $i"
done

while

while test command
do
  other commands
done

#/bin/bash
# while command test
var1=10
while [ $var1 -gt 0 ]
do
  echo $var1
  var1=$[ $var1 - 1 ]
done

until

util test commands
do
  other commands
done

#!/bin/bash
# using the until command
num=100
until [ $num -eq 0 ]
do
    echo $num
    num=$[ $num - 25 ]
done

break

跳出单体循环

#!/bin/bash
# breaking out of a for loop
for num in 1 2 3 4 5 6 7 8 9 10
do
  if [ $num -eq 5 ]
  then
    break
  fi
  echo "Iteration number: $num"
done
echo "The for loop is completed"

跳出内部循环

#!/bin/bash
# breaking out of an inner loop
for (( a = 1; a < 4; a++ ))
do
  echo "Outer loop: $a"
  for (( b = 1; b < 100; b++ ))
  do
    if [ $b -eq 5 ]
    then
      break
    fi
    echo "Inner loop: $b"
  done
done

跳出外部循环

break n

#!/bin/bash
# breaking out of an outer loop
for (( a = 1; a < 4; a++ ))
do
  echo "Outer loop: $a"
  for (( b = 1; b < 100; b++ ))
  do
    if [ $b -gt 4 ]
    then
      break 2
    fi
    echo "Inner loop: $b"
  done
done

continue

#!/bin/bash
# using the continue command
for (( num = 1; num < 15; num++ ))
do
  if [ $num -gt 5 ] && [ $num -lt 10 ]
  then
    continue
  fi
  echo "Iteration number: $num"
done

#!/bin/bash
# improperly using the continue command in a while loop
num=0
while echo "while iteration: $num"
  [ $num -lt 15 ]
do
  if [ $num -gt 15 ] && [ $num -lt 10 ]
  then
    continue
  fi
  echo "Inside iteration number: $num"
  num=$[ $num + 1 ]
done

嵌套循环

#!/bin/bash
# nesting for loops
for (( a = 1; a <= 3; a++ ))
do
  echo "Starting loop $a"
  for (( b = 1; b <= 3; b++ ))
  do
    echo " Inside loop: $b"
  done
done

#!/bin/bash
# placing a for loop inside a while loop
num1=5
while [ $num1 -ge 0 ]
do
  echo "Output loop: $num1"
  for (( num2 = 1; num2 < 3; num2++ ))
  do
    num3=$[ $num1 * $num2 ]
    echo " Inner loop: $num1 * $num2 = $num3"
  done
  $num1=$[ $num1 - 1 ]
done

处理循环输出

for file in /home/rich/*
  do
    if [ -d "$file" ]
    then
      echo "$file is a direcoty"
    elif
      echo "$file is a file"
    fi
done > output.txt

实践应用

查找可执行文件

#!/bin/bash
# finding files in the PATH
IFS=:
for folder in $PATH
do
  echo "$folder:"
  for file in $folder/*
  do
    if [ -x $file ]
    then
      echo "    $file"
    fi
  done
done

创建多个用户账户

userid,user name

#!/bin/bash
# process new user accounts
input="users.csv"
while IFS=',' read -r userid name
do
  echo "adding $userid"
  useradd -c "$name" -m $userid
done < "$input"