• read
• $ISF

read

read [options] [variable...]

options:

• -p prompt 提示语句
• -t timeout 超时
• -s slient 不显示用户输入（用于输入密码）
• -i 用于提供默认值，必须和 -e 一起使用

# 提供默认值 $USER
# 读取输入，保存到变量名 name
[admin@localhost ~]$ read -e -p "Your name: " -i $USER  name && echo $name
Your name: admin
admin

$ cat test-integer.sh
#!/bin/bash

read -p "Enter an integer: " num

# 判断是否整数
if [[ "$num" =~ -?[[:digit:]]+ ]]; then

# 判断正负
  if (( num > 0 )); then
    printf "%s is positive\n" $num
  elif (( num < 0 )); then
    printf  "%s is negative\n" $num
  else
    printf "%s is zero\n" $num
  fi

# 判断奇偶
  if (( num % 2 == 0 )); then
    printf "%s is even\n" $num
  else
    printf "%s is odd\n" $num
  fi

# 不是整数，重定向到 stderr
else
  echo "$num is not an integer" >&2
  exit 1
fi

IFS

环境变量 IFS (internal field separator): 默认值是空白字符；
作用：作为 read 的分隔符

$ ./user_info.sh
Enter a username: admin

username: admin
group: admin
uid: 1000
gid: 1000
home: /home/admin:/bin/bash

$ cat ./user_info.sh
#!/bin/bash

# root:x:0:0:root:/root:/bin/bash

read -p "Enter a username: " -ei $USER user

user_info=$(grep "^$user" /etc/passwd)

if [ -n "$user_info" ]; then
    # 临时改变分隔符为 :
    # 用 <<< 将变量 user_info 的值作为标准输入传给 read
    IFS=":" read username passwd uid gid group home <<< "$user_info"
    printf "\nusername: %s" $username
    printf "\ngroup: %s" $group
    printf "\nuid: %s" $uid
    printf "\ngid: %s" $gid
    printf "\nhome: %s" $home
    printf "\n\n"
else
    echo "No such user: $user">&2
    exit 1
fi

操作符 >>>

>>> 用于重定向字符串
command1 | command2：管道将 command1 的输出作为 command2 的输输入，但是管道会创建一个 subshell （复制 command1 的环境变量） 来执行 comand2，command2 执行完毕后销毁 subshell （清除环境变量）

# $variable 的值为空
$ echo "Hello" | read variable; echo $variable

# $variable 的值为 "Hello"
$ read variable <<< "Hello"; echo $variable
Hello