bash脚本编程---循环

#!/bin/bash
if ! (grep "^$1\>" /etc/passwd &> /dev/null);then
     useradd $1
     echo $1 | passwd --stdin $1 &>/dev/null
     echo "add user $1 finished."
else
     echo "the user $1 is exists."
fi
#如果给出的用户不存在，就添加，用户帐号与密码相同，如果存在就提示存在

#!/bin/bash
if [ $# -eq 1 ];then
   echo "At least one username."
   exit 2
fi
#先判断是否输入了一个参数，如果没有或者输入了多个参数就退出
#如果是“-lt”，则表示最少一个参数，即使输入多个也只取第一个
if ! (grep "^$1\>" /etc/passwd &> /dev/null);then
     useradd $1
     echo $1 | passwd --stdin $1 &>/dev/null
     echo "add user $1 finished."
else
     echo "the user $1 is exists."
fi

#!/bin/bash
if [ $# -lt 2 ];then
   echo "Must give two num."
   exit 2
fi
if [ $1 -ge $2 ];then
   echo "Max num is $1."
else
   echo "Max num is $2."
fi

#!/bin/bash
if [ $# -lt 2 ];then
   echo "Must give two num."
fi
declare -i max=$1
if [ $1 -lt $2 ];then
   max=$2
fi
echo "The max num is $max"
#先对max进行赋值，然后进行比较

#!/bin/bash
if [ $# -lt 1 ];then
   echo "Must give a username."
fi
num=$(id -u $1)
let num2=$num%2
#echo "num is $num"
#echo "num2 is $num2"
if [ $num2 -eq 1 ];then
   echo "the uid is ji"
else
   echo "the uid is ou"
fi

#!/bin/bash
if [ $# -lt 2 ];then
   echo "Must give two filename."
   exit 2
fi
if [ -f $1 ];then
   num1=$(cat $1 |wc -l)
   echo "the file $1 is exists and hangshu is $num1."
else
   echo "the file $1 is not exists."
   exit 4
fi
if [ -f $2 ];then
   num2=$(cat $2 |wc -l)
   echo "the file $2 is exists and hangshu is $num2."
else
   echo "the file $2 is not exists."
   exit 3
fi
if [ $num1 -lt $num2 ];then
   echo "$2 hangshu is more."
else
   echo "$1 hangshu us more."

if 测试条件；then
   为真时执行；
fi

if 测试条件；then
   为真时执行
else
   为假时执行
fi

if 测试条件；then
   条件1为真时执行
elif condition-2;then
   条件2为真时执行
elif condition-3;then
   条件3为真时执行
。。。。
elif condition-n;then
   条件n为真时执行
else 
   所有条件都不满足时执行的分支
fi

#!/bin/bash
#
if [ $# -lt 1 ];then
   echo "Must give a path."
   exit 2
fi
if [ -L $1 ];then
  echo "Symbolic link."
elif [ -b $1 ];then
  echo "block special."
elif [ -c $1 ];then
  echo "Character special file."
elif [ -S $1 ];then
  echo "Socket file."
elif [ -f $1 ];then
   echo "common file."
elif [ -d $1 ];then
   echo "Directory."lse
  echo "UNKNOW."
fi

#!/bin/bash
[ $# -lt 1 ] && echo "At least one username."&& exit 1  #先判断参量是否存在
! id $1 &>/dev/null && echo "Bo such user."&& exit 2    #判断用户是否存在
uid=$(id -u $1)
if [ $uid -eq 0 ];then
   echo "the user is root."
elif [ $uid -le 1000 ];then
   echo "the user is system user."
else
   echo "the user is login user."
fi

for VARIABLE in LIST;do
    循环体
done

while CONDITION;do
     循环体
     循环控制变量修正表达式
done

until CONDITION;do
    循环体
    循环控制变量修正表达式
done

#!/bin/bash
declare -i sum=0
declare -i i=1
#until [ $i -gt 100 ];do
#判断i的值是否大于100，为假时循环
while [ $i -le 100 ];do
#判断i的值是否小于等于100，为真时循环
     let sum+=$i
     let i++
done
echo $sum

#!/bin/bash
declare -i sum=0
#for i in {1..100};do
for i in `seq 1 100`;do
    let sum=$sum+$i
done
echo "sum=$sum"

#!/bin/bash
for i in {101..103};do
    name="user${i}"
    ! id $name &> /dev/null && useradd $name && echo "add user ${name} succeed!"
    echo "$i" | passwd --stdin "$name" &> /dev/null && echo "set passwd ‘${name}‘ for $name succeed!"
done

#!/bin/bash
for j in {1..9};do
    for i in `seq 1 $j`;do
        echo -n -e "${i}x${j}=$[${i}*${j}]\t"
    done
    echo
done

#!/bin/bash
for s in {1..9};do
    j=$[10-$s]
    for i in $(seq 1 $j);do
        echo -n -e "${i}X${j}=$[${i}*${j}]\t"
    done
    echo
done

#!/bin/bash
declare -i s=9
until [ $s -eq 0 ];do
declare -i j=1         
#注意：每次循环，i的值都会发生变化，一定要重新定义
until [ $j -gt $s ];do
     echo -n -e "${s}X${j}=$[${s}*${j}]\t"
     let j++
     done
     let s--
     echo
done

#!/bin/bash
declare -i s=9
while [ $s -gt 0 ];do
   declare -i j=1
   while [ $j -le $s ];do
       echo -n -e "${s}X${j}=$[${s}*${j}]\t"
       let j++
   done
   let s--
   echo
done