【Linux】循环语句之for&while
1. 条件测试
1.1 条件测试常用语法
在 bash 编程里，条件测试常用的语法如下：
- 语法1：test <判断表达式>，test命令和"<判断表达式>"之间至少有一个空格。
- 语法2：[ <判断表达式> ] ，和test命令的用法相同，这是推荐方法。
[]的边界和内容之间至少有一个空格。 - 语法3：[[ <判断表达式> ]] ，是比test和[]更新的语法格式。
[[]]的边界和内容之间至少有一个空格。 - 语法4：(( <判断表达式> )) ，一般用于 if 语句。
(())(双小括号）两端不需要有空格。 - 语法5：comand，命令的返回值确定表达式的真值或假值。
条件测试常用于判断以下内容：
- 字符串
- 数值
- 文件、进程、服务、用户等
1.2 test帮助信息
bash
[harvy@CentOS7 ~ 09:42:41]$ man test
test: test [expr]
Evaluate conditional expression.
Exits with a status of 0 (true) or 1 (false) depending on
the evaluation of EXPR. Expressions may be unary or binary. Unary
expressions are often used to examine the status of a file. There
are string operators and numeric comparison operators as well.
The behavior of test depends on the number of arguments. Read the
bash manual page for the complete specification.
File operators:
-a FILE True if file exists.
-b FILE True if file is block special.
-c FILE True if file is character special.
-d FILE True if file is a directory.
-e FILE True if file exists.
-f FILE True if file exists and is a regular file.
-g FILE True if file is set-group-id.
-h FILE True if file is a symbolic link.
-L FILE True if file is a symbolic link.
-k FILE True if file has its `sticky' bit set.
-p FILE True if file is a named pipe.
-r FILE True if file is readable by you.
-s FILE True if file exists and is not empty.
-S FILE True if file is a socket.
-t FD True if FD is opened on a terminal.
-u FILE True if the file is set-user-id.
-w FILE True if the file is writable by you.
-x FILE True if the file is executable by you.
-O FILE True if the file is effectively owned by you.
-G FILE True if the file is effectively owned by your group.
-N FILE True if the file has been modified since it was last read.
FILE1 -nt FILE2 True if file1 is newer than file2 (according to
modification date).
FILE1 -ot FILE2 True if file1 is older than file2.
FILE1 -ef FILE2 True if file1 is a hard link to file2.
String operators:
-z STRING True if string is empty.
-n STRING
STRING True if string is not empty.
STRING1 = STRING2
True if the strings are equal.
STRING1 != STRING2
True if the strings are not equal.
STRING1 < STRING2
True if STRING1 sorts before STRING2 lexicographically.
STRING1 > STRING2
True if STRING1 sorts after STRING2 lexicographically.
Other operators:
-o OPTION True if the shell option OPTION is enabled.
-v VAR True if the shell variable VAR is set
! EXPR True if expr is false.
EXPR1 -a EXPR2 True if both expr1 AND expr2 are true.
EXPR1 -o EXPR2 True if either expr1 OR expr2 is true.
arg1 OP arg2 Arithmetic tests. OP is one of -eq, -ne,
-lt, -le, -gt, or -ge.
Arithmetic binary operators return true if ARG1 is equal, not-equal,
less-than, less-than-or-equal, greater-than, or greater-than-or-equal
than ARG2.
Exit Status:
Returns success if EXPR evaluates to true; fails if EXPR evaluates to
false or an invalid argument is given.1.3 判断文件
bash
# test文件是否存在，根据变量的复制判断不同的结果
[harvy@CentOS7 ~ 09:54:50]$ test -a $file && echo "${file} is exist" || echo "${file} is not exist"
/etc/fstab is exist
[harvy@CentOS7 ~ 09:57:16]$ file=/etc/fstab-aaa
[harvy@CentOS7 ~ 09:57:29]$ test -a $file && echo "${file} is exist" || echo "${file} is not exist"
/etc/fstab-aaa is not exist1.4 判断目录
bash
# 目录存在，显示目录
[harvy@CentOS7 ~ 09:50:25]$ test -a $dir_path && echo "${dir_path} is exist" || echo "${dir_path} is not exist"
/home/harvy/test_dir is exist
# 将test目录删除
[harvy@CentOS7 ~ 09:54:30]$ rm -rf /home/harvy/test_dir/
# 再次test显示文件不存在
[harvy@CentOS7 ~ 09:54:48]$ test -a $dir_path && echo "${dir_path} is exist" || echo "${dir_path} is not exist"
/home/harvy/test_dir is not exist1.5 其他示例
bash
[harvy@CentOS7 ~ 09:44:46]$ ls -ld /etc/fstab /etc/shadow
-rw-r--r-- 1 root root 541 Apr 10 14:26 /etc/fstab
---------- 1 root root 1262 Apr 10 14:01 /etc/shadow
[harvy@CentOS7 ~ 09:48:41]$ test -r /etc/fstab && tail -5 /etc/fstab || echo "You have no privilege for this file"
#
/dev/mapper/centos-root / xfs defaults 0 0
UUID=a90d05c3-2ade-4aa6-bd7e-0781907fb198 /boot xfs defaults 0 0
/dev/mapper/centos-home /home xfs defaults 0 0
/dev/mapper/centos-swap swap swap defaults 0 0
[harvy@CentOS7 ~ 09:50:10]$ test -r /etc/fstab && tail -5 /etc/shadow || echo "You have no privilege for this file"
tail: cannot open '/etc/shadow' for reading: Permission denied
You have no privilege for this file1.6 判断字符串
- -z，字符串为空，则为true
- -n，字符串非空，则为true
bash
# 测试变量是否定义
[harvy@CentOS7 ~ 10:07:41]$ test -z "$testvar" && echo "testvar is not defined" || echo "testvar is ${testvar}"
testvar is not defined
[harvy@CentOS7 ~ 10:13:18]$ testvar=name
[harvy@CentOS7 ~ 10:13:35]$ test -z "$testvar" && echo "testvar is not defined" || echo "testvar is ${testvar}"
testvar is name
# 取消变量定义
[harvy@CentOS7 ~ 10:13:37]$ unset testvar
[harvy@CentOS7 ~ 10:14:09]$ test -n "$testvar" && echo "testvar is ${testvar}" || testvar=zhangsan
# 再次打印，显示变量值
[harvy@CentOS7 ~ 10:15:01]$ test -n "$testvar" && echo "testvar is ${testvar}" || testvar=zhangsan
testvar is zhangsan-
STRING1 = STRING2,True if the strings are equal
-
STRING1 != STRING2,True if the strings are equal.
bash
# 检查当前运行的用户
# 如果不是root用户，则提示：请以root身份运行
# 如果是root用户，则执行创建用户zhangsan
#普通用户运行，则提示：请以root身份运行
[harvy@CentOS7 ~ 10:22:52]$ test "$USER" = "root" && useradd zhangsan || echo "Please run as root"
Please run as root
[harvy@CentOS7 ~ 10:31:13]$ id zhangsan
id: zhangsan: no such user
# root用户运行，创建用户成功
[root@CentOS7 ~ 10:32:36]# test "$USER" = "root" && useradd zhangsan || echo "Please run as root"
[root@CentOS7 ~ 10:32:41]# id zhangsan
uid=1001(zhangsan) gid=1001(zhangsan) groups=1001(zhangsan)
# 等效命令
# 先删除zhangsan2用户，连带文件目录一起删除
[root@CentOS7 ~ 10:35:14]# userdel -r zhangsan2
[root@CentOS7 ~ 10:35:36]# test "$USER" != "root" && echo "Please run as root" || useradd zhangsan2
[root@CentOS7 ~ 10:35:40]# id zhangsan2
uid=1002(zhangsan2) gid=1002(zhangsan2) groups=1002(zhangsan2)1.7 数值判断
arg1 OP arg2 Arithmetic tests. OP is one of -eq, -ne, -lt, -le, -gt, or -ge.
bash
# 检查当前运行的用户
# 如果不是root用户，则提示：请以root身份运行
# 如果是root用户，则执行删除用户zhangsan
# 普通用户运行，则提示：请以root身份运行
[harvy@CentOS7 ~ 10:39:42]$ test $UID -eq 0 && userdel -r zhangsan || echo "Please run as root"
Please run as root
# root用户运行，删除用户成功,zhangsan原本就不存在
[root@CentOS7 ~ 10:38:04]# test $UID -eq 0 && userdel -r zhangsan || echo "Please run as root"userdel: user 'zhangsan' does not exist
Please run as root1.8 多条件测试
- ！ EXPR True if expr is false
- EXPR1 -a EXPR2 True if both expr1 AND expr2 are true.
- EXPR1 -o EXPR2 True if either expr1 OR expr2 is true.
bash
# 逻辑非：提交取反满足
[harvy@CentOS7 ~ 11:21:28]$ test ! -a $file && touch $file && echo "create file success."
create file success.
[harvy@CentOS7 ~ 11:21:37]$ ls $file
/tmp/test.log
# 逻辑与：两个条件同时满足
[harvy@CentOS7 ~ 11:22:29]$ test -w $file -a -a $file && date > $file
[harvy@CentOS7 ~ 11:23:03]$ cat $file
Mon Apr 13 11:23:03 CST 2026
# 逻辑或：两个条件任一满足
[harvy@CentOS7 ~ 11:23:51]$ file1=/etc/yum.conf
[harvy@CentOS7 ~ 11:24:03]$ file2=/etc/yum.repos.d/
[harvy@CentOS7 ~ 11:24:29]$ test -a $file1 -o -a $file2 && echo "yum repo is exist"
yum repo is exist2. IF 语句
2.1 单分支语法
bash
# 单分支
if 条件判断;then
command
fi
if 条件判断
then
command
fi
条件判断 && command2.2双分支语法
bash
# 单分支
if 条件判断;then
command1
else
command2
fi
条件判断 && command1 || command22.3 多分支语法
bash
# 单分支
if 条件1判断;then
command1
elif 条件2判断;then
command2
else
command3
fi2.4 实践
双分支
需求：
- 如果sshd服务正常运行，则提示服务正在运行
- 提示服务未运行，并启动服务，启动后提示服务正常运行
开发脚本if_sshd.sh, 内容如下：
bash
[root@CentOS7 ~ 14:15:47]# vim if_sshd.sh
#!/bin/bash
systemctl is-active sshd &> /dev/null
if (($?==0));then
echo sshd is running
else
echo sshd is not running,auto restart it. && systemctl start sshd
fi运行结果：
bash
[root@CentOS7 ~ 14:30:52]# systemctl stop sshd
[root@CentOS7 ~ 14:31:04]# bash if_sshd_running2.sh
sshd is not running,auto restart it.
[root@CentOS7 ~ 14:31:15]# bash if_sshd_running2.sh
sshd is running多分支：
需求：开发if_sshd.sh脚本。
- 脚本使用方法："Usage：$0 start|stop|status|restart|reload"
- 如果参数数量个数不是1个，则输出脚本使用方法。
- 如果$1是start，则执行'systemctl start sshd'
- 如果$1是stop，则执行'systemctl stop sshd'
- 如果$1是status，则执行'systemctl status sshd'
- 如果$1是restart，则执行'systemctl restart sshd'
- 如果$1是reload，则执行'systemctl reload sshd'
- 如果$1是其他子命令，则输出脚本使用方法。
开发脚本if_sshd.sh，内容如下：
bash
#!/bin/bash
service_name=sshd
if (($#!=1));then
echo "Usage: $0 start|stop|status|restart|reload"
exit
fi
if [ "$1" == "start" ];then
systemctl start ${service_name}
elif [ "$1" == "stop" ];then
systemctl stop ${service_name}
elif [ "$1" == "restart" ];then
systemctl restart ${service_name}
elif [ "$1" == "status" ];then
systemctl status ${service_name}
elif [ "$1" == "reload" ];then
systemctl reload ${service_name}
else
echo "Usage: $0 start|stop|status|restart|reload"
fi运行结果：
bash
# 缺乏参数
[root@CentOS7 ~ 14:52:40]# bash if_sshd.sh
Usage: if_sshd.sh start|stop|status|restart|reload
# 参数过多
[root@CentOS7 ~ 14:53:06]# bash if_sshd.sh start stop
Usage: if_sshd.sh start|stop|status|restart|reload
# 正常传参
[root@CentOS7 ~ 14:53:13]# bash if_sshd.sh status
● sshd.service - OpenSSH server daemon
Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: enabled)
Active: active (running) since Mon 2026-04-13 14:31:15 CST; 22min ago
Docs: man:sshd(8)
......略
# 参入错误参数
[root@CentOS7 ~ 14:54:14]# bash if_sshd.sh statrtrtr
Usage: if_sshd.sh start|stop|status|restart|reload优化代码
bash
#!/bin/bash
service_name="$2"
action="$1"
if (($#!=2));then
echo "Usage：$0 start|stop|status|restart|reload ${service_name}"
exit
fi
if [ "$action" == "start" -o "$action" == "stop" -o "$action" == "status" -o "$action" == "restart" -o "$action" == "reload" ];then
systemctl $action ${service_name}
else
echo "Usage：$0 start|stop|status|restart|reload"
fi3. FOR 语句
3.1 Shell 自带语法
bash
for 变量名 in 清单
do
command
done3.2 C语言格式语法
bash
for ((num=1;num<=10;num++))
do
echo $num
done3.3 实践
从1+2+...+n-1+n的和
bash
vim cal_for.sh代码具体内容
bash
#!/bin/bash
maxnum="$1"
sum=0
num_n=0
for ((num=1;num<=$maxnum;num++))
do
sum=$[ sum + num ]
done
num_n=$[ maxnum-1 ]
echo "1+2+3+...+$num_n+$maxnum=$sum"执行效果
bash
[root@CentOS7 ~ 15:32:45]# bash cal_for.sh 10
1+2+3+...+9+10=55
[root@CentOS7 ~ 15:33:06]# bash cal_for.sh 55
1+2+3+...+54+55=1540批量修改文件名称
bash
# 准备文件
[root@CentOS7 ~ 15:33:22]# mkdir Pictures
[root@CentOS7 ~ 15:48:46]# touch Pictures/snap-{01..10}.jpg
[root@CentOS7 ~ 15:49:01]# ls /root/Pictures/
snap-01.jpg snap-03.jpg snap-05.jpg snap-07.jpg snap-09.jpg
snap-02.jpg snap-04.jpg snap-06.jpg snap-08.jpg snap-10.jpg
bash
# 代码内容
#!/bin/bash
for file in Pictures/snap*
do
old_name=$file
new_name=${old_name/snap/pic}
mv ${old_name} ${new_name}
done
bash
# 执行效果
[root@CentOS7 ~ 15:49:45]# vim mod_filename.sh
[root@CentOS7 ~ 15:53:17]# bash mod_filename.sh
[root@CentOS7 ~ 15:53:27]# ls ~/Pictures/
pic-01.jpg pic-03.jpg pic-05.jpg pic-07.jpg pic-09.jpg
pic-02.jpg pic-04.jpg pic-06.jpg pic-08.jpg pic-10.jpg为公司新入职的员工，创建一批账号
- 员工姓名清单保存在staff.txt中，每行一个。
- 为每个员工创建一个初始密码，初始密码是8位随机值，要求登录后必须修改密码。
- 创建的结果要保存到users_info.conf中，格式为username: password。
bash
# 代码内容如下：
#!/bin/bash
staff_name_file=staff.txt
staff_info_file=uaer_info.conf
if [ -a ${staff_name_file} -a -r ${staff_name_file} ];then
for staff in $(cat $(staff_name_file))
do
useradd $staff
password=$(date +%N| md5sum | head -c8)
echo $password | passwd --stdin $staff &> /dev/null
echo "$staff:$password" >> ${staff_info_file}
done
else
echo "${staff_name_file} is not exist."
fi运行结果
bash
[root@CentOS7 ~ 16:32:23]# bash add_staff.sh
[root@CentOS7 ~ 16:32:25]# cat ~/user_info.conf
Tom:b5e15742
Jerry:018095af
Jack:ac661597
Rose:0b5c5dce优化代码：如果用户已经存在，则跳过创建
bash
# 代码内容
#!/bin/bash
staff_name_file=staff.txt
staff_info_file=user_info.conf
if [ -a ${staff_name_file} -a -r ${staff_name_file} ];then
for staff in $(cat ${staff_name_file})
do
id $staff &> /dev/null
flag=$?
if (($flag != 0));then
useradd $staff
password=$(date +%N | md5sum | head -c8)
echo $password | passwd --stdin $staff &> /dev/null
echo "$staff:$password" >> ${staff_info_file}
else
echo "${staff} is exist,pass and go on"
fi
done
else
echo "${staff_name_file} is not exist."
fi
bash
# 运行结果
[root@CentOS7 ~ 17:10:05]# bash staff_v2.1.sh
Tom is exist,pass and go on
Jerry is exist,pass and go on
Jack is exist,pass and go on
Rose is exist,pass and go on3.4 生成随机值常见方法：
- 利用时间生成纳秒级别值：
date +%N
bash
[root@centos7 bin 16:04:25]# date +%N
456812617- mktemp命令创建一个临时目录，目录名是随机值。
bash
[root@centos7 bin 16:05:14]# echo $(mktemp)
/tmp/tmp.ncZkKK9TYD- 利用环境变量RANDOM生成0~32767随机值。
4. while/until 语句
4.1 基本语法
while
条件满足，一直运行。
bash
while 条件判断
do
command
doneuntil
条件不满足，一直运行。满足条件，则终止运行。
bash
until 条件判断
do
command
done4.2 实践demo
监控 sshd 服务
开发脚本monitor_sshd.sh 实时监控sshd服务，实现以下功能：
- 如果sshd服务没有运行，则报告sshd服务状态到/tmp/sshd_status.log日志；同时启动sshd服务，启动的结果也写入/tmp/sshd_status.log日志。
- 如果sshd服务正常运行，则报告sshd服务状态，并写入/tmp/sshd_status.log日志
- 每隔3秒执行一次监控。
bash
#!/bin/bash
# 定义日志文件路径，所有监控信息都会记录到这个文件里
log_file=/tmp/monitor_sshd.log
# 无限循环：一直执行监控，永不退出
while true
do
# 获取 sshd 服务的当前状态（active：运行中，inactive：停止，failed：故障）
status=$(systemctl is-active sshd)
# 判断：如果 sshd 状态是 active（正在运行）
if [ "$status" = "active" ];then
# 输出日志：打印当前时间 + 服务正常信息，同时写入日志文件（tee -a）
echo "$(date):sshd is running,status is active." | tee -a ${log_file}
else
# 否则：sshd 没有运行，先输出提示日志
echo "$(date):sshd is not running,status is not active." | tee -a ${log_file}
# 尝试启动 sshd 服务，屏蔽所有输出信息
systemctl start sshd &> /dev/null &&\
# 如果启动成功（&& 代表上一条命令成功才执行），打印成功日志
echo "$(date):Start sshd success." | tee -a ${log_file} ||\
# 如果启动失败（|| 代表上一条命令失败才执行），打印失败日志
echo "$(date):Start sshd failed." | tee -a ${log_file}
fi
# 每次检查完，休眠 3 秒，再进行下一次循环
sleep 3
done运行结果
bash
[root@CentOS7 ~ 17:21:54]# bash moniter_sshd.sh
Mon Apr 13 17:22:22 CST 2026:sshd is running,status is active.
Mon Apr 13 17:22:25 CST 2026:sshd is not running,status is not active.
Mon Apr 13 17:22:25 CST 2026:Start sshd success.
Mon Apr 13 17:22:28 CST 2026:sshd is running,status is active.