[Linux Shell学习系列五]Shell编程基础-Bash的内部变量，位置参数和特殊参数

Posted 2021-02-19 workingdiary

1. Bash的内部变量

1）$BASH变量：用于引用Bash实例的全路径名；

$ echo $BASH 
/bin/bash

2）$HOME变量：当前用户的HOME目录，通常是/home/<username>；

$ echo $HOME
/home/user1

3）$IFS变量：内部字段分隔符的缩写；

$ set x y z
$ echo "$*"
x y z
$ set IFS=":;-" #可以修改
$ echo "$*"
x:y:z

4）$OSTYPE变量：操作系统的类型；

$ echo $OSTYPE
linux-gnu

5）$SECONDS变量：脚本已经运行的秒数；

$ cat echotime.sh 
#!/bin/bash
#2020-05-18

TIME_LIMIT=5
INTERVAL=1

echo 
echo "Use Ctrl+C to leave before $TIME_LIMIT seconds."
echo 

while [ "$SECONDS" -le "$TIME_LIMIT" ]
do
        sleep $INTERVAL
        echo $(date) >> /home/ntrade/20200511study/time.log
        if [ "$SECONDS" -eq 1 ]
        then
                units=second
        else
                units=seconds
        fi
        echo "This script has been running for $SECONDS $units."
done


$ ./echotime.sh  #执行脚本

Use Ctrl+C to leave before 5 seconds.

This script has been running for 1 second.
This script has been running for 2 seconds.
This script has been running for 3 seconds.
This script has been running for 4 seconds.
This script has been running for 5 seconds.
This script has been running for 6 seconds.

6）$TMOUT变量：如果该变量被设置为非0，则Bash将其作为read默认的超时秒数；

$ cat readtimeout.sh 
#!/bin/bash
#2020-05-18

set -o nounset #Treat unset variables as an error.
TMOUT=3

echo "Are you sure? (Y/N)"
read input

if [ "$input" == "Y" ]
then 
        echo "Continue..."
else 
        echo "Exit!"
fi

$ ./readtimeout.sh  #执行脚本
Are you sure? (Y/N) #此时如果一直不输入，则3秒后自动退出
Exit!

7）$UID变量：当前用户的帐号标识码（ID号），/etc/passwd中记录值（只读）；输出真实ID，即使通过su获得了其他用户的权限。

$ cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
... #省略内容
user1:x:1001:1001::/home/user1:/bin/bash

$ echo $UID #使用user1执行
1001

 

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