1,$$:显示当前的进程id号
ghostwu@dev:~/linux/shell/how_to_use_var$ cat show_pid.sh #!/bin/bash echo $$ sleep 3000 ghostwu@dev:~/linux/shell/how_to_use_var$ bash show_pid.sh & [1] 9401 ghostwu@dev:~/linux/shell/how_to_use_var$ 9401 ghostwu@dev:~/linux/shell/how_to_use_var$ ps -ef | grep show_pid ghostwu 9401 2792 0 06:07 pts/18 00:00:00 bash show_pid.sh ghostwu 9404 2792 0 06:07 pts/18 00:00:00 grep --color=auto show_pid
ghostwu@dev:~/linux/shell/how_to_use_var$ ps -ef | grep show_pid | grep -v grep ghostwu 9401 2792 0 06:07 pts/18 00:00:00 bash show_pid.sh
ghostwu@dev:~/linux/shell/how_to_use_var$ kill 9401 ghostwu@dev:~/linux/shell/how_to_use_var$ ps -ef | grep show_pid ghostwu 9478 2792 0 06:08 pts/18 00:00:00 grep --color=auto show_pid [1]+ Terminated bash show_pid.sh
2,(())用于整数的常用运算符
>把两个整数的运算结果赋值给一个变量,前面要加$
ghostwu@dev:~/linux/shell/how_to_use_var$ a=((10+20)) bash: syntax error near unexpected token `(' ghostwu@dev:~/linux/shell/how_to_use_var$ a=$((10+20)) ghostwu@dev:~/linux/shell/how_to_use_var$ echo $a 30
3,四则运算
ghostwu@dev:~/linux/shell/how_to_use_var$ bash calc.sh 10 2 a+b=12 a-b=12 a*b=12 a/b=12 a**b=100 a%b=0 ghostwu@dev:~/linux/shell/how_to_use_var$ cat calc.sh #!/bin/bash a=$1 b=$2 echo "a+b=$(($a+$b))" echo "a-b=$(($a+$b))" echo "a*b=$(($a+$b))" echo "a/b=$(($a+$b))" echo "a**b=$(($a**$b))" echo "a%b=$(($a%$b))"
4,let用于整数运算,类似(())
ghostwu@dev:~/linux/shell/how_to_use_var$ i=10 ghostwu@dev:~/linux/shell/how_to_use_var$ let i=i+10 ghostwu@dev:~/linux/shell/how_to_use_var$ echo $i 20
不使用let,是不会计算变量的值
ghostwu@dev:~/linux/shell/how_to_use_var$ i=2 ghostwu@dev:~/linux/shell/how_to_use_var$ i=i+8 ghostwu@dev:~/linux/shell/how_to_use_var$ echo $i i+8
5,bash内置命令read,通过参数-p 提示信息,读入变量的值
ghostwu@dev:~/linux/std_err_out$ read -p "pls input 2 number:" a b pls input 2 number:10 20 ghostwu@dev:~/linux/std_err_out$ echo $a $b 10 20
10,test -f 判断普通文件是否存在
ghostwu@dev:~/linux/std_err_out$ ls ghostwu.txt output_error.txt std_out1.txt std_out.txt ghostwu@dev:~/linux/std_err_out$ test -f ghostwu.txt && echo 1 || echo 0 1 ghostwu@dev:~/linux/std_err_out$ test -f ghostwu2.txt && echo 1 || echo 0 0
test -z 测试字符串长度是否为0
ghostwu@dev:~/linux/std_err_out$ test -z "hello" && echo 1 || echo 0 0 ghostwu@dev:~/linux/std_err_out$ test -z "" && echo 1 || echo 0 1
中括号[]与test一样.
ghostwu@dev:~/linux/std_err_out$ [ -f ghostwu.txt ] && echo 1 || echo 0 1 ghostwu@dev:~/linux/std_err_out$ [ -f ghostwu2.txt ] && echo 1 || echo 0 0
11,判断一个变量值或者字符串是否为整数?
利用expr做计算时变量或者字符串必须是整数的规则,把一个变量或字符串和一个已知的整数(非0)相加,看命令返回的值是否为0。如果为0,就认为做加法的变量或字符串为整数,否则不是整数。
ghostwu@dev:~/linux/shell/flow_control$ i=10 ghostwu@dev:~/linux/shell/flow_control$ expr $i + 1 >/dev/null ghostwu@dev:~/linux/shell/flow_control$ echo $? 0 ghostwu@dev:~/linux/shell/flow_control$ i='a' ghostwu@dev:~/linux/shell/flow_control$ expr $i + 1 >/dev/null 2>&1 ghostwu@dev:~/linux/shell/flow_control$ echo $? 2