【tips】linux中单双引号，单双括号与反引号的区别

# 双引号不输出带特殊字符的字符，单引号输出全部
echo "my $like is apple"
my  is apple

echo 'my $like is apple' 
my $like is apple

# 不带引号会认为空格后的字符是shell命令
[service@NDL-BASS-YS-VM-129-152 wy]$ str=my $like is apple
-bash: is: command not found
[service@NDL-BASS-YS-VM-129-152 wy]$ str='my $like is apple'
[service@NDL-BASS-YS-VM-129-152 wy]$ echo $str
my $like is apple

# 反引号有 命令替换的作用
[service@NDL-BASS-YS-VM-129-152 wy]$ echo today is date
today is date
[service@NDL-BASS-YS-VM-129-152 wy]$ echo today is `date`
today is Wed Nov 29 16:12:44 CST 2017

# a=$(date)    【等同于】    a=`date`
echo $(date)
Wed Nov 29 16:14:39 CST 2017

echo `date`
Wed Nov 29 16:14:45 CST 2017

# 实例（4个结果一样）
#!/bin/bash
echo '当前时间-系统变量:' `date +'%Y%m%d %H:%M:%S' -d @$(date +%s)`
echo '当前时间-系统变量:' $(date +'%Y%m%d %H:%M:%S' -d @`date +%s`) 

btime=`date +%s`
echo '当前时间-自定义变量:' `date +'%Y%m%d %H:%M:%S' -d @$btime`
echo '当前时间-自定义变量:' $(date +'%Y%m%d %H:%M:%S' -d @$btime)

# a=$((1+2))    【等同于】    a=`expr 1 + 2`
echo $((1+2))
3
echo `expr 1 + 2`
3