shell variables {} ()

The prototype 1.Shell variable: $ {var}

Prototype variables: $ {var}, that is, add a brace to define the scope of the variable name

 

[root@bogon sh]# aa='ajax'

[root@bogon sh]# echo $aa

ajax

[root@bogon sh]# echo $aa_AA

 

[root@bogon sh]# echo ${aa}_AA

ajax_AA

 

2.$(cmd)

[root@bogon t]# ls 1.txt 2.txt

[root@bogon t]# echo $(ls)

1.txt 2.txt

echo $ (ls) execution

shell command line scan again found $ (cmd) structure, put $ cmd execution (cmd) in the first, to obtain its standard output,

This command is then output into the original echo $ (LS) in $ (LS) position, i.e., replacing the $ (LS), and then execute the echo command

as follows:

echo $ (ls) is replaced by echo 1.txt 2.txt

Here we must note that the error output $ (cmd) in the command will not be replaced, but the replacement of the standard output

3, a series of command execution ()} and {

 

(), And {} is a string of commands for execution, but differ:

Same point:

(), And {} is a string of commands in brackets, and with between the command; spaced No.

 

difference:

() Is a string of commands to re-open a sub-shell execution, {} in the current chain of commands performed on the shell

 

() Can not last command semicolon, the last command} {semicolons

[Root @ dev opt] # var = test

[Root @ dev opt] # (var = 'notest'; echo $ var;)

notest

[Root @ dev opt] # (var = 'notest'; echo $ var)

notest

[Root @ dev opt] # {var = notest; echo $ var;}

notest

 

There must be a space between the first command and left parenthesis () in the first command and left brackets do not have the space, {the}

[Root @ dev opt] # {var = notest; echo $ var;}

notest

 

()和{}中括号里面的某个命令的重定向只影响该命令,但括号外的重定向则影响到括号里的所有命令

 

从1加到100的和：

[root@dev opt]# cat for.sh

#!/bin/bash

sum=0

for  ((i=1; i<=100; i++))

do

  sum=`expr $sum + $i`

done

echo $sum