Shell Parenthesis Usage Introduction

• Medium brackets [

and the test command equivalent, the user Boolean judgment, such as:

1 a='abc'2if'abc'  ] 3  Then 4    " Yes " 5 Else 6    " No " 7 Fi

Equivalent to

1 if ' ABC ' 2  Then 3     " Yes " 4 Else 5     " No " 6 Fi

When using [], be aware that:

1. [] left and right brackets to leave a space, or will error

2. If the value on the left side of = = is empty, it will cause the command to go wrong, in order to prevent this, usually add an extra string to the left or right, for example:

if [${a}x = = Abcx]

Also, note the difference between the parentheses and the double brackets:

• Parentheses (

Parentheses are used in arithmetic operators, which are similar to the Let command when used for assignment operations, and are referenced here

Let a=5((a=ten)) echo $a, $b

((a= $a +7))         # Add 7 to a((a = a + 7))      # Add 7 to a.  Identical to the previous command. ((A + = 7))         # ADD 7 to a.  Identical to the previous command. ((a = random% + 1))     # Choose A random number from 1 to ten.                             # is modulus, as in C.   

Double parentheses to the left plus the $ symbol to replace the result, such as:

echo $ (a>)

A single () is used to execute the corresponding command in the new Subshell, such as:

(CD..) /.. /)

It is important to note that this command does not change the current directory after it has been executed

Resources:

Http://mywiki.wooledge.org/ArithmeticExpression

Http://stackoverflow.com/questions/2188199/how-to-use-double-or-single-bracket-parentheses-curly-braces

Shell Parenthesis Usage Introduction