Parentheses and instances in a shell script

1, single parenthesis and double parenthesis

The double parenthesis command allows advanced mathematical expressions to be put into a comparison. The format is as follows:

((expression))

In addition to the standard mathematical operators used by the test command (If-then []), double parentheses support the following common operators:

val++/val--post-increase/post-reduction

++val//--val first increase/decrease first

！ Logical negation

-Position Negation

XX Power operation

<< Left displacement

>> Right Displacement

& Bit Boolean and

| Bit Boolean or

&& Logic and

|| Logical OR

#!/bin/bashcount=0for line1 in $ (cat "$WORK _dir/rpm_list_file") do       cat $line 1        count=$ (($count + 1))       echo "# # # $count # # "       if [$count-ge 6];then           echo" Test finished! "           Break       Fidone

2, the square brackets and the two-sided brackets

[[Expression]]

Expression in both brackets is compared using the standard string used in the test command, but also provides a "pattern match (matching)"

if [[] $USER = = c*]];  then    Echo " Hello, $USER " Else    Echo " Sorry. I don ' t know you! " fi

Parentheses and instances in a shell script