Shell programming (II)- Branching structure

This is the 7th day of my participation in Gwen Challenge

Branching structure

The test command

The test command is used to judge conditions

No standard output is produced and the result must be judged by its return value
Integers, strings, and files can be judged
Expression TRUE returns 0 (TRUE)
FALSE 1 (FALSE)

test expression
# or
[ expression ]  There must be a space between # [] and expression
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There must be a space between [] and expression

1. The integer

The operator	The semantic
-lt	Less than (<)
-le	Less than or equal to (<=)
-gt	Greater than (>)
-ge	Greater than or equal to (>=)
-eq	Equals (== =)
-ne	Does not equal (! =)

demo

[root@localhost sh]# test 3 -lt 4
[root@localhost sh]# echo $? # return the result of the last judgment
0
[root@localhost sh]# [ 3 -gt 4 ]
[root@localhost sh]# echo $?
1
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2. The string

The operator	The semantic
=	Is equal to the
! =	Is not equal to

[root@localhost sh]# a=ff
[root@localhost sh]# c=ff
[root@localhost sh]# test $a=$c
[root@localhost sh]# echo $?
0
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3. The file

The operator	The semantic
-f	True if the file exists and is a regular file (not a directory or device)
-s	The file exists and the number of bytes is greater than 0
-r	The file exists and is readable as true
-w	The file exists and is writable to true
-x	The file exists and is executable true
-d	The directory exists and is true

4. Other

The operator	The semantic
-a	Logic and
-o	Logic or
!	Logic is not
\( \)	The grouping parentheses must use \(and) and must have Spaces between them and the expression

If statement

If/THEN /fi are independent statements. If they are placed on a line, separate them with a semicolon (;)

if [ expression ];then
	Todo
elif [ expression ];then
    Todo
else
	Todo
fi  # end
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Single branch

#! /bin/sh
read filename  # input file name
if [ -d $filename ];then
        echo "is dir"
fi
exit 0
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Double branch

#! /bin/sh
read filename
if [ -d $filename ]
then echo $filename "is a directory"
else
        echo $filename "is't a DIRECTOR"
fi
exit 0
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Many branches

#! /bin/sh
read filename  Enter a file name
if [ -d $filename ];then  Check whether the directory exists
        ls $filename   Output directory
elif [ -x $filename ]  Determine whether an executable file is available
then 	echo "is"
else
        echo "not is"
fi
exit 0
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Enter a directory and output the files in that directory

The select statement

The SELECT statement can make a list of options similar to a directory, interactively selecting the data from the list and passing it into the select body for use

Select the variableinThe list ofdo... [break]
done
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The select statement is essentially a loop statement. Without the break keyword, the program cannot exit the SELECT structure
List elements are separated by Spaces

The instance

Choose the course you want to study

#! /bin/sh
echo "what do you want tu study?"
select sub in "Java" "C++" "WEB"   # List elements separated by Spaces
do
  echo "you have selected $sub" # use the sub
  break # Out of the loop
done
exit 0
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If you do not break, the loop will not break

A case statement

The case statement matches the contents of a variable with multiple options. If the match is successful, the statement corresponding to that condition is executed.

casevariableinOption 1)... ;;'option 2')... ;;"Option 3")... ;; ... *)...esac  # end
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The matching conditions of the case statement can be multiple, multiple conditions of each match is connected with “|”