Chapter 5 Study Notes of Linux Command Line and shell script Programming

Chapter 2: using structured commands

Use the if-then statement

if commandthen    commandsfi

If the exit status code of the command after if is 0, execute then.

Another Form

if command; then    commandsfi

 

If-then-else statement

if commandthen    commandselse    commandsfi

If the exit status code of the command after if is 0, execute then. Otherwise, run Else.

 

Nested if (else if)

if command1then    commandselif command2then    commandsfi

 

Test command

The test command can determine three types of conditions.

1. Numerical Comparison

2. String comparison

3. file comparison

Format:

TestCondition

In the if statement, square brackets [] can be used.

if test conditionthen    commandsfi

Or

if [ command ]then    commandsfi

Note: a space must be added on both sides of square brackets. Otherwise, an error will be reported.

 

Numerical Comparison

Comparison	Description
N1-EQ N2	Check whether N1 is equal to N2
N1-ge N2	Check whether N1 is greater than or equal to N2
N1-GT N2	Check whether N1 is greater than N2
N1-Le N2	Check whether N1 is raining or equal to N2
N1-lt N2	Check whether N1 is less than N2
N1-ne N2	Check whether N1 is not equal to N2

It can be used on variables.

Note: floating point numbers cannot be used.

 

String comparison

Comparison	Description
Str1 = str2	Check if str1 is the same as str2
Str1! = Str2	Check if str1 is different from str2
Str1 <str2	Check if str1 is smaller than str2
Str1> str2	Check if str1 is larger than str2
-N str1	Check if the str1 length is not 0
-Z str1	Check whether the str1 length is 0

 

Note the following for sequence comparison:

1. Escape if the value is greater than or less than the symbol, otherwise it will be recognized as a redirection symbol.

2. The test command is sorted in ASCII order, which is different from the sort command.

 

 String Length

If the variable is a null string or is not initialized, the variable length is 0.

Empty strings or uninitialized variables may have disastrous effects on the program. before using them, you 'd better use-N and-Z in the test command to confirm

 

File comparison

Comparison	Description
-D File	Check whether the file exists and is a directory
-E File	Check whether the file exists
-F File	Check whether the file exists and is a file
-R File	Check whether the file exists and is readable
-S	Check whether the file exists.
-W	Check whether the file exists and can be written
-X	Check whether the file exists and can be executed
-O	Check whether the file exists and belongs to the current user
-G	Check whether the file exists and the default group is the same as the current user.
File1-nt file2	Check if file1 is newer than file2
File1-ot file2	Check if file1 is older than file2

Take the check directory as an example:

if [ -d $HOME ]then    cd $HOMEelse    echo "$HOME is not a directory"fi

 

Composite condition test

[Condition1] | [condition2]

[Condition1] & [condition2]

 

Advanced features of if-then

1. parentheses used for mathematical expressions

2. brackets used for advanced string processing

Parentheses allow advanced mathematical expressions to be placed in comparison

Format:

(Expression ))

Command symbols

Symbol	Description
Val ++	Post-Increment
Val --	Subtraction
++ Val	Add first
-- Val	Minus
!	Logic inversion
~	Bitwise Inversion
**	Power Operation
<	Left Shift
>	Right Shift
&	Boolean and
|	Boolean OR
&&	Logic and
|	Logic or

 

Brackets

[[Expression]

Provides the pattern matching feature, where you can use regular expressions

if [ $USER == r* ]

 

Case command

Command Format:

case variable inpattern1 | pattern2) commands1;;pattern3) commands2;;*) default commands;;esac

In Bash, you can check strings in case.

From Java to bash, the syntax is still very large, but the idea will not change.

The following is a simple example:

case $test inpork | beef | lamb )    echo "$test is meat!"    echo "I like it!";;potato | tomato | eggplant ）    echo "$test is a vegetable"    echo "who likes it?";;*)    echo "what's this?";;esac

Before Java 7, this write is not allowed. The switch case does not support the string type.

 

 

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