Fundamentals of shell programming (Part 7: branch statement -if)

Preface

In the execution stream , Execute different statements according to conditions , Branch statement , stay shell Programming , Mainly if、case in Use , Let's study together

if sentence

1、 Need semicolons to separate

if true;then
    echo hello world
fi

because true The exit status code of the command is 0, So included in then And fi Statements between will execute

2、 No semicolons are required to separate

if true
then
    echo hello world
fi

because true And then Not in the same line , therefore ; It can be omitted , Which of the two formats is used , It's the same with all of them

if-else sentence

1、 Need semicolons to separate

if true; then
    echo hello world
else
    echo what
fi

because true The exit status code of is always 0, therefore else And fi Statements Between cannot be executed

2、 No semicolons are required to separate

if true
then
    echo hello world
else
    echo what
fi

because then And true Not in the same line , No need to use ; separate

if-elif sentence

1、 Need semicolons to separate

if false; then
    echo what
elif true; then
    echo hello world
fi

because false The exit status code of the command is fixed as 1, therefore if The code in the statement will not execute , Turn to judgment elif Exit status code in

2、 No semicolons are required to separate

if false
then
    echo what
elif true
then
    echo hello world
fi

because false And then、true And then Are not on the same line , So you can omit ; separate

if-elif-else sentence

1、 The same line should be separated by semicolons

if true; then
    echo hello world
elif false; then
    echo what
else
    echo haha
fi

because true The exit status code of is 0, therefore if The statements in execute ,elif、else The statements in will not have a chance to be OK

2、 No semicolons are required to separate

if true
then
    echo hello world
elif false
    echo what
else
    echo haha
fi

The characteristics of conditional statements

if true; then
    echo hello world
elif false; then
    echo what
else
    echo haha
fi

Once a qualified statement is executed , Other statements will not execute

if Rule of judgement

if true; then
    echo hello world
fi

Be careful ：

1、if Statement only determines the exit status code , There is no... In other languages boolean value ,true It's also an order , because true The exit status code of is always 0, So the condition is always true ！

2、 Any order , As long as there is an exit status code, it can be placed in if perhaps elif As a condition , However, a common condition command is test

Common conditional commands test

1、 Judging the equality of strings

if test "a" = "a"; then
    echo hello world
fi

because a And a equal , therefore test The exit status code of the command is 0, here then And fi Statements between will be executed

2、 Judge that the file exists

if test -f "hello.txt"; then
    echo hello world
fi

-f And "hello.text" Are passed to test Arguments to the command , Therefore, the principle of word separation should be strictly implemented , Use white space characters to separate

3、test There are many other ways to judge commands , Do you think test tedious ？？？ So the author provides test Shorthand for commands

test Shorthand for commands []

1、 Judging the equality of strings

if [ "a" = "a" ];then
    echo hello world
fi

because [] by test Shorthand for commands , therefore [] The space between characters cannot be omitted , Represents separation test Commands and parameters

2、 Judge whether the file exists

if [ -f "hello.txt" ]; then
    echo hello world
fi

3、 More conditional judgments （ String judgment ）

• [ string ]： Judge string Not empty
• [ -n string ]： Judge string Length greater than zero
• [ -z string ]： Judge string The length is zero
• [ string1 = string2 ]： Judge string1 and string2 Are they the same?
• [ string1 == string2 ] ： And [ string1 = string2 ] equally
• [ string1 != string2 ]： Judge string1 And string2 Is it different
• [ string1 '>' string2 ]： In dictionary order string1 Whether in string2 after
• [ string1 '<' string2 ]： In dictionary order string1 Arrange in string2 Before

When the condition is true ,test The exit status code of the command is 0

4、 More conditional judgments （ File to judge ）

• [ -a file ]：file There is
• [ -d file ]：file Exists and is a directory
• [ -e file ]： If file Exists and is an executable
• [ -f file ]： If file It exists and is a normal file

………… There are still a lot of it

5、 More conditional judgments （ Integer judgment ）

• [ integer1 -eq integer2 ]：integer1 be equal to integer2
• [ integer1 -ne integer2 ]：integer1 It's not equal to integer2
• [ integer1 -le integer2 ]：teger1 Less than or equal to integer2
• [ integer1 -lt integer2 ]：integer1 Less than integer2
• [ integer1 -ge integer2 ]：integer1 Greater than or equal to integer2
• [ integer1 -gt integer2 ]：integer1 Greater than integer2

When the judgment condition is true ,test The command will return the exit status code 0 

test Another shorthand for commands [[]]

1、 Judging the equality of strings

if [[ "a" = "a" ]]; then
    echo hello world
fi

2、 Judge whether the file exists

if [[ -f "hello.txt" ]]; then
    echo hello world
fi

 3、 And [] The difference between , regular expression

[[ string1 =~ regex ]]

regex A regular expression ,=~ It's a regular comparison operator

if Write in a single line

if true; then echo hello world;fi

Use ; Just separate

Alternative usage ： Write multiple commands

if false; true; then
    echo hello world
fi

false Command and true The orders are written in if Behind , here if Only the exit status code of the last command will be judged

Conditions and &&

if [ a = a ] && [ b = b ]; then
    echo hello world
fi

Judge multiple commands , When the exit status code of all commands is 0 when , Will execute then And fi The statement in

Condition or ||

if [ a = b ] || [ c = c ] ;then
    echo hello world
fi

Judge multiple commands , When the exit status code of any command is 0 when , Will execute then And fi The sentence between

Skillfully use conditions to execute statements

$ command1 && command2
 Execute first command1, Only command1 Successful implementation （ Exit status code is 0） after , To perform command2


$ command1 || command2
 Execute first command1, Only command1 Execution failure （ Exit status code is not 0） after , To perform command2

summary

1、shell Programming if The statement does not check boolean value , It only judges the exit status code

2、 As long as there is a command with exit status code , Include built-in commands 、 Executable program 、 function , Can be used as if Later conditions

3、shell The reason why programming is difficult to learn is ： Too much code ……