Shell script
Shell script （shell script）, It's a kind of shell Scripted program .
What the industry says shell Usually it means shell script .
Shell variable
Using variables
Use a defined variable , Just add the dollar sign before the variable name
Curly braces outside variable names are optional , Add it or not , Curly braces are used to help the interpreter identify the boundaries of variables
read-only variable
use readonly Command to define a variable as a read-only variable , The value of a read-only variable cannot be changed
Delete variable
use unset Command to delete variables .
Variable type
function shell Time , There will be three variables at the same time ：
(1) local variable Local variables are defined in scripts or commands , Current only shell Valid in instance , other shell Started program cannot access local variables .
(2) environment variable All procedures , include shell Program started , Can access environment variables , Some programs need environment variables to ensure their normal operation . When necessary shell Scripts can also define environment variables .
(3) shell variable shell Variables are created by shell
Special variables for program settings .shell Some of the variables are environment variables , Some of them are local variables , These variables guarantee shell Normal operation of
Shell character string
String is shell The most commonly used and useful data types in programming （ Except numbers and strings , There's nothing else to use ）, Strings can be quoted in single quotes , You can also use double quotes , You can also use no quotes .
Shell array
bash Supports one dimensional array （ Multidimensional arrays are not supported ）, And does not limit the size of the array .
Similar to C language , The subscript of an array element is determined by 0 Start number . To get the elements in an array, use Subscripts , Subscripts can be integers or arithmetic expressions , Its value should be greater than or equal to 0.
Shell Pass parameters
We can execute Shell When scripting , Pass parameters to script , The format of getting parameters in the script is ：$n.n Represents a number ,1 Is the first parameter to execute the script ,2
Is the second parameter to execute the script , and so on ……
Shell Basic operators
Arithmetic operator
The following table lists the common arithmetic operators , Assumed variable a by 10, variable b by 20：
+
addition
`expr $a + $b` The result is 30.
-
subtraction
`expr $a - $b` The result is -10.
*
multiplication
`expr $a \* $b` The result is 200.
/
division
`expr $b / $a` The result is 2.
%
Take surplus
`expr $b % $a` The result is 0.
=
assignment
a=$b The variable will be b Value assigned to a.
==
equal . Used to compare two numbers , Return if same true.
[ $a == $b ] return false.
!=
Unequal . Used to compare two numbers , Return if different true.
[ $a != $b ] return true.
Relational operators
Relational operators only support numbers , String not supported , Unless the value of the string is a number .
The following table lists the common relational operators , Assumed variable a by 10, variable b by 20：
-eq
Check whether two numbers are equal , Equal return true.
[ $a -eq $b ] return false.
-ne
Check whether two numbers are equal , Unequal return true.
[ $a -ne $b ] return true.
-gt
Check whether the number on the left is greater than that on the right , If it is , Then return true.
[ $a -gt $b ] return false.
-lt
Check whether the number on the left is less than that on the right , If it is , Then return true.
[ $a -lt $b ] return true.
-ge
Check whether the number on the left is greater than or equal to that on the right , If it is , Then return true.
[ $a -ge $b ] return false.
-le
Check whether the number on the left is less than or equal to that on the right , If it is , Then return true.
[ $a -le $b ] return true.
Boolean operator
The following table lists common Boolean operators , Assumed variable a by 10, variable b by 20：
!
Non operation , Expression is true Then return false, Otherwise return true.
[ ! false ] return true.
-o
Or operation , There is an expression true Then return true.
[ $a -lt 20 -o $b -gt 100 ] return true.
-a
And operation , Both expressions are true To return true.
[ $a -lt 20 -a $b -gt 100 ] return false.
Logical operators
Here's how Shell Logical operators for , Assumed variable a by 10, variable b by 20:
&&
Logical AND
[[ $a -lt 100 && $b -gt 100 ]] return false
||
Logical OR
[[ $a -lt 100 || $b -gt 100 ]] return true
String operator
The following table lists common string operators , Assumed variable a by "abc", variable b by "efg"：
=
Check if two strings are equal , Equal return true.
[ $a = $b ] return false.
!=
Check if two strings are equal , Unequal return true.
[ $a != $b ] return true.
-z
Check whether the string length is 0, by 0 return true.
[ -z $a ] return false.
-n
Check whether the string length is 0, Not for 0 return true.
[ -n $a ] return true.
str
Check if the string is empty , Not null return true.
[ $a ] return true.
Shell echo command
Shell Of echo Instructions are output for Strings .
Shell printf command
printf Command imitation C Library （library） Inside printf() program .
printf from POSIX As defined in the standard , So use printf Using echo Good transplantation .
printf Parameters separated by reference text or spaces , It can be outside printf Format string used in , You can also specify the width of the string , Left right alignment, etc .
Shell test command
Shell In test Command to check if a condition holds , It can be numerical , Character and file testing .
Numerical test
-eq
Equal is true
-ne
True if not equal
-gt
Greater than is true
-ge
True if greater than or equal to
-lt
Less than is true
-le
True if less than or equal to
String test
=
Equal is true
!=
True if unequal
-z character string
True if string length is zero
-n character string
True if string length is not zero
Shell Process control
if else
if
if Statement syntax format ：
if condition then command1 command2 ... commandN fi
if else
if else Grammatical format ：
if condition then command1 command2 ... commandN else
command fi
if else-if else
if else-if else Grammatical format ：
if condition1 then command1 elif condition2 then command2 else
commandN fi
for loop
for The general format of the loop is ：
forvarin item1 item2 ... itemN do command1 command2 ...
commandN done
while sentence
while condition do command done
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