Linux Basic Series-day9

Arithmetic operators

Set Variable a=10,b=4

+ Addition

[[Email protected] ~] # echo $[$a + $b]14

-Subtraction

[[Email protected] ~] # echo $[$a-$b]6

* Multiplication

[[Email protected] ~] # echo $[$a * $b]40

/division, because is divisible, so only equals 2

[[Email protected] ~] # echo $[$a/$b]2

% take-up

[[Email protected] ~] # echo $[$a% $b]2

In scripts, you can use expr to call

[[Email protected] ~] # echo ' expr $a + $b '~]#  echo ' expr $a-$b '6~]#  echo ' expr $a \* $b '+~]#  echo ' expr $a/$b '2~]#  echo ' expr $a% $b ' /c15>2

Assignment operators

= Assignment, used to assign values to variables

[[Email protected] ~] # a=5 [[Email protected] ~] # b= $a [[Email protected] ~] # Echo $b5

+ = variable is assigned after addition operation

[[Email protected] ~] # x=0 [[Email protected] ~] # ((x+=5)) [[Email protected] ~] # Echo $x5

*= variable is assigned after multiplication method

[[Email protected] ~] # x=5 [[Email protected] ~] # ((x*=5)) [[Email protected] ~] # Echo $x25

/= variables are assigned after the division method is evaluated

[[Email protected] ~] # x=5 [[Email protected] ~] # ((x/=2)) [[Email protected] ~] # Echo $x2

%= variables are assigned after the remainder operation

[[Email protected] ~] # x=5 [[Email protected] ~] # ((x%=2)) [[Email protected] ~] # echo $x1

String relational operators

= = Compares two strings for equality, equals true

[[Email protected] ~] # A=ABC [[Email protected] ~] # B=ABC [[Email protected] ~] # [[$a = = $b]] [[Email protected] ~] #　　　　　　echo $? #  $? Returns the execution status of the previous command, 0 is true  0

! = Compare two strings do not want to wait, do not want to wait for the true

[[Email protected] ~] # A=ABC [[Email protected] ~] # B=abb [[Email protected] ~] # [[$a = = $b]] [[Email protected] ~] # echo $? 1~]#  [[$a! = $b]][[email protected] ~]#  echo $? 0

Integer relational operators

Format: [integer operator integer]

-eq equals

[[Email protected] ~] # [5-eq 5] [[Email protected] ~] # echo $? 0

-GT Greater than

[[Email protected] ~] # [8-GT 5] [[Email protected] ~] # echo $? 0

-lt less than

[[Email protected] ~] # [3-lt 5] [[Email protected] ~] # echo $? 0

-ge greater than or equal to

[[Email protected] ~] # [5-ge 5] [[Email protected] ~] # echo $?  ~]#  [5-ge 3][[email protected] ~]#  echo $? 0

-le less than or equal to

[[Email protected] ~] # [3-le 3] [[Email protected] ~] # echo $?  ~]#  [3-le 5][[email protected] ~]#  echo $? 0

-ne Not equal to

[[Email protected] ~] # [3-ne 5] [[Email protected] ~] # echo $? 0

Comparison operations for files

-nt (newer than) to determine whether File1 is newer than file2

[Email protected] ~]# [abc.txt-nt aaa.txt][[email protected] ~]# echo $?1

-ot (older than) determine if file is older than file2

[Email protected] ~]# [Abc.txt-ot aaa.txt][[email protected] ~]# echo $?0

-ef judge whether File1 and File2 are the same file, can be used in judging hard link

[Email protected] ~]# [Abc.txt-ef aaa.txt][[email protected] ~]# echo $?1

logical operators

-A or &&: logical AND, "and" means, before and after two expressions are all established, the entire test result is true, otherwise false

[[Email protected] ~]#a=3[[Email protected] ~]#b=6[[Email protected] ~]#[$a-ne 6] && [$b-gt 9][[Email protected] ~]#echo $? #因为a不等于6, B is not greater than 9, so the left is set up, the right is not set up, so the overall is not established 1[[Email protected]~]#[$a-ne 6] && [$b-gt 4][[Email protected] ~]#echo $? #因为a不等于6, B is more than 4, around are set up, so the overall establishment 0
[[Email protected] ~]# [$a-ne 6-a $b-gt 4]
[[Email protected] ~]# echo $?
0

-O or | | : Logical OR, "or" means that at least one of the two sides of the operator is true, the result is true, and no is false

[[Email protected] ~]#a=3[[Email protected] ~]#b=6[[Email protected] ~]#[$a-ne 6] | | [$b-GT 9][[Email protected] ~]#echo $? #因为左边a不等于6成立, so the whole establishment 0[[email protected]~]#[$a-ne 3] | | [$b-GT 4][[Email protected] ~]#echo $? #因为右边b大于4成立, so the whole establishment 0[[email protected]~]#[$a-ne 3] | | [$b-GT 9][[Email protected] ~]#echo $? #因为左右两边都不成立, so the whole is not established 1

! : Logical No, when the conditions are not set, the return result is true

[[Email protected] ~] # [! $a-ne 3] | | [$b-GT 9] [[Email protected] ~] #　　　　echo $? #因为左边a等于3, after-ne judgment for false, and after! The negation of the false, so for the true establishment  0

Test action

Test can be replaced with [], such as Test-e Abc.txt can be written as [-e Abc.txt]

File type judgment

-e Whether the file name exists

-f The filename exists and is file

-D whether the file name exists and is a directory

-B Whether the file name exists and is a block

-C whether the file name exists and is a character device

-S The file name exists and is a socket file

-P The file name exists and is a FIFO (pipe) file

-l The file name exists and is a connection file

Example:

[[Email protected] ~] # Touch Abc.txt [[Email protected] ~] #  [[email protected] ~]#  echo $?  ~]#[[email protected] ~]#  echo $? 1~]#[[email protected] ~]#  echo $? 0

File Permission Detection

-R detects if the file name exists and has "readable" permissions

-W detects if the file name exists and has "writable" permissions

-X detects if the file name exists and has "executable" permission

-U detects if the file name exists and has "SUID" permission

-G detects if the file name exists and has "SGID" permission

-K detects if the file name exists and has the "Sticky bit" permission

-S detects if the file name exists and is "not a blank file"

Example:

[[Email protected] ~] #  [[email protected] ~]#  echo $?  ~]#[[email protected] ~]#  echo $? 1

String Data judgment

-Z to determine if the string is 0, or null to return True (0)

[[Email protected] ~] # test-z "ABC" [[Email protected] ~] # echo $? 1

-N to determine if the string is 0, or null to return False (0)

[[Email protected] ~] # test-n "ABC" [[Email protected] ~] # echo $? 0

Process Control

If single branch: performs the appropriate action when the condition is set.

if condition test operation then command sequence fi

Example: Output True when 100 is greater than 80 o'clock

if [100-gt ];then     " True " fi

If double branch: performs different actions when the condition is set and the condition is not valid.

if condition Test Command then    command sequence 1else     command sequence 2fi

Example: Output true when checking to presence abc.txt, otherwise output false

# !/bin/bash if [-e abc.txt];then     " True " Else  "False" fi     

If multi-branch: Equivalent If statement nesting, performing different operations on multiple conditions.

if condition Test command 1; then    command sequence 1elif  Condition test command 2;then    command sequence 2elif  ...  Else     command sequence NFI

For loop: Performs a set of command operations repeatedly based on the different values of the scalar.

 for inch Value list do    command sequence done

Example 1: Printing numbers 1 through 5 sequentially

# !/bin/bash  for inch [1..5]do    

While condition loop: repeats the condition of the test instruction, and executes the corresponding command operation repeatedly, as long as the condition is true, until the condition is false. If used true as a loop condition can produce an infinite loop.

 while command-expression do    command list done

Example: adding 20 system accounts in bulk user name is USER1~20

# !/bin/bashi=1 while [$i-le ]do    useradd user$1    "123456 " | passwd--stdin user$i &>/dev/null    i= ' expr $i + 1' done

Linux Basic Series-day9