Day09_shell Basic grammar and Process Control

I. Shell's basic syntax-variables

1. Naming rules
   1. Start with a letter
      
   2. Connect words with an underscore or an underscore
      
   3. The same type is distinguished by numbers
      
   4. For file best plus extension
      
2. System variables
   • Set and env differences
     
   • Set: Show All variables
     
   • ENV: System variable is environment variable
     
3. Assigning values to variables
   • unset VARNAME: Deleting variables
     
4. Common system Variables
   • PATH
     
   • Pwd
     
   • Lang
     
   • HOME
     
   • Histsize: Number of history log lines
     
   • PS1: Storing user information
     
   • Ifs
     
   • Export: Defining global variables
     
5. Define the bounds of a variable name
   [email protected] ~]# rest_mem=20
   [email protected] ~]# echo ${rest_mem}%
   20%
   
6. Data type

Two. Shell's basic syntax-operator

2. Arithmetic operators
   • +
     
   • -
     
   • *
     
   • /
     
   • %
     
4. Relational operations
   • <
     
   • >
     
   • <=
     
   • >=
     
   • ==
     
   • !=
     
   • &&
     
   • ||
     • Above with (())
       
   • Test command related, [] can achieve the same effect
     
6. Assignment operators
   • =
     
   • +=
     
   • *=
     
   • /=
     
   • %=
     
   • [email protected] ~]# ((x%=3))
     [email protected] ~]# echo $x
     
8. all calculators in the shell
   • $[]
     
   • (())
     
   • $ (())
     
   • expr
     
   • floating-point arithmetic b C Software
     • yum install bc-y
       
     • echo ' SCALE=2;1/3 ' |bc-l (reserved two bit, 1 divided by 3 operation)
       
10. Test action
    1. When the command executes, it returns to a system variable of $?
       A value of 0 indicates that the command executed successfully or failed.
       
    2. Testing command test [] [[]] (())
       1. Test file status
          • -D Directory
            
          • -E
            
          • -H Link File
            
          • -S file length > 0, non-empty
            
          • -F Normal File
            
          • -W Writable
            
          • -R Readable
            
          • -X Executable
            
          • -L Symbolic connection
            
          • -u file has suid bit settings
            
          • -B Block File
            
       2. String test
          • = Two strings equal
            
          • ! = two strings are not equal
            
          • -Z Empty string
            
          • -N Non-empty string
            
       3. Test values
          • -eq equals
            
          • -ne Not equal to
            
          • -GT Greater than
            
          • -lt less than
            
          • -ge greater than or equal to
            
          • -le less than or equal to
            
       4. Expand Test Symbols

Three. Process Control

1. The IF statement for Process Control

   #!/bin/bashvar='/etc/init.d'#var='/dev/sda'if-d $var ]    then        "$var is directory"elif-b $var ]    then        "$var is block"elif-f $var ]    then        "$var is regular file"else        'unknow'fi

• Passing parameters to a script

  #test. Shecho $0echo $1echo $2echo $3Echo ${ One}echo'$$'$ $echo'$*'$*Echo' [email protected] '[Email Protected]echo'$#'$#Echo'$?'$?'''test: Python test.sh 1 2 3 4 5 6 7 8 9Output Result:./test.sh2 One$$ 14312$* 1 2 3 4 5 6 7 8 9[email protected] 1 2 3 4 5 6 7 8 9$#$? 0* $ A file name to run* $$ for process PID* $* for all parameters of the input* [email protected] with $** $# number of input parameters* $? Whether the previous command was executed successfully'''

• Modify the script so that it can receive the arguments from the caller

  [[Email protected]-R3-Srv~]# cat Test_file.sh#!/bin/bashif[-D $1] then Echo"is directory"elif[-B $1] then Echo"is block"elif[-F $1] then Echo"is regular file"ElseEcho' Unknown 'Fi[[email protected]-R3-Srv~]#./TEST_FILE.SH/ETC/PASSWD/etc/passwd isRegularfile

• Detects if Nginx is started, and if not, starts it

  #! /bin/bashps aux|grep nginx|-v grepif-1 ]    then        systemctl start nginxfi

Day09_shell Basic grammar and Process Control