Centos system special symbol description

1. System special symbols

• Commonly used symbols

  • 1) $ Retrieve variable information and distinguish user types
  • 2) # Indicates that the configuration file is annotated, and that the user information is a super administrator user
  • 3)! But realize the effect of negation and coercion
  • 4) | Realize the pipeline function
  • 5) = Assignment symbol, assign the following value to the previous variable dusays=com

• Quotation Mark Series

  • 1) "" is basically similar to the single quote function, but it can parse some special symbols $ 命令==$ (command)
  • 2) The content written in '' is what content is output, what you see is what you get
  • 3) `` ($()) Pass the execution result of the command inside the quotation mark to the command outside the quotation mark for use
  • 4) No quotation marks can be directly wildcarded

• Path symbol

  • 1). Hidden files
  • 2) .. switch to the superior list
  • 3) ~ Switch to home directory
  • 4)-Switch to the last cd path

• Logical symbol

  • 1) $$ When a command is executed successfully, execute the following command COMMAND1 && COMMAND2
  • 2) || The execution of the previous command failed, and the following command COMMAND1 || COMMAND2 is being executed
  • 3); Commands are executed in sequence, the success of the previous execution has no effect on the latter COMMAND1; COMMAND2

• Directional symbol

  • 1) 1> Standard output redirection symbol
  • 2) 1>> Add redirection symbol to standard output
  • 3) 2> Error output redirection symbol
  • 4) 2>> Add redirection symbol to error output
  • 5) <standard input redirection symbol
  • 6) << Standard input with redirection symbol added
  • 7) &> Standard output error output redirection symbol

• The system's unification symbol (wildcard is the main user to find the file, according to the file name)

  • 1) * Match all characters, the number is not limited, for example: rm -f FILE*
  • 2) {} Set a range interval Example: touch FILE{1…10}

• Regular symbols of the system (the main user searches for the contents of the file)

  • Basic regularity

    • 1) ^ Filter out information starting with what
    • 2) Filter out the information ending with $
    • 3) ^$ Take out the blank line information in the file
    • 4). Means to match any one and only one character
    • 5) * matches 0 or more consecutive occurrences of a character before the asterisk rm -f FILE
    • 6) .* matches all information
    • 7) \ No meaning becomes meaningful and meaningful becomes meaningless, advanced symbols-become ordinary
    • 8) [] matches every character in the brackets, and the matching relationship is an or relationship
      • grep "oldb[oe]y" test.txt to find out the two words oldboy and oldbey in the file
      • grep "[0-9a-zA-Z]" test.tx to find the alphanumeric information in the file
    • 9) [^] Exclude the matched character information in the brackets
      • grep "[^0-9a-zA-Z]" test.txt will exclude all letters and numbers, leaving only symbolic information
    • 10) ^{1 Use the} matching characters in the brackets as the word at the beginning of a line

  • Extended regular

    • 1) + matches one or more consecutive occurrences of the character before the plus sign
    • 2) | or relation symbol
      • egrep "oldboy|oldbey" test.txt matches oldboy or oldbey
    • 3) () Collect multiple character information into a whole
    • 4) {} Specify how many consecutive matches of the first character of the expansion sign
      • {n,m} n means the least number of consecutive matches and m means the most consecutive matches\
      • {n} n means only match n times in a row
      • {n,} n means match at least n times in a row, at most there is no limit
      • {,m} m means at most n consecutive matches, at least 0 times
      • 5)? Means to match the character before the question mark 0 times or 1 time rm -f FILE?

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