Some details about Unix shell programming

• The test statement must have spaces before and after the equal sign, and there can be more than one space. To promote it, there must be no spaces before and after the equal sign of the value assignment statement, non-assignment statements require spaces (I dare not say so)
• Double quotation marks are equivalent to double quotation marks under printf. single quotation marks are a string bracket, which contains the string content.
• True. For example, if a successful match in test is returned, 0 is returned, and if is the valid true value.
• Declare a numeric parameter: declare-I X. FOR MORE DECLARE usage, click here.
• A script running in shell is a sub-terminal running on the login terminal. The sub-terminal has its own environment, and the login terminal shares environment variables with the sub-terminal. the export data in the sub-terminal does not change in the parent terminal, but in turn is valid.
• Shell first processes pipe and redirection, and then performs variable substitution. Sometimes the command must be replaced first (especially for processing special symbols), and the eval command
• Jing @ Jing-LAPTOP :~ $ E =; echo $ E; echo $ {e:-/bin}; echo $ E

  /Bin

  Jing @ Jing-LAPTOP :~ $ E =; echo $ E; echo $ {e: =/bin}; echo $ E

  /Bin
  /Bin
  Note:-And: = both set a default value. When the variable is empty, the default value is substituted. The former is only a temporary replacement, and the latter is a value assignment to the variable.

• Shell Command Prompt function: press TAB twice, you can only press tab, then you can output all commands, or enter some prompt words, and then press tab, such as O + TAB, list all commands starting with "O"