Linux Shell study note 9

12.3 option analysis in shell script

The first method is to use the case statement for manual processing.

Method 2: Use the getopts command to analyze the options

Syntax format:

Getopts option-string variable

Option-string is a string that contains all single-character options. These characters are analyzed by getopts.

These options should be assigned a variable. The variable is the variable name assigned to the variable.

 

Getopts analyzes the options provided by the command line. The analysis process is as follows:

1) The getopts option checks all command line parameters and finds the characters starting.

2) When a parameter starting with "-" is found, the character following "-" is compared with the character given in Option-string.

 

Relatively.

3) if a match is found, the specified variable is set to an option; otherwise, variable is set "? "Character.

4) Repeat 1-3 until the search option is traversed.

5) After the analysis, getopts returns a non-zero value and releases it.

Another feature of getopts is that it can specify options that require additional parameters. Add ":" After the option in Option-string ":"

Character to achieve this function. In this case, after the option is analyzed, the additional parameter is set to the value of the variable optarg.

 

Use getopts

While F: O: V option:

Do

Case "$ option" in

F) F = $ option ;;

O) O = $ option ;;

V) V = $ option ;;

*) Echo "$ usage ";

Exit 1

;;

Esac

Done

 

 

12th hours input/output

When a command generates a write-to-terminal output, the program prints the output to the standard output (stdout ).

Error messages are not written to stdout. They are written to a special type output, which is called stderr ),

It is reserved for error information.

 

13.1.1 output to the terminal

1) echo

2) printf

 

13.1.2 output redirection

In shell scripts, commands are often captured and stored in files. When the output is stored in a file, you can easily

 

Edit and modify.

Redirection operator ">", "<"

 

">" Symbol redirection overwrites the data in the original file. You must pay attention to it when using it.

If you need to add data on the original basis, you can use ">" to append the data to the end of the file.

 

 

Redirection output to files and screens

In some cases, you must redirect the output of scripts to files and terminals at the same time. You can use the tee command.

Example: $ date | tee Now

Output the current date on the terminal and write the data to the file nowzhong.

 

13.2 Input

1) redirect the input to a file to read the input from the file.

2) read the input from the user.

3) redirects the output of one command to the input of another command.

 

13.2.1 input redirection

Command <File

 

13.2.2 read user input

One of the common tasks of shell scripts is to prompt the user to input and then read the user's response.

Format:

Read name

It reads a whole row of user input until the user enters the Enter key, which makes the row the value of the variable specified by name.

For example:

YN = Yes

Printf "do you want to play a gane [$ YN]? "

Read YN

Case $ YN in

[YY]) echo yes ;;

*) Echo "Maybe later .";;

Esac

 

13.2.3 MPs queue

In UNIX, most of the commands used to process files can read input from stdin, which allows users to use one program to filter input from another program.

 

Output.

You can use pipelines to redirect the output of one command to the input of another command. Several commands can be connected together with pipelines.

Example: $ who | grep sudo

Use the output of the WHO command as the input of the next command, and then filter the output to the terminal.

 

13.3 file descriptor

Standard input (stdin) 0

Standard output (stdout) 1

Standard Error (stderr) 2

 

Command 1> file1 2> file2

Stdout of the given command is redirected to file1, and stderr (error message) is redirected to file2

 

For file in $ files

Do

Ln-S $ file./docs>/tmp/ln. log 2>/dev/null

Here, the stdout of LN is added to the file/tmp/ln. log, and stdout is redirected to the/dev/null file.

 

Note: File/dev/null is a reshuffle file that can be used in all UNIX systems to delete the output.