The echo command in Linux writes content to a file __linux

Source: Internet
Author: User
Tags echo command echo message strlen

Turn from:

Overlay writing (the original content in the file is covered) echo "AAA" > A.txt
echo AAA > A.txt

Add style (The new content is added after the original content)

echo "AAA" >> a.txt
echo AAA >> A.txt

where Echo <string> >or>> file,
In the user space, the value is assigned to the string+ line break in buf,
When incoming to kernel space, buf is a string+ line break, strlen (BUF) = strlen (string) + 1;

The function of the echo command is to display a piece of text on the monitor, which generally acts as a hint.

The general format for this command is: the echo [-n] string where option n indicates that the output text does not wrap, and the string can be enclosed in quotes or without quotes. Output a quoted string with the Echo command, outputting the string as it is, and using the echo command to output a string without quotes, each word in the string is output as a string, separated by a space between the strings.
Function Description: Display text. Syntax: Echo [-ne][string] or echo [--help][--version] Supplemental Note: Echo will send the input string to standard output. The output string is separated by a white space character and the last line number is added. Parameters:-n do not wrap at the last word-e if the following characters appear in the string, it is handled in particular, not as normal text output: The \a emits a warning sound; \b Deletes the previous character; the \c ends with a newline symbol; \f the line wraps but the cursor is still The old stays in the original position; \ n Wraps and the cursor moves to the beginning of the line; \ r The cursor moves to the beginning of the line, but does not wrap; \ t inserts tab; \v is the same as \f; \ insert \ character; \nnn insert nnn (octal) represents ASCII characters; –help Display Help –version display version information

The echo command is a child command of a DOS batch command familiar to you, but some of its features and usage maybe you don't know all about it, you see:

1. The switch format in which the command line itself is displayed as a control batch command at execution time: ECHO [on| OFF] If you want to turn off the display of the "ECHO off" command line itself, you need to precede the command line with "@".

2. Displays the current ECHO settings status format: Echo

3. Output hint Information format: Echo message content The above are three common uses of the echo command and are familiar and available, but as a DOS command miner you should also know the following tips:

4. Turn off the DOS command prompt by typing echo off at a DOS prompt, you can turn off the display of the DOS prompt so that the screen leaves only the cursor until you type echo on and the prompt will appear again.

5. Output blank line, which is equivalent to entering a carriage return format: ECHO. It is important to note that in the command line. "There must be no spaces in the middle of the echo, otherwise". "will be output to the screen as a cue message. In addition ". "may use,:;"/[/]+, such as any symbol substitution. In the following example, Echo. The output carriage return, which is turned into a DOS pipe as input to the time command, is equivalent to giving a carriage return after the time command has been executed. Therefore, the system automatically returns to the DOS prompt after the current time is displayed: c:〉echo.| The time echo command outputs another application instance of a blank row: the echo. Add to the automatic batch file so that the cue screen originally displayed at the bottom of the screen appears above the screen.

6. Question format in reply to command: Echo Reply | command file name the above format can be used to simplify some of the commands that require man-machine conversation (such as: Chkdsk/f;format Drive:;del *.*) operations, It is the input to the Man-machine conversation command using the preset reply of the echo command output through the DOS pipe command. The following example is equivalent to entering "Y" carriage return when the calling command appears in Man-machine conversation: C:〉echo y| chkdsk/f C:〉echo y| DEL A:*. *

7. Create a new file or add a file content format: Echo file contents > filename echo file content-filename For example: C:〉echo @ECHO off〉autoexec. BAT establishes an automatic batch processing file C:〉echo c:/cpav/bootsafe〉〉autoexec. BAT appends content C:type AUTOEXEC to an automatic batch file. BAT displays the automatic batch file @ECHO off C:/cpav/bootsafe

8. Output print to the printer or print control code format: Echo Printer control code >prn echo Print content >prn The following example is to enter the print control code to the M-1724 printer. 156 is to hold down the ALT key to type 156 in the keypad, and so on: c:〉echo〈alt〉+156〈alt〉+42〈alt〉+116〉prn (Enter underline command fs*t) c:〉echo〈alt〉+155@〉 PRN (Input Initialization command esc@) c:〉echo.〉prn (newline)

9. Make the Horn sound c:〉echo ^g "^g" is input with ctrl+g or alt+007, input multiple ^g can produce multiple beeps. The use method is to add it directly to a batch file or to make a batch file call.

10. Perform ESC control sequence modify screen and keyboard settings we know that DOS device driver Ansi.sys provides a set of ESC control sequences to modify screen and keyboard settings. A batch program that performs the following can define the function key F12 as a DOS command "dir/w" and modify the screen color to a white character blue background. @ECHO "←[0;134;" dir/w 13p @ECHO ←[1;37;44m (Note: The Input method for "←" characters in a batch file is to press ALT 27 on the keypad in edit state)

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