Redirection and piping

I. Interpreting redirects

The program must be able to read input and then be processed to display the results, the program to obtain a variety of data, the program produced a variety of results. In other words, the program must have the output to read.

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Read-in data: Input

Outputs data: Output

everything in Linux is connected to files. This means that any device has a corresponding file in the system. So we can access the device through a file. For users, access to files is through the file name, the user sees the file name, but, for the kernel, all is a number, The number is the file descriptor to access the file. When you open an existing file or a new file, the kernel returns an FD. Both read and write files require the use of FD to write and read files. fd:file Descriptor (file descriptor)

Linux provides three I/O devices to Programs

Standard input (STDIN)-0 accepts input from the keyboard by default

Standard output (STDOUT)-1 Default output to terminal window

Standard error (STDERR)-2 default output to terminal window

I/O redirection: Changing the default location

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when we perform the redirect we see that the results of the LS display are output to a new file, and if the file is not in TestDir, the file is created automatically.

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and I redirect ll results to the file, then we will find that the previous results are not, are overwritten, so the file is created by.

1: If the file does not exist, the system will automatically create the file.

2: When this file is present, the system will empty the contents of the file, so that the data will be written later.

3: If the output of > to an existing file, the file will be overwritten.

if we don't want to overwrite the file use >> (accumulate) on it.

Second, redirect specific usage

stdout and Tderr can be redirected to a file:

Command Action Symbol file name

The supported operating symbols are:

> redirect stdout to a file

2> redirect stderr to file

&> Redirect all output to file

> file content will be overwritten

#set-C: Prohibit overwriting of existing files, but may append mandatory Lid: >|

#set +c: Allow overlay

>> additional content based on the original content

redirect output and errors to a file

2>: Overwrite redirect error output data stream;

2>>: Append redirect error output data stream;

The standard output and error outputs are directed to different locations:

COMMAND >/path/to/file.out >/path/to/error.out

Merge standard output and error output redirect for the same data stream

&>: Overwrite redirect

&>>: Append redirect

COMMAND >/path/to/file.out 2>&1 (Order important)

COMMAND >>/path/to/file.out 2>>&1 (Order important)

Find/etc-name passwd 2>/dev/null

/dev/null Garbage bin black hole usage

If we know there will be a lot of error messages and don't want it to show up, this time use a black hole device/dev/null. It can eat all the information that guides it.

(): Merging stdout of multiple programs

(Cal 2007;cal2008) >all.txt

Standard input:< and <<

< is to say that the data that needs to be entered by the keyboard is replaced by the file contents. As shown in.

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< is very helpful, especially if you are sending mail in this command. So what does << mean, it represents the end of the input meaning. As

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You can see that end is the ending input and he doesn't appear in the file.

Use < to redirect standard input

Some commands can accept stdin imported from a file:

$tr ' A-Z ' A-Z ' </etc/issue

Converts a lowercase letter in a file to uppercase.

Tr-d abc</etc/issue

Remove any letters from all ABC in the issue file.

#cat > Filea

Press Ctrl+d to leave, you can use the file instead of the keyboard input

#Cat > Filea < Fileb

tr-converting and deleting characters

Tr[option] ... Set1[set2]

Options:

-C or--complerment: a complement to the character set

-D or--delete: Deletes all characters belonging to the first character set

-S or-squeeze-repeats: the consecutive repeating character is represented by a single character

-T or--truncate-set1: Converts the corresponding character of the first character set to paragraph two character set corresponding to characters

Piping

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Pipe (using the symbol "|" Used to connect commands

Command 1 | Command 2 | Command 3 | ...

Send stdout of command 1 to stdin of command 2, stdout of command 2 to stdin of command 3

StdErr cannot be piped forward by default and can be implemented with 2>&1 or |&

The last command executes in the child shell process of the current shell process to

The ability to combine multiple tools

ls | Tr ' A-Z ' A-Z

Less: one page, one page to view the input:

$ ls-l/etc | Less

Mail: Send input by e-mail:

$ echo "Test Email" | Mail \ S "Test" [email protected]

Redirection and piping