What operator redirects stdout to a file creating or overwriting an existing file?

Redirection can be defined as changing the way from where commands read input to where commands sends output. You can redirect input and output of a command.

For redirection, meta characters are used. Redirection can be into a file (shell meta characters are angle brackets '<', '>') or a program ( shell meta characters are pipesymbol '|').

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Standard Streams In I/O Redirection

The bash shell has three standard streams in I/O redirection:

• standard input (stdin) : The stdin stream is numbered as stdin (0). The bash shell takes input from stdin. By default, keyboard is used as input.
• standard output (stdout) : The stdout stream is numbered as stdout (1). The bash shell sends output to stdout. Output goes to display.
• standard error (stderr) : The stderr stream is numbered as stderr (2). The bash shell sends error message to stderr. Error message goes to display.

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Redirection Into A File

Each stream uses redirection commands. Single bracket '>' or double bracket '>>' can be used to redirect standard output. If the target file doesn't exist, a new file with the same name will be created.

Overwrite

Commands with a single bracket '>' overwrite existing file content.

• > : standard output
• < : standard input
• 2> : standard error

Note: Writing '1>' or '>' and '0<' or '<' is same thing. But for stderr you have to write '2>'.

Syntax:

Example:

Look at the above snapshot, command "cat > sample.txt" has created 'sample.txt' with content 'a, b, c'. Same file 'sample.txt' is created again with command "cat > sample.txt" and this time it overwrites earlier file content and only displays 'd, e, f '.

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Append

Commands with a double bracket '>>' do not overwrite the existing file content.

• >> - standard output
• << - standard input
• 2>> - standard error

Syntax:

Example:

Look at the above snapshot, here again we have created two files with the same name using '>>' in command "cat >> sample.txt". But this time, content doesn't overwrite and everything is displayed.

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Redirection Into A Program

Pipe redirects a stream from one program to another. When pipe is used to send standard output of one program to another program, first program's data will not be displayed on the terminal, only the second program's data will be displayed.

Although the functionality of pipe may look similar to that of '>' and '>>' but has a significance difference. Pipe redirects data from one program to another while brackets are only used in redirection of files.

Example:

Look at the above snapshot, command "ls *.txt | cat > txtFile" has put all the '.txt' files into a newly created file 'txtFile'.

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CONCEPT: Every program you run from the shell opens three files: Standard input, standard output, and standard error. The files provide the primary means of communications between the programs, and exist for as long as the process runs.

The standard input file provides a way to send data to a process. As a default, the standard input is read from the terminal keyboard.

The standard output provides a means for the program to output data. As a default, the standard output goes to the terminal display screen.

The standard error is where the program reports any errors encountered during execution. By default, the standard error goes to the terminal display.

CONCEPT: A program can be told where to look for input and where to send output, using input/output redirection. Unix uses the "less than" and "greater than" special characters (< and >) to signify input and output redirection, respectively.

Redirecting input

Using the "less-than" sign with a file name like this: < file1 in a shell command instructs the shell to read input from a file called "file1" instead of from the keyboard.

EXAMPLE:Use standard input redirection to send the contents of the file /etc/passwd to the more command:

more < /etc/passwd

Many Unix commands that will accept a file name as a command line argument, will also accept input from standard input if no file is given on the command line.

EXAMPLE: To see the first ten lines of the /etc/passwd file, the command:

head /etc/passwd will work just the same as the command: head < /etc/passwd

Redirecting output

Using the "greater-than" sign with a file name like this: > file2 causes the shell to place the output from the command in a file called "file2" instead of on the screen. If the file "file2" already exists, the old version will be overwritten.

EXAMPLE: Type the command

ls /tmp > ~/ls.out to redirect the output of the ls command into a file called "ls.out" in your home directory. Remember that the tilde (~) is Unix shorthand for your home directory. In this command, the ls command will list the contents of the /tmp directory.

Use two "greater-than" signs to append to an existing file. For example:

>> file2 causes the shell to append the output from a command to the end of a file called "file2". If the file "file2" does not already exist, it will be created.

EXAMPLE: In this example, I list the contents of the /tmp directory, and put it in a file called myls. Then, I list the contents of the /etc directory, and append it to the file myls:

ls /tmp > myls
ls /etc >> myls

Redirecting error

Redirecting standard error is a bit trickier, depending on the kind of shell you're using (there's more than one flavor of shell program!). In the POSIX shell and ksh, redirect the standard error with the symbol "2>".

EXAMPLE: Sort the /etc/passwd file, place the results in a file called foo, and trap any errors in a file called err with the command:

sort < /etc/passwd > foo 2> err

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Which operator is used to redirect output to an existing file?

What operator can be used to redirect standard input to a file while overwriting an existing file contents?

Which symbol should I use to redirect the standard output to a file overwrite the file )?

What is the 2 redirection operator used for?