Linux redirect all streams

Linux Fundamentals – I/O, Standard Streams, and Redirection.

In Linux there are three standard input streams, standard input (0 – stdin), standard output (1 – stdout), and standard error (2 – stderr). In this article we will discuss practical uses of redirection, the standard streams, and their file descriptors in the bash command line and give examples of how to use each including redirection.

Table of Contents

Standard Streams

In Linux and computer programming in general, standard streams are input and output (I/O) communication channels between a program and it’s environment. The three standard streams are standard input (stdin), standard output (stdout), and standard error (stderr). Streams are usually connected to the terminal in which they are executed, but that can be changed using redirection operators and/or pipes.

Below is a graphic to help visualize the data streams and their file descriptors.

File Descriptors

Linux often represents the three standard streams as file descriptors, here is a table to help understand the relationship.

File descriptors are not limited to the three standard streams, but these are the most commonly used predefined streams. It is possible to create custom file descriptors, however we will not cover that in this article.

Redirection Operators

Redirection operators are a subset of control operators. They allow you to direct the input or output (stream) of your command.

The Input Operator

The input operator gives input to a command or process. The left angle bracket ( The Input Operator (rw)

The input operator (rw) does the same as the input operator only it opens the file in read/write mode. The left and right angle bracket ( <> ) represents this operator. If the file does not exist, it will be created. Since most input is from the keyboard or read from a file, this is rarely used.

The Output Operator

The output operator controls the command output stream. The right angle bracket ( > ) represents this operator. If the file you are redirecting to does not exist, it will be created. If it does exist, it will be overwritten.

The Output Append Operator

The output append operator does the same as the output operator only it appends data to a file. Two right angle brackets ( >> ) represent this operator. If the file you are redirecting to does not exist, it will be created. If it does exist, this command will append data to it instead of overwriting.

The Non-Standard Operators

Non-standard operators direct both standard output (stdout) and standard error (stderr). The ampersand sign followed by a right bracket or double right brackets represent these operators. One overwrites ( &> ) and one appends ( &>> ) data to the output file.

Pipe Operator

The pipe operator is used to pass the output of a command to the input of another command. The vertical bar ( | ) represents this operator. The aptly named pipe can be thought of similar to a physical plumbing pipe which moves liquid from one vessel to another. Output from a command enters the pipe on one side and leaves the pipe as input for another command on the other side.

Examples for using the pipe operator are in the sections below.

Standard Input – STDIN

Standard input is a data stream going into a program. This is often text input from a keyboard, but can also be data piped or redirected to a program.

Standard Input From Keyboard

A simple example of a program that uses standard input is the cat command. By default cat takes standard input and prints it to the display (standard output).

To test this we can use the cat command. Open a terminal and type cat, then type a line from your favorite song and hit enter.
NOTE: The cat program will do this forever, when you are finished hit CTRL+D to exit.

Standard Input from an Input File

Standard input can also come from an input file. If you provide cat with a file as an argument, it will replace or redirect standard input from the keyboard to the data coming from that file.

To test this, enter the following command in your terminal and hit enter.

The above command takes the argument (

/.bashrc) as standard input and outputs the content of the file on the screen.

Standard Input from Redirection

Standard input can also come from any file output using the input redirection operator which is represented by a less than sign or left pointing angle bracket ( Standard Input from a Data Pipe

So far we learned how to provide standard input from a keyboard or from a file by argument or redirection. But, you can also provide standard input from the output of another program on the input stream. This is called piping, and even if you are new to Linux you probably already encountered this concept. A pipe represented by a vertical line ( | ), basically takes output from one command and puts it into the pipe, then it comes out of the pipe on the other side and into the next command.

To test this enter the following command in your terminal.

The head program outputs the first 10 lines of a file and pipes it to cat as standard input (stdin), which in turn prints it to standard out (stdout).

Standard Output – STDOUT

Standard output is a standardized stream that is sent to a display monitor by default. Often the terminal is standard output, but you can pipe or redirect output to a file, another program or device.

Standard Output to Display

You can use the output of almost any command as an example of standard output (stdout). Here we will use the stat command to get status information about a file and display it on the monitor.

As you can see the stat command took the filename as an argument, did some processing and then displayed the statistics of that file to the monitor (stdout).

Standard Output Redirected to a File

You can redirect standard output (stdout) to a file. This is useful if you want to save the output for later use, or as a log of a script.

You can redirect stdout to a file using the output redirection operator represented by a greater than sign or right pointing angle bracket ( > ). For example, you can save the output of the stat command to a file named output.txt like so:

This created a file named output.txt, and places the standard output (stdout) of the stat command into it. We can verify this by reading the file.

Using this output redirection will overwrite any data that may exist in the file already. Use the output append operator if you want to append, represented by two angle brackets ( >> ).

Standard Output Through a Data Pipe

The data pipe sends output from one command to another. Here we will pipe the output of the stat command into grep to pull only last modified time of the file.

The stat commands output enters the pipe and passed to grep for processing. When grep finishes processing the data, it sends the output to standard output (stdout).

Standard Error – STDERR

Standard error (stderr) is the destination for all error messages from a process. By default, this is the display screen but just like standard out, it can be redirected.

Standard Error to Display

Mistyping will certainly lead to an error displayed on the screen if you use the command line often. This is an example of standard error (stderr). Here we will feed the ls command an argument that doesn’t exist, and it will tell us using stderr.

An error was displayed on the screen as stderr by default.

Standard Error Redirected to a File

Just like the other streams, you can redirect stderr to a file. Often used in scripting, but handy to know for anyone using the command line. To redirect stderr, you can use the same output redirection operator as you did with stdout ( > ). But this time, we have to specify that we want to redirect stderr by using the file descriptor. As shown in the table and diagram above, the file descriptor for stderr is 2. So we would redirect it like so:

The file errors.txt now contains the errors from the output of the above command.

As we discussed in the stdout section, using the single bracket redirection will result in overwriting any data that is currently in the file. To append instead of overwrite, you have to use the append redirection ( >> ).

Standard Error Through a Data Pipe

You cannot directly pipe stderr to another command. This is by design to allow for complete error reporting. Here is an example of trying (and failing) to pipe stderr into grep.

The ls command prints errors to the screen and the grep command acts as if it received no input.

However, as with most things Linux, there are multiple ways to accomplish the same task. You can redirect stderr to stdout using their file descriptors, then pipe it through grep.

We will discuss more about file descriptors in the next section.

Using File Descriptors for Redirection

Non-Standard redirection requires file descriptors. We covered all of the standard (input / output ) redirection in the above sections. Below we will outline some additional redirects using file descriptors.

File descriptors are used to redirect streams. However, you can do a lot more with file descriptors, much more than we will cover here. The intention of this section is to give you a elementary understanding of file descriptors and making you comfortable with their basic usage.

Redirect STDERR and STDOUT to Different Files.

You can redirect both stderr and stdout to a file. Here is an example:

The above command sends stdout (File Descriptor 1) to ouput.txt and stderr (2) to errors.txt. We can verify by reading those files.

Redirect STDERR and STDOUT to Same File.

You can redirect both stderr and stdout to a single file if you wish. For this we can either redirect stderr to stdout, then stdout to a file like so:

Or we can use a shorthand (non-standard) operator like so:

Either command produces the same result, all stdout and stderr were sent to the alloutput.txt file.

Using Permanent Redirection in Scripts

If you are writing a script and want to redirect data streams without appending the redirect operator or every line, you can use the exec command like this.

You can use one, both or any number of custom file descriptors in your script.

Conclusion

Knowing the standard streams, file descriptors and redirection are an important part of being proficient on the command line. In this article we covered a lot but still only scratched the surface. I hope I touched all the basics of these topics, but if you feel I need to be corrected or should include additional information please sound off in the comments.

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Linux redirect all streams

COMMAND_OUTPUT > # Redirect stdout to a file. # Creates the file if not present, otherwise overwrites it. ls -lR > dir-tree.list # Creates a file containing a listing of the directory tree. : > filename # The > truncates file «filename» to zero length. # If file not present, creates zero-length file (same effect as ‘touch’). # The : serves as a dummy placeholder, producing no output. > filename # The > truncates file «filename» to zero length. # If file not present, creates zero-length file (same effect as ‘touch’). # (Same result as «: >», above, but this does not work with some shells.) COMMAND_OUTPUT >> # Redirect stdout to a file. # Creates the file if not present, otherwise appends to it. # Single-line redirection commands (affect only the line they are on): # ——————————————————————— 1>filename # Redirect stdout to file «filename.» 1>>filename # Redirect and append stdout to file «filename.» 2>filename # Redirect stderr to file «filename.» 2>>filename # Redirect and append stderr to file «filename.» &>filename # Redirect both stdout and stderr to file «filename.» # This operator is now functional, as of Bash 4, final release. M>N # «M» is a file descriptor, which defaults to 1, if not explicitly set. # «N» is a filename. # File descriptor «M» is redirect to file «N.» M>&N # «M» is a file descriptor, which defaults to 1, if not set. # «N» is another file descriptor. #============================================================================== # Redirecting stdout, one line at a time. LOGFILE=script.log echo «This statement is sent to the log file, \»$LOGFILE\».» 1>$LOGFILE echo «This statement is appended to \»$LOGFILE\».» 1>>$LOGFILE echo «This statement is also appended to \»$LOGFILE\».» 1>>$LOGFILE echo «This statement is echoed to stdout, and will not appear in \»$LOGFILE\».» # These redirection commands automatically «reset» after each line. # Redirecting stderr, one line at a time. ERRORFILE=script.errors bad_command1 2>$ERRORFILE # Error message sent to $ERRORFILE. bad_command2 2>>$ERRORFILE # Error message appended to $ERRORFILE. bad_command3 # Error message echoed to stderr, #+ and does not appear in $ERRORFILE. # These redirection commands also automatically «reset» after each line. #=======================================================================

2>&1 # Redirects stderr to stdout. # Error messages get sent to same place as standard output. >>filename 2>&1 bad_command >>filename 2>&1 # Appends both stdout and stderr to the file «filename» . 2>&1 | [command(s)] bad_command 2>&1 | awk ‘‘ # found # Sends stderr through a pipe. # |& was added to Bash 4 as an abbreviation for 2>&1 |. i>&j # Redirects file descriptor i to j . # All output of file pointed to by i gets sent to file pointed to by j . >&j # Redirects, by default, file descriptor 1 (stdout) to j . # All stdout gets sent to file pointed to by j .

0 «>» , and often used in combination with it. # # grep search-word filename # Open file «filename» for reading and writing, #+ and assign file descriptor «j» to it. # If «filename» does not exist, create it. # If file descriptor «j» is not specified, default to fd 0, stdin. # # An application of this is writing at a specified place in a file. echo 1234567890 > File # Write string to «File». exec 3<> File # Open «File» and assign fd 3 to it. read -n 4 &3 # Write a decimal point there. exec 3>&- # Close fd 3. cat File # ==> 1234.67890 # Random access, by golly. | # Pipe. # General purpose process and command chaining tool. # Similar to «>» , but more general in effect. # Useful for chaining commands, scripts, files, and programs together. cat *.txt | sort | uniq > result-file # Sorts the output of all the .txt files and deletes duplicate lines, # finally saves results to «result-file» .

Multiple instances of input and output redirection and/or pipes can be combined in a single command line.

command output-file # Or the equivalent: output-file # Although this is non-standard. command1 | command2 | command3 > output-file

Multiple output streams may be redirected to one file.

ls -yz >> command.log 2>&1 # Capture result of illegal options «yz» in file «command.log.» # Because stderr is redirected to the file, #+ any error messages will also be there. # Note, however, that the following does *not* give the same result. ls -yz 2>&1 >> command.log # Outputs an error message, but does not write to file. # More precisely, the command output (in this case, null) #+ writes to the file, but the error message goes only to stdout. # If redirecting both stdout and stderr, #+ the order of the commands makes a difference.

Close input file descriptor n .

Close output file descriptor n .

Child processes inherit open file descriptors. This is why pipes work. To prevent an fd from being inherited, close it.

# Redirecting only stderr to a pipe. exec 3>&1 # Save current «value» of stdout. ls -l 2>&1 >&3 3>&- | grep bad 3>&- # Close fd 3 for ‘grep’ (but not ‘ls’). # ^^^^ ^^^^ exec 3>&- # Now close it for the remainder of the script. # Thanks, S.C.

For a more detailed introduction to I/O redirection see Appendix F .

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