Linux redirect all output to file

How to write the output into the file in Linux

How do I save terminal output to a file?

A command can receive input from a file and send output to a file.

Writing the output into the file

The syntax is
command > filename
For example, send output of the ls command to file named foo.txt
$ ls > foo.txt
View foo.txt using the cat command:
$ cat foo.txt
Please note that when you type ‘ls > foo.txt’, shell redirects the output of the ls command to a file named foo.txt, replacing the existing contents of the file. In other words, the contents of the file will be overwritten.

Appending the output or data to the file

The syntax is
command >> filename
For example the following will append data:

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Verify it:
cat /tmp/data.txt

How to save the output of a command to a file in bash using tee command

The tee command read from standard input and write to standard output and files. The syntax is as follows for writing data into the file:
command | tee file.txt
Want to append data? Try
command | tee -a output.txt

Examples

Display output of the date command on screen and save to the file named /tmp/output.txt. If the output.txt already exists, it gets overwritten:
$ date | tee /tmp/output.txt
$ cat /tmp/output.txt
Same as above but append to the given files, do not overwrite file:
$ pwd | tee -a /tmp/test.txt
$ echo «Today is $(date)» | tee -a /tmp/test.txt
$ hostnamectl | tee -a /tmp/test.txt
$ cat /tmp/test.txt

The above commands will append the output to the end of the file, just like the shell >> operator as explained earlier.

I/O redirection summary for bash and POSIX shell

Shell operator	Description	Overwrite existing file?
command > output.txt	Save terminal output (standard output) to a file named output.txt	Yes
command >> output.txt	Append terminal output (standard output) to a file named output.txt	No
command	Takes standard input from output.txt file	N/A
command 0	Takes standard input from output.txt file	N/A
command 1> output.txt	Puts standard output to output.txt file	Yes
command 1>> output.txt	Appends standard output to output.txt	No
command 2> output.txt	Puts standard error to output.txt	Yes
command 2>> output.txt	Appends standard error to output.txt file	No
command &> output.txt	Puts both standard error and output to output.txt	Yes
command > output.txt 2>&1	<POSIX> Puts both standard error and output to file named output.txt	Yes
command &>> output.txt	Appends both standard error and output to file named output.txt	No
command >> output.txt 2>&1	<POSIX> Appends both standard error and output to file called output.txt	No
command | tee output.txt	Puts standard output to output.txt while displaying output on screen	Yes
command | tee -a output.txt	Appends standard output to output.txt while displaying output on screen	No
command |& tee output.txt	Puts both standard output and error to output.txt while displaying output on terminal	Yes
command 2>&1 | tee output.txt	<POSIX> Puts both standard output and error to file named output.txt while displaying output on terminal	Yes
command |& tee -a output.txt	Append both standard output and error to file called output.txt while displaying output on terminal	No
command 2>&1 | tee -a output.txt	<POSIX> Append both standard output and error to file named output.txt while displaying output on terminal	No

Conclusion

You learned how to write the output to the file in Linux or Unix-like system when using bash or POSIX shell. We have:

1. /dev/stdin (standard input) — File descriptor 0 is duplicated.
2. /dev/stdout (standard output) — File descriptor 1 is duplicated.
3. /dev/stderr (standard error) — File descriptor 2 is duplicated.

See I/O redirection documentation for more information. We can read bash man page as follows using the man command:
man bash

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How Do I Redirect Top Output to a File in Linux?

This article will discuss three different procedures of redirecting the output of the top command to any file.

Method 1: Single File Output Redirection

For utilizing the redirection of bash, execute any script, then define the > or >> operator followed by the file path to which the output should be redirected.

• “>>” operator is used for utilizing the command’s output to a file, including the output to the file’s current contents.
• “>” operator is used to redirect the command’s output to a single file and replace the file’s current content.

We can say that technically, this is a file redirection of “stdout,” which is the normal display. Now, we will execute the sample example. The “ls” command displays the content of the current directory’s folders and files after its execution.

However, this command will save the output to the specified file in the following example rather than printing it to the terminal.

Utilize the given command syntax for checking the content of the file.

Now, write out the below-given command for printing the content of the “output file” in the terminal.

The operator “>” overwrites the file content with the command execution output. Instead, you can use the “>>” operator for saving the multiple commands output in a single file. For instance, the execution of the given command will add the system information to the specific file.

$ uname -a >> / home / linuxhint / outputfile

$ cat / home / linuxhint / outputfile

Method 2: Redirecting terminal output to a single file

Didn’t like the idea of using the”>” or “>>” operator for redirecting output? Don’t worry! The tee command is here to rescue you.

The below-given tee command will overwrite the file content with the command’s output similar to the “>” operator.

Method 3: The top command

System administrators also use the Linux top command to view real-time system statistics such as load average, system uptime, running tasks, used memory, specific information about each running process, and a summary of threads or processes. By utilizing the -b flag, this command helps to get the information about the currently executing processes in the system. The top command will permit the top to function in batch mode and the -n flag to determine the number of iterations the command should take as output.

All of the output resulting from the top command’s execution will be redirected to the specified file. Now, write out the “less” command for checking the content of the file.

The -n flag will send the single snapshot of executed command to the specified file. To retrieve only the first iteration, specify the “1” after the “-n” flag.

Utilize the “cat” command for viewing the running tasks information.

Conclusion:

In Linux, for redirecting output to a file, utilize the ”>” and ”>>” redirection operators or the top command. Redirection allows you to save or redirect the output of a command in another file on your system. You can use it to save the outputs and use them later for different purposes.

About the author

Talha Saif Malik

Talha is a contributor at Linux Hint with a vision to bring value and do useful things for the world. He loves to read, write and speak about Linux, Data, Computers and Technology.

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Input Output Redirection in Linux/Unix Examples

Updated October 7, 2021

What is Redirection?

Redirection is a feature in Linux such that when executing a command, you can change the standard input/output devices. The basic workflow of any Linux command is that it takes an input and give an output.

• The standard input (stdin) device is the keyboard.
• The standard output (stdout) device is the screen.

With redirection, the above standard input/output can be changed.

In this tutorial, we will learn-

Click here if the video is not accessible

Output Redirection

The ‘>‘ symbol is used for output (STDOUT) redirection.

Here the output of command ls -al is re-directed to file “listings” instead of your screen.

Note: Use the correct file name while redirecting command output to a file. If there is an existing file with the same name, the redirected command will delete the contents of that file and then it may be overwritten.”

If you do not want a file to be overwritten but want to add more content to an existing file, then you should use ‘>>‘ operator.

You can redirect standard output, to not just files, but also devices!

The cat command reads the file music.mp3 and sends the output to /dev/audio which is the audio device. If the sound configurations in your PC are correct, this command will play the file music.mp3

Input redirection

The ‘
Standard Input STDIN 0 Standard Output STDOUT 1 Standard Error STDERR 2

By default, error stream is displayed on the screen. Error redirection is routing the errors to a file other than the screen.

Why Error Redirection?

Error re-direction is one of the very popular features of Unix/Linux.

Frequent UNIX users will reckon that many commands give you massive amounts of errors.

• For instance, while searching for files, one typically gets permission denied errors. These errors usually do not help the person searching for a particular file.
• While executing shell scripts, you often do NOT want error messages cluttering up the normal program output.

The solution is to re-direct the error messages to a file.

Example 1

Above we are executing a program names myprogram.

The file descriptor for standard error is 2.

Using “2>” we re-direct the error output to a file named “errorfile”

Thus, program output is not cluttered with errors.

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Example 2

Here is another example which uses find statement –

Example 3: Let’s see a more complex example,

Server Administrators frequently, list directories and store both error and standard output into a file, which can be processed later. Here is the command.

• which writes the output from one file to the input of another file. 2>&1 means that STDERR redirects to the target of STDOUT (which is the file dirlist)
• We are redirecting error output to standard output which in turn is being re-directed to file dirlist. Hence, both the output is written to file dirlist

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How to redirect and append both standard output and standard error to a file with Bash

To redirect standard output to a truncated file in Bash, I know to use:

To redirect standard output in Bash, appending to a file, I know to use:

To redirect both standard output and standard error to a truncated file, I know to use:

How do I redirect both standard output and standard error appending to a file? cmd &>> file.txt did not work for me.

8 Answers 8

Bash executes the redirects from left to right as follows:

1. >>file.txt : Open file.txt in append mode and redirect stdout there.
2. 2>&1 : Redirect stderr to «where stdout is currently going». In this case, that is a file opened in append mode. In other words, the &1 reuses the file descriptor which stdout currently uses.

There are two ways to do this, depending on your Bash version.

The classic and portable (Bash pre-4) way is:

A nonportable way, starting with Bash 4 is

(analog to &> outfile )

For good coding style, you should

• decide if portability is a concern (then use classic way)
• decide if portability even to Bash pre-4 is a concern (then use classic way)
• no matter which syntax you use, not change it within the same script (confusion!)

If your script already starts with #!/bin/sh (no matter if intended or not), then the Bash 4 solution, and in general any Bash-specific code, is not the way to go.

Also remember that Bash 4 &>> is just shorter syntax — it does not introduce any new functionality or anything like that.

In Bash you can also explicitly specify your redirects to different files:

Appending would be:

In Bash 4 (as well as Z shell ( zsh ) 4.3.11):

just out of box.

This should work fine:

It will store all logs in file.txt as well as dump them in the terminal.

Your usage of &> x.file does work in Bash 4. Sorry for that: (

Here comes some additional tips.

0, 1, 2, . 9 are file descriptors in bash.

0 stands for standard input, 1 stands for standard output, 2 stands for standard error. 3

9 is spare for any other temporary usage.

Any file descriptor can be redirected to other file descriptor or file by using operator > or >> (append).

Usage: >

If using older versions of Bash where &>> isn’t available, you also can do:

This spawns a subshell, so it’s less efficient than the traditional approach of cmd >> file.txt 2>&1 , and it consequently won’t work for commands that need to modify the current shell (e.g. cd , pushd ), but this approach feels more natural and understandable to me:

1. Redirect standard error to standard output.
2. Redirect the new standard output by appending to a file.

Also, the parentheses remove any ambiguity of order, especially if you want to pipe standard output and standard error to another command instead.

To avoid starting a subshell, you instead could use curly braces instead of parentheses to create a group command:

(Note that a semicolon (or newline) is required to terminate the group command.)

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