Linux output to file and console

How to redirect output to a file and stdout

In bash, calling foo would display any output from that command on the stdout.

Calling foo > output would redirect any output from that command to the file specified (in this case ‘output’).

Is there a way to redirect output to a file and have it display on stdout?

10 Answers 10

The command you want is named tee :

For example, if you only care about stdout:

If you want to include stderr, do:

2>&1 redirects channel 2 (stderr/standard error) into channel 1 (stdout/standard output), such that both is written as stdout. It is also directed to the given output file as of the tee command.

Furthermore, if you want to append to the log file, use tee -a as:

2>&1 dumps the stderr and stdout streams. tee outfile takes the stream it gets and writes it to the screen and to the file «outfile».

This is probably what most people are looking for. The likely situation is some program or script is working hard for a long time and producing a lot of output. The user wants to check it periodically for progress, but also wants the output written to a file.

The problem (especially when mixing stdout and stderr streams) is that there is reliance on the streams being flushed by the program. If, for example, all the writes to stdout are not flushed, but all the writes to stderr are flushed, then they’ll end up out of chronological order in the output file and on the screen.

It’s also bad if the program only outputs 1 or 2 lines every few minutes to report progress. In such a case, if the output was not flushed by the program, the user wouldn’t even see any output on the screen for hours, because none of it would get pushed through the pipe for hours.

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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 to save terminal output to a file under Linux/Unix

H ow do I save the terminal output to a file when using BASH/KSH/CSH/TCSH under Linux, macOS, *BSD or Unix-like operating systems?

Yes, we can save command output by redirecting it to a file. The standard streams for input, output, and error are as follows (also known as file descriptors):

Tutorial details
Difficulty level	Easy
Root privileges	No
Requirements	Linux, macOS, or Unix
Est. reading time	3 minutes

1. stdin (numeric value 0) – Keyboard
2. stdout (numeric value 1) – Screen/display
3. stderr (numeric value 2) – Screen/display

• Redirect stdout/stderr to a file
• Redirect stdout to a stderr OR redirect stderr to a stdout
• To redirect stderr and stdout to a file
• We can redirect stderr and stdout to stdout too
• And finally you can redirect stderr and stdout to stderr

How to save terminal output to a file

By default, the command sends outputs to stdout and can be redirected to the file using the following syntax:
command > filename.txt
For example, save the date command output to a file named mydate.txt, run:
date > mydate.txt
To view file contains use the cat command:
cat mydate.txt

Feed data to our commands (input redirection)

We can read input from a file using the following simple syntax and the file must already exist:
command

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Append output to a file

If the filename.txt/mydate.txt (file) already exists, it will get overwritten. To append output, run:
command >> filename.txt
echo «——————» >> mydate.txt
ls -l /etc/resolv.conf >> mydate.txt
Verify it:
cat mydate.txt

Please note that the file such as mydate.txt is overwritten unless the bash noclobber option is set using the set command. For example, turn off noclobber option:
set -o noclobber
echo «some data» > mydata.txt
Sample outputs:

We can turn on noclobber option as follows:
set +o noclobber
echo «foo bar» > mydata.txt

How to redirect stderr to a file

The syntax is as follows:
command &> file.txt
command &>> file.txt
OR
command 2> file.txt
command 2>> file.txt
Above works with bash and other modern shell. For POSIX version try:
command >output.txt 2>&1
command >>output.txt 2>&1
In this example, send the find command errors to a file named err.log:
find / -iname «*.conf» &>err.log
## OR ##
find / -iname «*.conf» 2>err.log
## POSIX version ##
find . -iname «*.conf» >err.log 2>&1
Verify it:
cat err.log
Sample outputs:

How to suppress error messages

Use the following syntax:
command 2>&-
find . -iname «*.txt» 2>&-
We can also redirect error messages (stderr) to standard output (stdout), run:
command 2>&1
echo «foo» 2>&1
kill $target_pid 2>&1 > /dev/null

How to redirect both stdout and stderr to a file

The syntax is as follows to redirect both stdout and stderr to a file:
command 2>&1 | tee output.txt
For example:
find . -iname «*.txt» 2>&1 | tee cmd.log
cat cmd.log
To append text to end of file use the following syntx:
find . -iname «*.conf» 2>&1 | tee -a cmd.log
cat cmd.log

How to combine redirections

The following command example simply combines input and output redirection. The file resume.txt is checked for spelling mistakes, and the output is redirected to an error log file named err.log:
spell error.log

How to redirect screen output (stdout) and errors (stderr) to /dev/null

Try the following syntax
command > /dev/null 2>&1
/path/to/script.py > /dev/null 2>&1

Redirect both standard error and standard out messages to a log file

command > log.txt 2>&1
/path/to/my-appname.py > my-appname.log 2>&1

Conclusion

You learned how to save terminal output to a file when using Linux or Unix-like operating system with modern shell such as Bash or KSH including POSIX syntax. See bash docs here for more info or type the following man command:
man bash

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How do I save terminal output to a file?

How do I save the output of a command to a file?

Is there a way without using any software? I would like to know how.

9 Answers 9

Yes it is possible, just redirect the output (AKA stdout ) to a file:

Or if you want to append data:

If you want stderr as well use this:

or this to append:

if you want to have both stderr and output displayed on the console and in a file use this:

(If you want the output only, drop the 2 above)

To write the output of a command to a file, there are basically 10 commonly used ways.

Overview:

_{Please note that the n.e. in the syntax column means «not existing».
There is a way, but it’s too complicated to fit into the column. You can find a helpful link in the List section about it.}

The standard output stream will be redirected to the file only, it will not be visible in the terminal. If the file already exists, it gets overwritten.

The standard output stream will be redirected to the file only, it will not be visible in the terminal. If the file already exists, the new data will get appended to the end of the file.

command 2> output.txt

The standard error stream will be redirected to the file only, it will not be visible in the terminal. If the file already exists, it gets overwritten.

command 2>> output.txt

The standard error stream will be redirected to the file only, it will not be visible in the terminal. If the file already exists, the new data will get appended to the end of the file.

Both the standard output and standard error stream will be redirected to the file only, nothing will be visible in the terminal. If the file already exists, it gets overwritten.

Both the standard output and standard error stream will be redirected to the file only, nothing will be visible in the terminal. If the file already exists, the new data will get appended to the end of the file..

command | tee output.txt

The standard output stream will be copied to the file, it will still be visible in the terminal. If the file already exists, it gets overwritten.

command | tee -a output.txt

The standard output stream will be copied to the file, it will still be visible in the terminal. If the file already exists, the new data will get appended to the end of the file.

Bash has no shorthand syntax that allows piping only StdErr to a second command, which would be needed here in combination with tee again to complete the table. If you really need something like that, please look at «How to pipe stderr, and not stdout?» on Stack Overflow for some ways how this can be done e.g. by swapping streams or using process substitution.

command |& tee output.txt

Both the standard output and standard error streams will be copied to the file while still being visible in the terminal. If the file already exists, it gets overwritten.

command |& tee -a output.txt

Both the standard output and standard error streams will be copied to the file while still being visible in the terminal. If the file already exists, the new data will get appended to the end of the file.

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