How to save terminal output to a file under Linux/Unix

last updated April 19, 2020 in CategoriesBASH Shell, KSH Shell, Linux, UNIX

How 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):

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

So 1 represents stdout and 2 represents stderr. Based upon the information we can:

• 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 < input.txt
cat < /etc/passwd
wc -l < /etc/passwd

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:

bash: mydata.txt: cannot overwrite existing file

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:

find: ‘./snap.chromium’: Permission denied
find: ‘./systemd-private-timesyncd.service-KOh0jg’: Permission denied
find: ‘./snap.demo’: Permission denied
find: ‘./snap.lxd’: Permission denied
find: ‘./.vbox-root-ipc’: Permission denied

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 < resume.txt > 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