Standard error (also known as stderr) is the default error output device. Use stderr to write all system error messages. The number (FD – File Descriptors) two (2) denotes the stderr. The default stderr is the screen or monitor. Standard output (also known as stdout) is used by a command to writes (display) its output. The default stdout is the screen. It is denoted by one number (1).
2> is input redirection symbol and syntax is:
- To redirect stderr (standard error) to a file:
command 2> errors.txt - Let us redirect both stderr and stdout (standard output):
command &> output.txt - Finally, we can redirect stdout to a file named myoutput.txt, and then redirect stderr to stdout using 2>&1 (errors.txt):
command > out 2>errors.txt
Make sure you use >> for appending data/log if the file already has data. For instance:
# overwrite existing file # command1 &> output.txt # append existing file # command2 &>> output.txt # append stdout and stderr to differnet files # my_command 2>err.log 1>out.log
You must replace command with the command you want to run. Let us see some examples that explains redirection of standard error in bash.
How to redirect standard error in bash
Run find command and save all error messages to find.error.txt file:
find / -name "*.conf" 2> find.error.txt
You can view find.error.txt with the cat command:
cat find.error.txt
Sample outputs:
find: ‘/boot/grub2’: Permission denied find: ‘/proc/tty/driver’: Permission denied find: ‘/proc/1/task/1/fd’: Permission denied find: ‘/proc/1/task/1/fdinfo’: Permission denied ... .. find: ‘/proc/963/task/963/ns’: Permission denied find: ‘/proc/963/task/1078/fd’: Permission denied find: ‘/proc/963/task/1078/fdinfo’: Permission denied .... .. find: ‘/var/spool/postfix/public’: Permission denied find: ‘/var/spool/postfix/saved’: Permission denied find: ‘/var/spool/postfix/trace’: Permission denied find: ‘/usr/share/polkit-1/rules.d’: Permission denied find: ‘/usr/libexec/initscripts/legacy-actions/auditd’: Permission denied
You need to use “2>” when you want to redirect stderr to a file. You can redirect stdout to file named results.txt and stderr to file named errors.txt:
find / -name "*.conf" >results.txt 2>error.txt
Verify results with the cat command:
cat results.txt
cat error.txt
This is useful in shell scripts or any other purpose.
How to redirect standard error and standard output in bash
You can send both stdout and stderr to a file named output.txt
command &>output.xt
find / -name "*.pl" &>filelist.txt
Please note that both errors and actual output of the find command stored into a file:
cat filelist.txt
Sample outputs:
/usr/bin/rsyslog-recover-qi.pl /usr/lib/grub/i386-pc/gmodule.pl /usr/lib/firmware/dsp56k/concat-bootstrap.pl /usr/share/doc/GeoIP-1.5.0/fetch-geoipdata-city.pl /usr/share/doc/GeoIP-1.5.0/fetch-geoipdata.pl /usr/share/doc/postfix-2.10.1/examples/smtpd-policy/greylist.pl find: ‘/usr/share/polkit-1/rules.d’: Permission denied find: ‘/usr/libexec/initscripts/legacy-actions/auditd’: Permission denied
Summary
| Command | Description/Purpose |
|---|---|
| command 2>filename | Redirect stderr to filename |
| command >output.txt 2>error.log cat output.txt error.txt |
Redirect stderr to file named error.log and stdout to file named output.txt |
| command &> filename | Redirect stderr and stdout to filename |
| command 2>&- | Just suppress error messages. No file created. No error message displayed on screen |
| command 1>&2 Sample code: die(){ echo "$1" 1>&2 # add exit code 1 i.e. an unspecified error exit 1 } die "File not found" Another example: error_print() { local m=$1 local c=${2-1} echo >&2 -e "${m-}" exit "$code" } error_print "Failed to create directory" 2 [ ! -f "$CONFIG_FILE" ] && error_print "${CONFIG_FILE} not found." |
Redirect error messages to standard output. Useful in shell script when you need to forcefully display error messages on screen |
For more info see bash man page online or read it at the cli by using the man command:
man bash
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| Category | List of Unix and Linux commands |
|---|---|
| File Management | cat |
| Firewall | Alpine Awall • CentOS 8 • OpenSUSE • RHEL 8 • Ubuntu 16.04 • Ubuntu 18.04 • Ubuntu 20.04 |
| Network Utilities | dig • host • ip • nmap |
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| WireGuard VPN | Alpine • CentOS 8 • Debian 10 • Firewall • Ubuntu 20.04 |
The numbers are actually file descriptors.
yes they are. thanks for comment.
Hi, Vitek!
It seems there is a mistake in your article, specifically the fifth example in your conclusive summarizing table is wrong in my opinion. I had tried to post you a reply, but it turned to be too long and the site refused to accept it. So I designed it as a text file, zipped it as a tar archive and uploaded to a filehosting. Please take some time, download it and read my message. Perhaps I’m wrong, still it seems to me, I’ve found a flaw in your article.
I can’t find your zip file. However, I updated my 5th example.
Hi, Vivek!
I have read your article “How to Redirect Standard Error in Bash” on the link https://www.cyberciti.biz/faq/how-to-redirect-standard-error-in-bash/ dedicated to the redirection of standard input/output streams,
however I couldn’t understand the meaning of your last instruction in the article and the example below.
Redirect error messages to standard output. Useful in shell script when you need
to forcefully display error messages on screen
die(){ echo "$1" 2>&1 } die "File not found"The 2>&1 command really redirects stderr to stdout, but it seems in most cases this construction won’t work as it was thought out. In your example you redirect the stderr of echo, but echo always sends its output to stdout, not stderr, thus this redirection will take no effect and the “File not found” message will be sent to stdout any case. On the other hand, if the standard output has been already redirected to some file, say “msg.txt”, the 2>&1 construction will redirect stderr to the same “msg.txt” file and put all your error messages together with normal output.
I think what you really meant was the construction 1>&2 which redirects stdout to stderr. Then the standard output of the script may be redirected to some file or a pipe, still the error messages will appear on the screen or a terminal. So your instruction possibly should be rewritten in some way like:
~~~BEGIN~~~
Redirect error messages from standard output to standard error stream. Useful in a shell script when you need to forcefully display error messages on the screen, no matter was the script’s output redirected to a file or a pipe or not. The construction is useful with commands like echo or printf, which always print the text to the standard output, and makes them print error messages to the standard error stream.
die(){ echo "$1" 1>&2 } die "File not found"Per contra, the 2>&1 operator is useful when you want your script’s or program’s error messages to get to the same stream as normal output. In that case it must be preceded by the stdout redirection, otherwise it won’t work. For example,
As far as I know, it’s a common practice to use in modern shells a brief simplified notation:
instead of the old traditional
but the meaning of both notations (modern simplified and the old verbose one)
is quite the same. Please look through my post and comment it. I’m not a shell programming expert, so I may be wrong, but it seems I’ve found a small mistake in your article.
Thanks. I fixed the page. I appreciate your feedback.