Standard error (also known as stderr) is the default error output device. Use stderr to write all system error messages. The number 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 - To redirect both stderr and stdout (standard output):
command &> output.txt
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 2>filename | Redirect stderr to file named filename and stdout to file named output | |
| command &> filename | Redirect stderr and stdout to filename | |
| command 2>&- | Just suppress error messages. No file created. No error message displayed on screen | |
command 2>&1
| 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:
man bash
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