• stdin – Get input from keyboard or program. In other words data going into a program.
• stdout – Write information on screen or file.
• stderr – Show error message on screen or file

Understanding I/O streams numbers

The Unix / Linux standard I/O streams with numbers:

Redirecting output

Say you want to save output of the date command to a file. Try:
$ command > output.txt
$ date > today.txt
Use the cat command to view contain of today.txt file, run:
$ cat today.txt

In above example, a file called ‘today.txt’ will be created and it will contain date (what you would see on the screen) if you type the command ‘date’ and execute it. Please note that above command also creates the file today.txty if not present, otherwise overwrites it. If you wish to add/append try the following syntax:
$ command1 >> filename
$ command2 >> filename
$ ls -l >> output.txt
$ date >> output.txt
In short we can redirect stdout to a file simply using the > or >> symbol.

Redirecting the standard error stream to a file

The following will redirect program error message to a file called error.log:
$ program-name 2> error.log
$ command1 2> error.log
For example, use the grep command for recursive search in the $HOME directory and redirect all errors (stderr) to a file name grep-errors.txt as follows:
$ grep -R 'MASTER' $HOME 2> /tmp/grep-errors.txt
$ cat /tmp/grep-errors.txt
Sample outputs:

grep: /home/vivek/.config/google-chrome/SingletonSocket: No such device or address
grep: /home/vivek/.config/google-chrome/SingletonCookie: No such file or directory
grep: /home/vivek/.config/google-chrome/SingletonLock: No such file or directory
grep: /home/vivek/.byobu/.ssh-agent: No such device or address

Redirecting the standard error (stderr) and stdout to file

Use the following syntax:
$ command-name &>file
We can als use the following syntax:
$ command > file-name 2>&1
We can write both stderr and stdout to two different files too. Let us try out our previous grep command example:
$ grep -R 'MASTER' $HOME 2> /tmp/grep-errors.txt 1> /tmp/grep-outputs.txt
$ cat /tmp/grep-outputs.txt

Redirecting stderr to stdout to a file or another command

Here is another useful example where both stderr and stdout sent to the more command instead of a file:
# find /usr/home -name .profile 2>&1 | more

Redirect stderr to stdout

Use the command as follows:
$ command-name 2>&1
$ command-name > file.txt 2>&1
## bash only ##
$ command2 &> filename
$ sudo find / -type f -iname ".env" &> /tmp/search.txt
Redirection takes from left to right. Hence, order matters. For example:
command-name 2>&1 > file.txt ## wrong ##
command-name > file.txt 2>&1 ## correct ##

How to redirect stderr to stdout in Bash script

A sample shell script used to update VM when created in the AWS/Linode server:

#!/usr/bin/env bash
# Author - nixCraft under GPL v2.x+
# Debian/Ubuntu Linux script for EC2 automation on first boot
# ------------------------------------------------------------
# My log file - Save stdout to $LOGFILE
LOGFILE="/root/logs.txt"
 
# My error file - Save stderr to $ERRFILE
ERRFILE="/root/errors.txt"
 
# Start it 
printf "Starting update process ... \n" 1>"${LOGFILE}"
 
# All errors should go to error file 
apt-get -y update 2>"${ERRFILE}"
apt-get -y upgrade 2>>"${ERRFILE}"
printf "Rebooting cloudserver ... \n" 1>>"${LOGFILE}"
shutdown -r now 2>>"${ERRFILE}"

Our last example uses the exec command and FDs along with trap and custom bash functions:

#!/bin/bash
# Send both stdout/stderr to a /root/aws-ec2-debian.log file
# Works with Ubuntu Linux too.
# Use exec for FD and trap it using the trap
# See bash man page for more info
# Author:  nixCraft under GPL v2.x+
# ---------------------------------------------
exec 3>&1 4>&2
trap 'exec 2>&4 1>&3' 0 1 2 3
exec 1>/root/aws-ec2-debian.log 2>&1
 
# log message
log(){
	local m="$@"
	echo ""
	echo "*** ${m} ***"
	echo ""
}
 
log "$(date) @ $(hostname)"
## Install stuff ##
log "Updating up all packages"
export DEBIAN_FRONTEND=noninteractive
apt-get -y clean
apt-get -y update
apt-get -y upgrade
apt-get -y --purge autoremove
 
## Update sshd config ##
log "Configuring sshd_config"
sed -i'.BAK' -e 's/PermitRootLogin yes/PermitRootLogin no/g' -e 's/#PasswordAuthentication yes/PasswordAuthentication no/g'  /etc/ssh/sshd_config
 
## Hide process from other users ##
log "Update /proc/fstab to hide process from each other"
echo 'proc    /proc    proc    defaults,nosuid,nodev,noexec,relatime,hidepid=2     0     0' >> /etc/fstab
 
## Install LXD and stuff ##
log "Installing LXD/wireguard/vnstat and other packages on this box"
apt-get -y install lxd wireguard vnstat expect mariadb-server 
 
log "Configuring mysql with mysql_secure_installation"
SECURE_MYSQL_EXEC=$(expect -c "
set timeout 10
spawn mysql_secure_installation
expect \"Enter current password for root (enter for none):\"
send \"$MYSQL\r\"
expect \"Change the root password?\"
send \"n\r\"
expect \"Remove anonymous users?\"
send \"y\r\"
expect \"Disallow root login remotely?\"
send \"y\r\"
expect \"Remove test database and access to it?\"
send \"y\r\"
expect \"Reload privilege tables now?\"
send \"y\r\"
expect eof
")
 
# log to file #
echo "   $SECURE_MYSQL_EXEC   "
# We no longer need expect 
apt-get -y remove expect
 
# Reboot the EC2 VM
log "END: Rebooting requested @ $(date) by $(hostname)"
reboot

Want both stderr and stdout to the terminal and a log file too?

Try the tee command as follows:
command1 2>&1 | tee filename
Here is how to use it insider shell script too:

#!/usr/bin/env bash
{
   command1
   command2 | do_something
} 2>&1 | tee /tmp/outputs.log

Conclusion

In this quick tutorial, you learned about three file descriptors, stdin, stdout, and stderr. We can use these Bash descriptors to redirect stdout/stderr to a file or vice versa. See bash man page here: