redirecting to /dev/null

Question

I'm reading an example bash shell script:

#!/bin/bash

# This script makes a backup of my home directory.

cd /home

# This creates the archive
tar cf /var/tmp/home_franky.tar franky > /dev/null 2>&1

# First remove the old bzip2 file.  Redirect errors because this generates some if the archive
# does not exist.  Then create a new compressed file.
rm /var/tmp/home_franky.tar.bz2 2> /dev/null
bzip2 /var/tmp/home_franky.tar

# Copy the file to another host - we have ssh keys for making this work without intervention.
scp /var/tmp/home_franky.tar.bz2 bordeaux:/opt/backup/franky > /dev/null 2>&1

# Create a timestamp in a logfile.
date >> /home/franky/log/home_backup.log
echo backup succeeded >> /home/franky/log/home_backup.log

I'm trying to understand the use of /dev/null 2>&1 here. At first, I thought this script uses /dev/null in order to gracefully ignore errors, without causing the script to crash (kind of like try catch exception handling in programming languages). Because I don't see how using tar to compress a directory into a tar file could possibly cause any type of errors.

Answer 1

No, this will not prevent the script from crashing. If any errors occur in the tar process (e.g.: permission denied, no such file or directory, ...) the script will still crash.

This is because of using > /dev/null 2>&1 will redirect all your command output (both stdout and stderr) to /dev/null, meaning no outputs are printed to the terminal.

By default:

stdin  ==> fd 0
stdout ==> fd 1
stderr ==> fd 2

In the script, you use > /dev/null causing:

stdin  ==> fd 0
stdout ==> /dev/null
stderr ==> fd 2

And then 2>&1 causing:

stdin  ==> fd 0
stdout ==> /dev/null
stderr ==> stdout

Answer 2

To understand "redirecting to /dev/null" easily, write it out explicitly. Below is an example command that tries to remove a non-existent file (to simulate an error).

rm nonexisting.txt 1> /dev/null 2> /dev/null

1. is for stdout. Sends info logs to /dev/null
2. is for stderr. Sends error logs to /dev/null

Below are couple of enhancements.

Enhancement 1: You can replace 1> with just >. This is because 1 is the default stdout and you can ignore mentioning defaults.

rm nonexisting.txt > /dev/null 2> /dev/null

Enhancement 2: You can replace the 2nd file redirect (> /dev/null) with a file descriptor duplication (>& 1). This is because /dev/null is already pointed to by stdout 1.

rm nonexisting.txt 1> /dev/null 2>& 1

Enhancement 3: This is such a common operation, that many shells have a shortened form of this as a single &> operator.

rm nonexisting.txt &> /dev/null

My suggestion: Stick to the first option. Write out the commands explicitly instead of using pointers. Takes little to no extra effort but much easier to understand and explain.

Answer 3

I'm trying to understand the use of "> /dev/null 2>&1" here.

(note that I added the redirection before /dev/null in your question.)

The above would redirect the STDOUT and STDERR to /dev/null. It works by merging the STDERR into the STDOUT. (Essentially all the output from the command would be redirected to the null device.)

... without causing the script to crash (kind of like try catch exception handling in programming languages).

It's not quite like a try/catch or anything. It simply silences any sort of output (including error) from the command.

Because I don't see how using tar to compress a directory into a tar file could possibly cause any type of errors.

It could result in errors for a number of reasons, including:

• Inadequate permissions on the file(s) you're attempting to archive or on the file that you're attempting to write to
• Lack of disk space in order to create the archive

Answer 4

When you run CMD > /dev/null 2>&1

STDOUT redirects to /dev/null, and then STDERR redirects to THE ADDRESS of STDOUT, which has been set to /dev/null , consequently both STDOUT and STDERR point to /dev/null

Oppositely, when you run CMD 2>&1 >/dev/null

STDERR redirects to THE ADDRESS of STDOUT (File descriptor 1 in that moment, or /proc/self/fd/1), and then STDOUT redirects to /dev/null , but STDERR keeps redirecting to fd1!! As a result the normal output from STDOUT is discarded, but the errors coming from STDERR are still being written onto the console.

Answer 5

cat nonexistantfile.txt &>/dev/null

This redirects both STDOUT both STDIN
And is equivalent to

cat nonexistantfile.txt >/dev/null 2>&1

Answer 6

Bash I/O Redirection

---

This code:

command > filename 2>&1

• > filename redirects stdout to filename
• (2>&1) redirects stderr to stdout (now filename)

(file descriptor 1 is the default, so > is short for 1>)

Here is the ABSG explanation (Ch. 20).

Another common example:

command >>/dev/null 2>&1

redirects stderr and stdout to /dev/null ... which means to nowhere. Things sent to /dev/null are not saved, cached, or remembered in any way.

They are just sent to 'nowhere' and forgotten. This is a way of running programs and making sure they produce NO output and will never be seen on the command line or in a log file.

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I see this type of question quite a bit ... mainly because I've had to look it up myself since I haven't been coding in years. Here is some handy information from the ABSG:

"Redirection simply means capturing output from a file, command, program, or script and sending it as input to another file, command, program, or script."

2>&1 
# Redirects stderr to stdout.

command >>filename 2>&1
# Appends both stdout and stderr
#+  to the file "filename" ...

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ABSG: Advanced Bash Scripting Guide: The Chapter 20 link above is a link to the I/O redirection page of the open source tldp.org document called the Advanced Bash Scripting Guide by Mendel Cooper. It is listed as "An in-depth exploration of the art of shell scripting " and I absolutely agree. It is a terrific resource and has tons of answers to all sorts of crazy situations.

Other Valuable Resources: There are many valuable resources in the current/maintained section (in several handy formats like html, pdf, text, etc) on the Linux Documentation Project Guides page. Here are a few I have found useful:

• Bash Guide for Beginners by Machtelt Garrels
• Introduction to Linux by Machtelt Garrels
• GNU/Linux Command-Line Tools Summary by Gareth Anderson
• Linux Filesystem Hierarchy by Binh Nguyen
• Securing & Optimizing Linux (pdf only): The Ultimate Solution by Gerhard Mourani