What does " 2>&1 " mean?

Question

To combine stderr and stdout into the stdout stream, we append this to a command:

2>&1

e.g. to see the first few errors from compiling g++ main.cpp:

g++ main.cpp 2>&1 | head

What does 2>&1 mean, in detail?

Stopper

@dbr I don't think it's just bash - I believe it's a bourne shell thing; hence sh, bash, ksh, ash, dash, etc. – Copeck

This is part of the redirection paragraph describing POSIX-compliant shells, or POSIX shell for short. ksh is a POSIX shell for example. See:pubs.opengroup.org/onlinepubs/009695399/utilities/… – Adulthood

This construct also works on Windows. – Socket

It's generally better doing 2>&1 than 2>/dev/null ;-) – Dispatcher

I thought I'd mention that |& is shorthand for 2>&1 | if you're using zsh. I can't speak to whether that applies to other bourne-like shells or if it's a zsh only feature. – Kenwrick

@Kenwrick |& is also in Bash, great feature. Not POSIX it seems though (?) – Solicitor

you can check this answer https://mcmap.net/q/22509/-what-is-dev-null-2-gt-amp-1-duplicate/… – Didache

@Kenwrick this is definitely needed in tcsh as 2>^1 doesn't work there – Scurlock

Does this answer your question? What is /dev/null 2>&1? – Didache

There's also the version without any file descriptors >& (unix.stackexchange.com/questions/521596/…). How does that work? – Denticle

Answer 1

File descriptor 1 is the standard output (stdout).
File descriptor 2 is the standard error (stderr).

At first, 2>1 may look like a good way to redirect stderr to stdout. However, it will actually be interpreted as "redirect stderr to a file named 1".

& indicates that what follows and precedes is a file descriptor, and not a filename. Thus, we use 2>&1. Consider >& to be a redirect merger operator.

Closefisted

but then shouldn't it rather be &2>&1? – Rainstorm

@Dominik: Nope, & is only interpreted to mean "file descriptor" in the context of redirections. Writing command &2>& is parsed as command & and 2>&1, i.e. "run command in the background, then run the command 2 and redirect its stdout into its stdout". – Eboh

Why did they pick such arcane stuff as this? Just curious. – Uzziel

But how would you redirect stderr to a file named '&1'? – Heterogenesis

@Martin: 2>'&1' – Vikiviking

any idea why this interacts with grep highlighting? nginx -V 2>&1 | grep auth_request, highlights the word auth_request in red but nginx -V | grep auth_request doesn't – Aby

so & meanings file descriptor and also go to the background?? Linux needs to stop using the same symbol too confusing!! – Piassava

In case this helps for mnemonics for someone, the way I think of this connects with the C-language & operator, "address of" -- so, 2>&1 as a whole I read in my head as something resembling "(file descriptor) 2 redirects to the address of (file descriptor) 1". I.e. to where fd1 is currently going, as of the time the shell reads that 4-character sequence. (Because it can change. For fun, try: ((echo 1/stdout; echo 2/stderr >&2) 2>&1 > /dev/tty) | grep --color . -- it'll colorize what started out as standard error ("2/stderr").) – Peony

What happens if I do 2> &1 (space after >) instead of 2>&1? I'm not trying it out myself over the fear of breaking stuff. – Scotism

@Scotism It just gives an error message. In bash: foo 2>&1 outputs "-bash: foo: command not found", and foo 2> &1 outputs "-bash: syntax error near unexpected token '&'". Shells are often picky about whitespace around symbols. @Aaron Franke Yes, foo 2>\&1 creates a new file named "&1", containing the above error text. – Ringed

Re "Why did they pick such arcane stuff as this?", It's actually good to use related tokens for related things. In bash, < redirects from a file; <& redirects from a fd; <<... redirects from a here-doc, <(...) redirects from a process; and <<< ... redirects from a string literal. – Hewet

@Uzziel Exactly. Everybody is telling me "Magic constants are bad in programming, avoid them, use names instead". And we have hard-coded file descriptor integers in the core of an operating system. Yack. Why couldn't they just use keywords like stderr > stdout? Another inconsistency - > usually means "replace". So, 2>&1 can be confused with replacing all output with error output. Why not use >> for appending, as we do for files? So, I vote for stderr >> stdout. – Different

As a mnemonic, if you know C, following pointer arithmetics, you can think of it as "contents of 2 (2) go (>) to where 1 is, i.e. to the memory address of 1 (&1)". – Erigena

@Different you can use 2>> /dev/stdout instead of 2>&1. I don't think there is a way to replace the 2>> with a keyword though since it would not be distinguishable from an argument to the command. You should use >> now because in this case the output file will be opened again instead of just duplicating the file descriptor, and > truncates the file after opening (which you don't want if stdout is redirected to a log file). – Alternately

@Different They do have names. In POSIX they are defined in <unistd.h> STDIN_FILENO,(0) STDOUT_FILENO(1) STDERR_FILENO.(2). As for the ">" symbol, you are treating the side effect of the command as the primary. ">" redirects. Redirection to an existing file truncates and then replaces. The symbol is used consistently. Also, std(in|out|err) are streams (of bytes ). You'd expect them to mix, when you copy them together. Nothing arcane here, no need to use magic constants, just review the definitions in the system header files. It's all there. – Tananarive

@Uzziel Optimization of typing & in general - it was a common trait for text oriented OSs like Linux&Unix. The syntax was meant to be as short as possible but understandable to the enlightened. Don't forget when it was created & lookup the old ASM commands/Assembly language. In the long run, this created various mischiefs like grep for regexp search, mv for rename, the infamous 1-letter options nobody knows what they do like tar -cvzf => in light of all of that 1=stdout and 2=stderr are things I have the least problem with :D 😂 – Skittle

@Uzziel Btw if anybody wonders tar -cvzf is a real-life command for "zipping" files - tar means "tape archiver" (that speaks for itself, yes stuff was being archived on literal (magnetic) tapes) c means "create a new archive", "v" means "verbose"/"verbosely list files processed", z goes for "filter the archive through gzip" and finally f - "use archive file or device ARCHIVE". Nowadays these can be written as tar --create --gzip --verbose --file=ArchiveName.tar.gz /what/to/gzip = please use long option names wherever possible :-) – Skittle

But @MartínFixman, why would you name a file &1 :-D ? – Roush

Answer 2

To redirect stdout to file.txt:

echo test > file.txt

This is equivalent to:

echo test 1> file.txt

To redirect stderr to file.txt:

echo test 2> file.txt

So >& is the syntax to redirect a stream to another file descriptor:

• 0 is stdin
• 1 is stdout
• 2 is stderr

To redirect stdout to stderr:

echo test 1>&2   # equivalently, echo test >&2

To redirect stderr to stdout:

echo test 2>&1

Thus, in 2>&1:

• 2> redirects stderr to an (unspecified) file.
• &1 redirects stderr to stdout.

Intelligence

@Intelligence cmd 2>&1 >>file does not redirect stderr to the file, but cmd >> file 2>&1 does. Order matters. In the first case, stderr is redirected to the stdout of the shell (possibly a tty if the command is entered interactively), and then stdout is directed to the file. In the second case, stdout is directed to the file, and then stderr is directed to the same place. – Thou

I like the answer above, but it could be a touch clearer. "2>&1" redirects stderr to the target of stdout. So if you have something like "ls -l >> directoryContents 2>&1" The result will be a file named directoryContents will have the contents of the working directory appended to it. If there are any errors in execution: the error messages will also be appended to the directoryContents file, as they occur. – Nellynelms

Is 0(or 1,2)>&0(or 1,2) like an option to control the output? Is echo test >test.log 2>&1 same as echo test 2>&1 >test.log? – Linsk

for completeness (as top answer) - "redirect (and append) stdout and stderr to file and terminal and get proper exit status" command 2>&1 | tee -a file.txt – Necrology

Answer 3

Some tricks about redirection

Some syntax particularity about this may have important behaviours. There is some little samples about redirections, STDERR, STDOUT, and arguments ordering.

1 - Overwriting or appending?

Symbol > means redirection.

• > means send to as a whole completed file, overwriting target if exist (see noclobber bash feature at #3 later).
• >> means send in addition to would append to target if exist.

In any case, the file would be created if they not exist.

2 - The shell command line is order dependent!!

For testing this, we need a simple command which will send something on both outputs:

$ ls -ld /tmp /tnt
ls: cannot access /tnt: No such file or directory
drwxrwxrwt 118 root root 196608 Jan  7 11:49 /tmp

$ ls -ld /tmp /tnt >/dev/null
ls: cannot access /tnt: No such file or directory

$ ls -ld /tmp /tnt 2>/dev/null
drwxrwxrwt 118 root root 196608 Jan  7 11:49 /tmp

(Expecting you don't have a directory named /tnt, of course ;). Well, we have it!!

So, let's see:

$ ls -ld /tmp /tnt >/dev/null
ls: cannot access /tnt: No such file or directory

$ ls -ld /tmp /tnt >/dev/null 2>&1

$ ls -ld /tmp /tnt 2>&1 >/dev/null
ls: cannot access /tnt: No such file or directory

The last command line dumps STDERR to the console, and it seem not to be the expected behaviour... But...

If you want to make some post filtering about standard output, error output or both:

$ ls -ld /tmp /tnt | sed 's/^.*$/<-- & --->/'
ls: cannot access /tnt: No such file or directory
<-- drwxrwxrwt 118 root root 196608 Jan  7 12:02 /tmp --->

$ ls -ld /tmp /tnt 2>&1 | sed 's/^.*$/<-- & --->/'
<-- ls: cannot access /tnt: No such file or directory --->
<-- drwxrwxrwt 118 root root 196608 Jan  7 12:02 /tmp --->

$ ls -ld /tmp /tnt >/dev/null | sed 's/^.*$/<-- & --->/'
ls: cannot access /tnt: No such file or directory

$ ls -ld /tmp /tnt >/dev/null 2>&1 | sed 's/^.*$/<-- & --->/'

$ ls -ld /tmp /tnt 2>&1 >/dev/null | sed 's/^.*$/<-- & --->/'
<-- ls: cannot access /tnt: No such file or directory --->

Notice that the last command line in this paragraph is exactly same as in previous paragraph, where I wrote seem not to be the expected behaviour (so, this could even be an expected behaviour).

Well, there is a little tricks about redirections, for doing different operation on both outputs:

$ ( ls -ld /tmp /tnt | sed 's/^/O: /' >&9 ) 9>&2  2>&1  | sed 's/^/E: /'
O: drwxrwxrwt 118 root root 196608 Jan  7 12:13 /tmp
E: ls: cannot access /tnt: No such file or directory

Note: &9 descriptor would occur spontaneously because of ) 9>&2.

Addendum: nota! With the new version of bash (>4.0) there is a new feature and more sexy syntax for doing this kind of things:

$ ls -ld /tmp /tnt 2> >(sed 's/^/E: /') > >(sed 's/^/O: /')
O: drwxrwxrwt 17 root root 28672 Nov  5 23:00 /tmp
E: ls: cannot access /tnt: No such file or directory

And finally for such a cascading output formatting:

$ ((ls -ld /tmp /tnt |sed 's/^/O: /' >&9 ) 2>&1 |sed 's/^/E: /') 9>&1| cat -n
     1  O: drwxrwxrwt 118 root root 196608 Jan  7 12:29 /tmp
     2  E: ls: cannot access /tnt: No such file or directory

Addendum: nota! Same new syntax, in both ways:

$ cat -n <(ls -ld /tmp /tnt 2> >(sed 's/^/E: /') > >(sed 's/^/O: /'))
     1  O: drwxrwxrwt 17 root root 28672 Nov  5 23:00 /tmp
     2  E: ls: cannot access /tnt: No such file or directory

Where STDOUT go through a specific filter, STDERR to another and finally both outputs merged go through a third command filter.

2b - Using |& instead

Syntax command |& ... could be used as an alias for command 2>&1 | .... Same rules about command line order applies. More details at What is the meaning of operator |& in bash?

3 - A word about noclobber option and >| syntax

That's about overwriting:

While set -o noclobber instruct bash to not overwrite any existing file, the >| syntax let you pass through this limitation:

$ testfile=$(mktemp /tmp/testNoClobberDate-XXXXXX)

$ date > $testfile ; cat $testfile
Mon Jan  7 13:18:15 CET 2013

$ date > $testfile ; cat $testfile
Mon Jan  7 13:18:19 CET 2013

$ date > $testfile ; cat $testfile
Mon Jan  7 13:18:21 CET 2013

The file is overwritten each time, well now:

$ set -o noclobber

$ date > $testfile ; cat $testfile
bash: /tmp/testNoClobberDate-WW1xi9: cannot overwrite existing file
Mon Jan  7 13:18:21 CET 2013

$ date > $testfile ; cat $testfile
bash: /tmp/testNoClobberDate-WW1xi9: cannot overwrite existing file
Mon Jan  7 13:18:21 CET 2013

Pass through with >|:

$ date >| $testfile ; cat $testfile
Mon Jan  7 13:18:58 CET 2013

$ date >| $testfile ; cat $testfile
Mon Jan  7 13:19:01 CET 2013

Unsetting this option and/or inquiring if already set.

$ set -o | grep noclobber
noclobber           on

$ set +o noclobber

$ set -o | grep noclobber
noclobber           off

$ date > $testfile ; cat $testfile
Mon Jan  7 13:24:27 CET 2013

$ rm $testfile

4 - Last trick and more...

For redirecting both output from a given command, we see that a right syntax could be:

$ ls -ld /tmp /tnt >/dev/null 2>&1

for this special case, there is a shortcut syntax: &> ... or >&

$ ls -ld /tmp /tnt &>/dev/null

$ ls -ld /tmp /tnt >&/dev/null

Nota: if 2>&1 exist, 1>&2 is a correct syntax too:

$ ls -ld /tmp /tnt 2>/dev/null 1>&2

4b- Now, I will let you think about:

$ ls -ld /tmp /tnt 2>&1 1>&2  | sed -e s/^/++/
++/bin/ls: cannot access /tnt: No such file or directory
++drwxrwxrwt 193 root root 196608 Feb  9 11:08 /tmp/

$ ls -ld /tmp /tnt 1>&2 2>&1  | sed -e s/^/++/
/bin/ls: cannot access /tnt: No such file or directory
drwxrwxrwt 193 root root 196608 Feb  9 11:08 /tmp/

4c- If you're interested in more information

You could read the fine manual by hitting:

man -Len -Pless\ +/^REDIRECTION bash

in a bash console ;-)

Dispatcher

Further reading: If you liked this, you may apreciate: How redirection abuse could give strange behaviours – Dispatcher

@fabs If you liked this, maybe would you like Open new window for input/output and/or Redirections from script himself – Dispatcher

Answer 4

I found this brilliant post on redirection: All about redirections

Redirect both standard output and standard error to a file

$ command &>file

This one-liner uses the &> operator to redirect both output streams - stdout and stderr - from command to file. This is Bash's shortcut for quickly redirecting both streams to the same destination.

Here is how the file descriptor table looks like after Bash has redirected both streams:

As you can see, both stdout and stderr now point to file. So anything written to stdout and stderr gets written to file.

There are several ways to redirect both streams to the same destination. You can redirect each stream one after another:

$ command >file 2>&1

This is a much more common way to redirect both streams to a file. First stdout is redirected to file, and then stderr is duplicated to be the same as stdout. So both streams end up pointing to file.

When Bash sees several redirections it processes them from left to right. Let's go through the steps and see how that happens. Before running any commands, Bash's file descriptor table looks like this:

Now Bash processes the first redirection >file. We've seen this before and it makes stdout point to file:

Next Bash sees the second redirection 2>&1. We haven't seen this redirection before. This one duplicates file descriptor 2 to be a copy of file descriptor 1 and we get:

Both streams have been redirected to file.

However be careful here! Writing

command >file 2>&1

is not the same as writing:

$ command 2>&1 >file

The order of redirects matters in Bash! This command redirects only the standard output to the file. The stderr will still print to the terminal. To understand why that happens, let's go through the steps again. So before running the command, the file descriptor table looks like this:

Now Bash processes redirections left to right. It first sees 2>&1 so it duplicates stderr to stdout. The file descriptor table becomes:

Now Bash sees the second redirect, >file, and it redirects stdout to file:

Do you see what happens here? Stdout now points to file, but the stderr still points to the terminal! Everything that gets written to stderr still gets printed out to the screen! So be very, very careful with the order of redirects!

Also note that in Bash, writing

$ command &>file

is exactly the same as:

$ command >&file

Redouble

The last two are different if "command" ends in a number, as then that is taken as optional file descriptor for >& – Cl

Very nice drawing and explanation! Could you elaborate what "duplicate" really means? You mentioned, "This one [2>&1] duplicates file descriptor 2 to be a copy of file descriptor 1". It sounds like stderr gets duplicated to stdout. But if it is the case, should I also see err though /dev/tty0? – Lob

This is a very nice explanation with visual. If I became the one who ask this question, I will mark this as accepted answer. – Howard

Answer 5

The numbers refer to the file descriptors (fd).

• Zero is stdin
• One is stdout
• Two is stderr

2>&1 redirects fd 2 to 1.

This works for any number of file descriptors if the program uses them.

You can look at /usr/include/unistd.h if you forget them:

/* Standard file descriptors.  */
#define STDIN_FILENO    0   /* Standard input.  */
#define STDOUT_FILENO   1   /* Standard output.  */
#define STDERR_FILENO   2   /* Standard error output.  */

That said I have written C tools that use non-standard file descriptors for custom logging so you don't see it unless you redirect it to a file or something.

Flume

Is it okay to just use your own "non-standard file descriptors"? How do you know there isn't an open file with the same FD? – Dramatize

Answer 6

That construct sends the standard error stream (stderr) to the current location of standard output (stdout) - this currency issue appears to have been neglected by the other answers.

You can redirect any output handle to another by using this method but it's most often used to channel stdout and stderr streams into a single stream for processing.

Some examples are:

# Look for ERROR string in both stdout and stderr.
foo 2>&1 | grep ERROR

# Run the less pager without stderr screwing up the output.
foo 2>&1 | less

# Send stdout/err to file (with append) and terminal.
foo 2>&1 |tee /dev/tty >>outfile

# Send stderr to normal location and stdout to file.
foo >outfile1 2>&1 >outfile2

Note that that last one will not direct stderr to outfile2 - it redirects it to what stdout was when the argument was encountered (outfile1) and then redirects stdout to outfile2.

This allows some pretty sophisticated trickery.

Winterkill

Although that last example would be much clearer as: foo >outfile2 2>outfile1 – Chummy

Clearer, yes, but that wouldn't show the "positional" nature of redirection. The example is contrived since it's not usually useful to do this in a single line - the method becomes really useful when different parties are responsible for the different parts of the redirection. For example, when a script does one bit of redirection and you run it with another bit. – Winterkill

I just realized that the last example also resolves a long standing confusion I had regarding why this: some_program 2>&1 > /dev/null does not work like this: some_program > /dev/null 2>&1. – Gullible

Your comment about the last example is worth its letters in gold :-) I never thought that these redirectional arguments are positional... I think this is pretty important to know. – Tupler

Answer 7

I found this very helpful if you are a beginner read this

Update:
In Linux or Unix System there are two places programs send output to: Standard output (stdout) and Standard Error (stderr).You can redirect these output to any file.

Like if you do this

ls -a > output.txt

Nothing will be printed in console all output (stdout) is redirected to output file.

And if you try print the content of any file that does not exits means output will be an error like if you print test.txt that not present in current directory

cat test.txt > error.txt

Output will be

cat: test.txt :No such file or directory

But error.txt file will be empty because we redirecting the stdout to a file not stderr.

so we need file descriptor( A file descriptor is nothing more than a positive integer that represents an open file. You can say descriptor is unique id of file) to tell shell which type of output we are sending to file .In Unix /Linux system 1 is for stdout and 2 for stderr.

so now if you do this

ls -a 1> output.txt means you are sending Standard output (stdout) to output.txt.

and if you do this

cat test.txt 2> error.txt means you are sending Standard Error (stderr) to error.txt .

&1 is used to reference the value of the file descriptor 1 (stdout).

Now to the point 2>&1 means “Redirect the stderr to the same place we are redirecting the stdout”

Now you can do this
<br

cat maybefile.txt > output.txt 2>&1

both Standard output (stdout) and Standard Error (stderr) will redirected to output.txt.

Thanks to Ondrej K. for pointing out

Entremets

+1 for "&1 is used to reference the value of the file descriptor 1 (stdout).". I always wondered why it wasn't just 2>1 – Spectator

Answer 8

2 is the console standard error.

1 is the console standard output.

This is the standard Unix, and Windows also follows the POSIX.

E.g. when you run

perl test.pl 2>&1

the standard error is redirected to standard output, so you can see both outputs together:

perl test.pl > debug.log 2>&1

After execution, you can see all the output, including errors, in the debug.log.

perl test.pl 1>out.log 2>err.log

Then standard output goes to out.log, and standard error to err.log.

I suggest you to try to understand these.

Reprove

The second sample is wrong: as order precedence STDERR is redirected to STDOUT, only default STDOUT will be written to debug.log (not STDERR) see my answer (the paragraph #2)! To ensure both to be redirected to same file, you have to invert redirections directives: perl test.pl > debug.log 2>&1 – Dispatcher

Answer 9

2>&1 is a POSIX shell construct. Here is a breakdown, token by token:

---

2: "Standard error" output file descriptor.

>&: Duplicate an Output File Descriptor operator (a variant of Output Redirection operator >). Given [x]>&[y], the file descriptor denoted by x is made to be a copy of the output file descriptor y.

1 "Standard output" output file descriptor.

The expression 2>&1 copies file descriptor 1 to location 2, so any output written to 2 ("standard error") in the execution environment goes to the same file originally described by 1 ("standard output").

---

Further explanation:

File Descriptor: "A per-process unique, non-negative integer used to identify an open file for the purpose of file access."

Standard output/error: Refer to the following note in the Redirection section of the shell documentation:

Open files are represented by decimal numbers starting with zero. The largest possible value is implementation-defined; however, all implementations shall support at least 0 to 9, inclusive, for use by the application. These numbers are called "file descriptors". The values 0, 1, and 2 have special meaning and conventional uses and are implied by certain redirection operations; they are referred to as standard input, standard output, and standard error, respectively. Programs usually take their input from standard input, and write output on standard output. Error messages are usually written on standard error. The redirection operators can be preceded by one or more digits (with no intervening characters allowed) to designate the file descriptor number.

Mev

Answer 10

To answer your question: It takes any error output (normally sent to stderr) and writes it to standard output (stdout).

This is helpful with, for example 'more' when you need paging for all output. Some programs like printing usage information into stderr.

To help you remember

• 1 = standard output (where programs print normal output)
• 2 = standard error (where programs print errors)

"2>&1" simply points everything sent to stderr, to stdout instead.

I also recommend reading this post on error redirecting where this subject is covered in full detail.

Duron

Answer 11

From a programmer's point of view, it means precisely this:

dup2(1, 2);

See the man page.

Understanding that 2>&1 is a copy also explains why ...

command >file 2>&1

... is not the same as ...

command 2>&1 >file

The first will send both streams to file, whereas the second will send errors to stdout, and ordinary output into file.

Alanna

Answer 12

People, always remember paxdiablo's hint about the current location of the redirection target... It is important.

My personal mnemonic for the 2>&1 operator is this:

• Think of & as meaning 'and' or 'add' (the character is an ampers-and, isn't it?)
• So it becomes: 'redirect 2 (stderr) to where 1 (stdout) already/currently is and add both streams'.

The same mnemonic works for the other frequently used redirection too, 1>&2:

• Think of & meaning and or add... (you get the idea about the ampersand, yes?)
• So it becomes: 'redirect 1 (stdout) to where 2 (stderr) already/currently is and add both streams'.

And always remember: you have to read chains of redirections 'from the end', from right to left (not from left to right).

Rebeccarebecka

Answer 13

Redirecting Input

Redirection of input causes the file whose name results from the expansion of word to be opened for reading on file descriptor n, or the standard input (file descriptor 0) if n is not specified.

The general format for redirecting input is:

[n]<word

Redirecting Output

Redirection of output causes the file whose name results from the expansion of word to be opened for writing on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created; if it does exist it is truncated to zero size.

The general format for redirecting output is:

[n]>word

Moving File Descriptors

The redirection operator,

[n]<&digit-

moves the file descriptor digit to file descriptor n, or the standard input (file descriptor 0) if n is not specified. digit is closed after being duplicated to n.

Similarly, the redirection operator

[n]>&digit-

moves the file descriptor digit to file descriptor n, or the standard output (file descriptor 1) if n is not specified.

Ref:

man bash

Type /^REDIRECT to locate to the redirection section, and learn more...

An online version is here: 3.6 Redirections

PS:

Lots of the time, man was the powerful tool to learn Linux.

Parenteral

Answer 14

unix_commands 2>&1

This is used to print errors to the terminal.

• When errors are produced, they are written to the "standard error" buffer at memory address &2, and 2 references and streams from that buffer.
• When outputs are produced, they are written to the "standard output" buffer at memory address &1, and 1 references and streams from that buffer.

So going back to the command. Anytime the program unix_commands produces an error, it writes that into the errors buffer. So we create a pointer to that buffer 2, and redirect > the errors into the outputs buffer &1. At this point we're done, because anything in the outputs buffer is read and printed by the terminal.

Empty

Answer 15

Provided that /foo does not exist on your system and /tmp does…

$ ls -l /tmp /foo

will print the contents of /tmp and print an error message for /foo

$ ls -l /tmp /foo >/dev/null

will send the contents of /tmp to /dev/null and print an error message for /foo

$ ls -l /tmp /foo 1>/dev/null

will do exactly the same (note the 1)

$ ls -l /tmp /foo 2>/dev/null

will print the contents of /tmp and send the error message to /dev/null

$ ls -l /tmp /foo 1>/dev/null 2>/dev/null

will send both the listing as well as the error message to /dev/null

$ ls -l /tmp /foo >/dev/null 2>&1

is shorthand

Kamenskuralski

Answer 16

This is just like passing the error to the stdout or the terminal.

That is, cmd is not a command:

$cmd 2>filename
cat filename

command not found

The error is sent to the file like this:

2>&1

Standard error is sent to the terminal.

Stodder

Answer 17

0 for input, 1 for stdout and 2 for stderr.

One Tip: somecmd >1.txt 2>&1 is correct, while somecmd 2>&1 >1.txt is totally wrong with no effect!

Mongolism

Answer 18

You need to understand this in terms of pipe.

$ (whoami;ZZZ) 2>&1  | cat
logan
ZZZ: command not found

As you can see both stdout and stderr of LHS of pipe is fed into the RHS (of pipe).

This is the same as

$ (whoami;ZZZ) |& cat
logan
ZZZ: command not found

Electroshock

Answer 19

Note that 1>&2 cannot be used interchangeably with 2>&1.

Imagine your command depends on piping, for example:
docker logs 1b3e97c49e39 2>&1 | grep "some log"
grepping will happen across both stderr and stdout since stderr is basically merged into stdout.

However, if you try:
docker logs 1b3e97c49e39 1>&2 | grep "some log",
grepping will not really search anywhere at all because Unix pipe is connecting processes via connecting stdout | stdin, and stdout in the second case was redirected to stderr in which Unix pipe has no interest.

Skellum

Your grep example only demonstrates how grep does not get stderr as input in Unix pipe. This does not however show that 1>&2 and 2>&1 cannot be used in an interchangable way. Do you have any other examples that would clarify your initial thesis? – Oxalis