Related
For documentation purposes I am trying to execute a shell script in a way that it looks as you typed it by hand in an interactive shell.
Script:
x=123
echo $x
Then execute:
PS4="$PS1"
set -x -v
. ./demo
Output:
. ./demo
user#host:~/tmp$ . ./demo
x=123
user#host:~/tmp$ x=123
echo $x
user#host:~/tmp$ echo 123
123
Desired output:
user#host:~/tmp$ x=123
user#host:~/tmp$ echo $x
123
It does not have to be bash. Any solution that simulates an interactive session is welcome.
How can I achieve the desired result?
Use script. It will record your session including timing information and play it back for you. eg:
$ script -r output /bin/sh
Script started, output file is output
sh-3.2$ x=123
sh-3.2$ echo "$x"
123
sh-3.2$ exit
exit
Script done, output file is output
$ script -p output
Script started on Sat Oct 29 20:00:16 2022
sh-3.2$ x=123
sh-3.2$ echo "$x"
123
sh-3.2$ exit
exit
Script done on Sat Oct 29 20:00:22 2022
In the above, the first section is entered interactively, and then played back with script -p
A pretty simple solution is to start an interactive shell and redirect your script file to the input stream:
bash -i </path/to/script-file
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?
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.
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.
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 ;-)
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
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.
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.
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
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.
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.
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.
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.
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).
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.
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.
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
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.
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!
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
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.
It looks like set -v doesn't work when in a subshell? Why?
(set -x does work in a subshell, but the output is a bit busier).
$ cat foo.sh
set -v
date
set +v > /dev/null 2>&1
$ bash foo.sh
date
Fri Mar 3 14:52:34 PST 2017
set +v > /dev/null 2>&1
$ cat foo_subshell.sh
(
set -v
date
)
$ bash foo_subshell.sh
Fri Mar 3 14:52:42 PST 2017
The documentation says:
-v Print shell input lines as they are read.
When you write a subshell in parentheses, the entire subshell is read, then it's executed. So the set -v command isn't executed until after all the commands are read, so it's too late for it to print them.
The script utility works like this:
$ script
Script started, file is typescript
$ ls
2 bin doubleline new1 play typescript
alok core filelist output pslist unix
$ ps
PID TTY TIME CMD
28149 pts/7 0:00 ksh.ms
$
Script done, file is typescript
After this contents of the typescript file are:
$ cat typescript
Script started on Wed Sep 07 05:56:26 2011
$ ls
2 bin doubleline new1 play typescript
alok core filelist output pslist unix
$ ps
PID TTY TIME CMD
28149 pts/7 0:00 ksh.ms
$
script done on Wed Sep 07 05:56:33 2011
$
I want to copy this behaviour with using other commands and I/O redirections.
The solution must be a one line command (may include pipelined commands).
Any help would be great, like if anyone can tell how can we redirect stdin, stdout and stderr to some file while all the data is still on the terminal.
$tee -a typescript | sh -i 2>&1 | tee -a typescript
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?
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.
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.
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 ;-)
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
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.
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.
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
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.
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.
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.
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.
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.
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).
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.
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
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.
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!
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
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.