In the shell what does 2 1 mean

Question

In a Unix shell  if I want to combine stderr and stdout into the stdout stream for further manipulation  I can append the following on the end of my command   2 gt  amp 1   So  if I want to use head on the output from g    I can do something like this   g   lots of errors 2 gt  amp 1   head   so I can see only the first few errors    I always have trouble remembering this  and I constantly have to go look it up  and it is mainly because I don t fully understand the syntax of this particular trick   Can someone break this up and explain character by character what 2 gt  amp 1  means

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unix commands 2 gt  amp 1  This is used to print errors to the terminal   The following illustrates the process   When errors are produced  they are written into the standard error memory address  amp 2  buffer   from which the standard error stream 2 references  When output is produced  it is written into the standard output memory address  amp 1  buffer   from which the standard output stream 1 references    So take the unix commands standard error stream 2  and redirect  gt  the stream  of errors  to the standard output memory address  amp 1  so that they will be streamed to the terminal and printed

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File descriptor 1 is the standard output  stdout   File descriptor 2 is the standard error  stderr   Here is one way to remember this construct  although it is not entirely accurate   at first  2 gt 1 may look like a good way to redirect stderr to stdout  However  it will actually be interpreted as  quot redirect stderr to a file named 1 quot    amp  indicates that what follows and precedes is a file descriptor and not a filename  So the construct becomes  2 gt  amp 1  Consider  gt  amp  as redirect merger operator

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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  lt 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  gt word       Moving File Descriptors      The redirection operator    n  lt  amp 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  gt  amp 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

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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  gt   dev null   will send the contents of  tmp to  dev null and print an error message for  foo    ls -l  tmp  foo 1 gt   dev null   will do exactly the same  note the 1     ls -l  tmp  foo 2 gt   dev null   will print the contents of  tmp and send the error message to  dev null    ls -l  tmp  foo 1 gt   dev null 2 gt   dev null   will send both the listing as well as the error message to  dev null    ls -l  tmp  foo  gt   dev null 2 gt   amp 1   is shorthand

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The numbers refer to the file descriptors  fd       Zero is stdin   One is stdout  Two is stderr   2 gt  amp 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

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2 gt  amp 1 is a POSIX shell construct  Here is a breakdown  token by token     2   Standard error  output file descriptor    gt  amp   Duplicate an Output File Descriptor operator  a variant of Output Redirection operator  gt    Given  x  gt  amp  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 gt  amp 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

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This is just like passing the error to the stdout or the terminal   That is  cmd is not a command    cmd 2 gt filename cat filename  command not found   The error is sent to the file like this   2 gt  amp 1   Standard error is sent to the terminal

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People  always remember paxdiablo s hint about the current location of the redirection target    It is important   My personal mnemonic for the 2 gt  amp 1 operator is this    Think of  amp  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 gt  amp 2    Think of  amp  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

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From a programmer s point of view  it means precisely this   dup2 1  2     See the man page   Understanding that 2 gt  amp 1 is a copy also explains why      command  gt file 2 gt  amp 1       is not the same as      command 2 gt  amp 1  gt file   The first will send both streams to file  whereas the second will send errors to stdout  and ordinary output into file

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echo test  gt  afile txt   redirects stdout to afile txt  This is the same as doing  echo test 1 gt  afile txt   To redirect stderr  you do   echo test 2 gt  afile txt    gt  amp  is the syntax to redirect a stream to another file descriptor - 0 is stdin  1 is stdout  and 2 is stderr   You can redirect stdout to stderr by doing   echo test 1 gt  amp 2   or echo test  gt  amp 2   Or vice versa   echo test 2 gt  amp 1   So  in short    2 gt  redirects stderr to an  unspecified  file  appending  amp 1 redirects stderr to stdout

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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  gt  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  gt  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 that 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 gt  output txt means you are sending Standard output  stdout  to output txt   and if you do this cat test txt 2 gt  error txt means you are sending Standard Error  stderr  to error txt     amp 1 is used to reference the value of the file descriptor 1  stdout    Now to the point 2 gt  amp 1 means    Redirect the stderr to the same place we are redirecting the stdout    Now you can do this   cat maybefile txt  gt  output txt 2 gt  amp 1  both Standard output  stdout  and Standard Error  stderr  will redirected to output txt   Thanks to Ondrej K  for pointing out

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I found this brilliant post on redirection  All about redirections  Redirect both standard output and standard error to a file       command  amp  file   This one-liner uses the  amp  gt  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  amp 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  amp 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  amp 1   is not the same as writing        command 2  amp 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  amp 1 so it duplicates stderr to stdout  The file descriptor table becomes     Now Bash sees the second redirect   gt 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  amp  file   is exactly the same as        command   amp file

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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 gt  amp 1   grep ERROR    Run the less pager without stderr screwing up the output  foo 2 gt  amp 1   less    Send stdout err to file  with append  and terminal  foo 2 gt  amp 1  tee  dev tty  gt  gt outfile    Send stderr to normal location and stdout to file  foo  gt outfile1 2 gt  amp 1  gt 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

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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 gt  amp 1   the standard error is redirected to standard output  so you can see both outputs together   perl test pl  gt  debug log 2 gt  amp 1   After execution  you can see all the output  including errors  in the debug log   perl test pl 1 gt out log 2 gt err log   Then standard output goes to out log  and standard error to err log   I suggest you to try to understand these

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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  amp 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

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Note that 1 gt  amp 2 cannot be used interchangeably with 2 gt  amp 1  Imagine your command depends on piping  for example  docker logs 1b3e97c49e39 2 gt  amp 1   grep  quot some log quot  grepping will happen across both stderr and stdout since stderr is basically merged into stdout  However  if you try  docker logs 1b3e97c49e39 1 gt  amp 2   grep  quot some log quot   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

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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  gt  means redirection    gt  means send to as a whole completed file  overwriting target if exist  see noclobber bash feature at  3 later    gt  gt  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  gt  dev null ls  cannot access  tnt  No such file or directory    ls -ld  tmp  tnt 2 gt  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  gt  dev null ls  cannot access  tnt  No such file or directory    ls -ld  tmp  tnt  gt  dev null 2 gt  amp 1    ls -ld  tmp  tnt 2 gt  amp 1  gt  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       lt --  amp  --- gt    ls  cannot access  tnt  No such file or directory  lt -- drwxrwxrwt 118 root root 196608 Jan  7 12 02  tmp --- gt     ls -ld  tmp  tnt 2 gt  amp 1   sed  s       lt --  amp  --- gt     lt -- ls  cannot access  tnt  No such file or directory --- gt   lt -- drwxrwxrwt 118 root root 196608 Jan  7 12 02  tmp --- gt     ls -ld  tmp  tnt  gt  dev null   sed  s       lt --  amp  --- gt    ls  cannot access  tnt  No such file or directory    ls -ld  tmp  tnt  gt  dev null 2 gt  amp 1   sed  s       lt --  amp  --- gt       ls -ld  tmp  tnt 2 gt  amp 1  gt  dev null   sed  s       lt --  amp  --- gt     lt -- ls  cannot access  tnt  No such file or directory --- gt   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      gt  amp 9   9 gt  amp 2  2 gt  amp 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   amp 9 descriptor would occur spontaneously because of   9 gt  amp 2  Addendum  nota  With the new version of bash   gt 4 0  there is a new feature and more sexy syntax for doing this kind of things    ls -ld  tmp  tnt 2 gt   gt  sed  s   E       gt   gt  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      gt  amp 9   2 gt  amp 1  sed  s   E      9 gt  amp 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  lt  ls -ld  tmp  tnt 2 gt   gt  sed  s   E       gt   gt  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  3 - A word about noclobber option and  gt   syntax That s about overwriting  While set -o noclobber instruct bash to not overwrite any existing file  the  gt   syntax let you pass through this limitation    testfile   mktemp  tmp testNoClobberDate-XXXXXX     date  gt   testfile   cat  testfile Mon Jan  7 13 18 15 CET 2013    date  gt   testfile   cat  testfile Mon Jan  7 13 18 19 CET 2013    date  gt   testfile   cat  testfile Mon Jan  7 13 18 21 CET 2013  The file is overwritten each time  well now    set -o noclobber    date  gt   testfile   cat  testfile bash   tmp testNoClobberDate-WW1xi9  cannot overwrite existing file Mon Jan  7 13 18 21 CET 2013    date  gt   testfile   cat  testfile bash   tmp testNoClobberDate-WW1xi9  cannot overwrite existing file Mon Jan  7 13 18 21 CET 2013  Pass through with  gt      date  gt    testfile   cat  testfile Mon Jan  7 13 18 58 CET 2013    date  gt    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  gt   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  gt  dev null 2 gt  amp 1  for this special case  there is a shortcut syntax   amp  gt      or  gt  amp    ls -ld  tmp  tnt  amp  gt  dev null    ls -ld  tmp  tnt  gt  amp  dev null  Nota  if 2 gt  amp 1 exist  1 gt  amp 2 is a correct syntax too    ls -ld  tmp  tnt 2 gt  dev null 1 gt  amp 2  4b- Now  I will let you think about    ls -ld  tmp  tnt 2 gt  amp 1 1 gt  amp 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 gt  amp 2 2 gt  amp 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  -

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0 for input  1 for stdout and 2 for stderr   One Tip  somecmd  gt 1 txt 2 gt  amp 1 is correct  while somecmd 2 gt  amp 1  gt 1 txt is totally wrong with no effect

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