How can I pipe stderr and not stdout

Question

I have a program that writes information to stdout and stderr  and I need to process the stderr with grep  leaving stdout aside  Using a temporary file  one could do it in two steps  command  gt   dev null 2 gt  temp file grep  something  temp file  But how can this be achieved without temp files  using one command and pipes

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In Bash  you can also redirect to a subshell using process substitution   command  gt   gt  stdlog pipe   2 gt   gt  stderr pipe    For the case at hand   command 2 gt   gt  grep  something    gt  dev null

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It s much easier to visualize things if you think about what s really going on with  redirects  and  pipes   Redirects and pipes in bash do one thing  modify where the process file descriptors 0  1  and 2 point to  see  proc  pid  fd       When a pipe or     operator is present on the command line  the first thing to happen is that bash creates a fifo and points the left side command s FD 1 to this fifo  and points the right side command s FD 0 to the same fifo    Next  the redirect operators for each side are evaluated from left to right  and the current settings are used whenever duplication of the descriptor occurs  This is important because since the pipe was set up first  the FD1  left side  and FD0  right side  are already changed from what they might normally have been  and any duplication of these will reflect that fact   Therefore  when you type something like the following   command 2 gt  amp 1  gt  dev null   grep  something    Here is what happens  in order    a pipe  fifo  is created   command FD1  is pointed to this pipe   grep FD0  also is pointed to this pipe  command FD2  is pointed to where  command FD1  currently points  the pipe   command FD1  is pointed to  dev null   So  all output that  command  writes to its FD 2  stderr  makes its way to the pipe and is read by  grep  on the other side  All output that  command  writes to its FD 1  stdout  makes its way to  dev null    If instead  you run the following   command  gt  dev null 2 gt  amp 1   grep  something    Here s what happens    a pipe is created and  command FD 1  and  grep FD 0  are pointed to it  command FD 1  is pointed to  dev null  command FD 2  is pointed to where FD 1 currently points   dev null    So  all stdout and stderr from  command  go to  dev null  Nothing goes to the pipe  and thus  grep  will close out without displaying anything on the screen   Also note that redirects  file descriptors  can be read-only   lt    write-only      or read-write   lt       A final note  Whether a program writes something to FD1 or FD2  is entirely up to the programmer  Good programming practice dictates that error messages should go to FD 2 and normal output to FD 1  but you will often find sloppy programming that mixes the two or otherwise ignores the convention

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Or to swap the output from standard error and standard output over  use   command 3 gt  amp 1 1 gt  amp 2 2 gt  amp 3   This creates a new file descriptor  3  and assigns it to the same place as 1  standard output   then assigns fd 1  standard output  to the same place as fd 2  standard error  and finally assigns fd 2  standard error  to the same place as fd 3  standard output    Standard error is now available as standard output and the old standard output is preserved in standard error  This may be overkill  but it hopefully gives more details on Bash file descriptors  there are nine available to each process

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You can use the rc shell   First install the package  it s less than 1 MB    This an example of how you would discard standard output and pipe standard error to grep in rc   find  proc   gt  1   dev null   2  grep task   You can do it without leaving Bash   rc -c  find  proc   gt  1   dev null   2  grep task    As you may have noticed  you can specify which file descriptor you want piped by using brackets after the pipe   Standard file descriptors are numerated as such    0   Standard input 1   Standard output 2   Standard error

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If you are using Bash  then use   command  gt  dev null   amp  grep  something    http   www gnu org software bash manual bashref html Pipelines

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This also works  and I find it a tiny bit easier to remember  command 2 gt   dev fd 1   grep  something   More info about  dev fd directory here

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Combining the best of these answers  if you do   command 2 gt   gt  grep -v something 1 gt  amp 2      then all stdout is preserved as stdout and all stderr is preserved as stderr  but you won t see any lines in stderr containing the string  something    This has the unique advantage of not reversing or discarding stdout and stderr  nor smushing them together  nor using any temporary files

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For those who want to redirect stdout and stderr permanently to files  grep on stderr  but keep the stdout to write messages to a tty     save tty-stdout to fd 3 exec 3 gt  amp 1   switch stdout and stderr  grep  -v  stderr for nasty messages and append to files exec 2 gt   gt  grep -v  nasty msg   gt  gt  std err   gt  gt  std out   goes to the std out echo  my first message   gt  amp 1   goes to the std err echo  a error message   gt  amp 2   goes nowhere echo  this nasty msg won t appear anywhere   gt  amp 2   goes to the tty echo  a message on the terminal   gt  amp 3

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This will redirect command1 stderr to command2 stdin  while leaving command1 stdout as is   exec 3 gt  amp 1 command1 2 gt  amp 1  gt  amp 3 3 gt  amp -   command2 3 gt  amp - exec 3 gt  amp -   Taken from LDP

User · Answer

I just came up with a solution for sending stdout to one command and stderr to another  using named pipes   Here goes   mkfifo stdout-target mkfifo stderr-target cat  lt  stdout-target   command-for-stdout  amp  cat  lt  stderr-target   command-for-stderr  amp  main-command 1 gt stdout-target 2 gt stderr-target   It s probably a good idea to remove the named pipes afterward

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First redirect stderr to stdout     the pipe  then redirect stdout to  dev null  without changing where stderr is going   command 2 gt  amp 1  gt  dev null   grep  something   For the details of I O redirection in all its variety  see the chapter on Redirections in the Bash reference manual  Note that the sequence of I O redirections is interpreted left-to-right  but pipes are set up before the I O redirections are interpreted   File descriptors such as 1 and 2 are references to open file descriptions   The operation 2 gt  amp 1 makes file descriptor 2 aka stderr refer to the same open file description as file descriptor 1 aka stdout is currently referring to  see dup2   and open      The operation  gt  dev null then changes file descriptor 1 so that it refers to an open file description for  dev null  but that doesn t change the fact that file descriptor 2 refers to the open file description which file descriptor 1 was originally pointing to     namely  the pipe

User · Answer

I try follow  find it work as well    command  gt   dev null 2 gt  amp 1   grep  something

[bash] How can I pipe stderr, and not stdout?

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