Redirect stderr and stdout in Bash

Question

I want to redirect both stdout and stderr of a process to a single file  How do I do that in Bash

User · Answer

For tcsh  I have to use the following command    command  gt  amp  file   If use command  amp  gt  file   it will give  Invalid null command  error

User · Answer

For situation  when  piping  is necessary you can use         amp    For example   echo -ne  15 n100 n  sort -c   amp  tee  gt sort result txt   or  TIMEFORMAT  R for i in  seq 1 20    do time kubectl get pods  grep node  gt  gt js log    done   amp  sort -h   This bash-based solutions can pipe STDOUT and STDERR separately  from STDERR of  sort -c  or from STDERR to  sort -h

User · Answer

You can redirect stderr to stdout and the stdout into a file   some command  gt file log 2 gt  amp 1    See http   tldp org LDP abs html io-redirection html  This format is preferred than the most popular  amp   format that only work in bash   In Bourne shell it could be interpreted as running the command in background  Also the format is more readable 2  is STDERR  redirected to 1  STDOUT      EDIT  changed the order as pointed out in the comments

User · Answer

Take a look here  Should be   yourcommand  amp  gt filename    redirects both stdout and stderr to filename

User · Answer

LOG FACILITY  local7 notice  LOG TOPIC  my-prog-name  LOG TOPIC OUT   LOG TOPIC-out      LOG TOPIC ERR   LOG TOPIC-err       exec 3 gt  amp 1  gt   gt  tee -a  dev fd 3   logger -p   LOG FACILITY  -t   LOG TOPIC OUT    exec 2 gt   gt  logger -p   LOG FACILITY  -t   LOG TOPIC ERR      It is related  Writing stdOut  amp  stderr to syslog   It almost work  but not from xinted

User · Answer

In situations when you consider using things like exec 2 gt  amp 1 I find easier to read  if possible rewriting code using bash functions like this   function myfunc               myfunc  amp  gt mylog log

User · Answer

I wanted a solution to have the output from stdout plus stderr written into a log file and stderr still on console  So I needed to duplicate the stderr output via tee   This is the solution I found   command 3 gt  amp 1 1 gt  amp 2 2 gt  amp 3 1 gt  gt logfile   tee -a logfile    First swap stderr and stdout  then append the stdout to the log file pipe stderr to tee and append it also to the log file

User · Answer

Curiously  this works   yourcommand  amp  gt  filename   But this gives a syntax error   yourcommand  amp  gt  gt  filename syntax error near unexpected token   gt     You have to use   yourcommand 1 gt  gt  filename 2 gt  amp 1

User · Answer

bash your script sh 1 gt file log 2 gt  amp 1   1 file log instructs the shell to send STDOUT to the file file log  and 2  amp 1 tells it to redirect STDERR  file descriptor 2  to STDOUT  file descriptor 1    Note  The order matters as liw fi pointed out  2  amp 1 1 file log doesn t work

User · Answer

do something 2 gt  amp 1   tee -a some file   This is going to redirect stderr to stdout and stdout to some file and print it to stdout

User · Answer

Close STDOUT file descriptor exec 1 lt  amp -   Close STDERR FD exec 2 lt  amp -    Open STDOUT as  LOG FILE file for read and write  exec 1 lt  gt  LOG FILE    Redirect STDERR to STDOUT exec 2 gt  amp 1  echo  This line will appear in  LOG FILE  not  on screen     Now  simple echo will write to  LOG FILE  Useful for daemonizing   To the author of the original post   It depends what you need to achieve  If you just need to redirect in out of a command you call from your script  the answers are already given  Mine is about redirecting within current script which affects all commands built-ins includes forks  after the mentioned code snippet     Another cool solution is about redirecting to both std-err out AND to logger or log file at once which involves splitting  a stream  into two  This functionality is provided by  tee  command which can write append to several file descriptors files  sockets  pipes  etc  at once  tee FILE1 FILE2       cmd1    cmd2       exec 3 gt  amp 1 4 gt  amp 2 1 gt   gt  tee  gt  logger -i -t  my script tag    gt  amp 3  2 gt   gt  tee  gt  logger -i -t  my script tag    gt  amp 4  trap  cleanup  INT QUIT TERM EXIT   get pids of ppid         local ppid   1       RETVAL        local pids  ps x -o pid ppid   awk     2        ppid      print    1         RETVAL   pids        Needed to kill processes running in background cleanup         local current pid element     local pids               running pids     pids            while    do         current pid    running pids 0              -z   current pid     amp  amp  break          running pids     running pids    1            get pids of ppid  current pid         local new pids   RETVAL            -z   new pids     amp  amp  continue          for element in  new pids  do             running pids     element               pids    element     pids               done     done      kill   pids     2 gt  dev null     So  from the beginning  Let s assume we have terminal connected to  dev stdout FD  1  and  dev stderr FD  2   In practice  it could be a pipe  socket or whatever    Create FDs  3 and  4 and point to the same  location  as  1 and  2 respectively  Changing FD  1 doesn t affect FD  3 from now on  Now  FDs  3 and  4 point to STDOUT and STDERR respectively  These will be used as real terminal STDOUT and STDERR  1         redirects STDOUT to command in parens parens sub-shell  executes  tee  reading from exec s STDOUT pipe  and redirects to  logger  command via another pipe to sub-shell in parens  At the same time it copies the same input to FD  3 terminal  the second part  very similar  is about doing the same trick for STDERR and FDs  2 and  4    The result of running a script having the above line and additionally this one   echo  Will end up in STDOUT terminal  and  var log messages       is as follows       my script Will end up in STDOUT terminal  and  var log messages    tail -n1  var log messages Sep 23 15 54 03 wks056 my script tag 11644   Will end up in STDOUT terminal  and  var log messages   If you want to see clearer picture  add these 2 lines to the script   ls -l  proc self fd  ps xf

User · Answer

Short answer  Command  gt filename 2 gt  amp 1 or Command  amp  gt filename    Explanation   Consider the following code which prints the word  stdout  to stdout and the word  stderror  to stderror       echo  stdout   echo  stderror   gt  amp 2  stdout stderror   Note that the   amp   operator tells bash that 2 is a file descriptor  which points to the stderr  and not a file name  If we left out the   amp    this command would print stdout to stdout  and create a file named  2  and write stderror there    By experimenting with the code above  you can see for yourself exactly how redirection operators work  For instance  by changing which file which of the two descriptors 1 2  is redirected to  dev null the following two lines of code delete everything from the stdout  and everything from stderror respectively  printing what remains        echo  stdout   echo  stderror   gt  amp 2  1 gt  dev null stderror    echo  stdout   echo  stderror   gt  amp 2  2 gt  dev null stdout   Now  we can explain why the solution why the following code produces no output    echo  stdout   echo  stderror   gt  amp 2   gt  dev null 2 gt  amp 1   To truly understand this  I highly recommend you read this webpage on file descriptor tables  Assuming you have done that reading  we can proceed  Note that Bash processes left to right  thus Bash sees  gt  dev null first  which is the same as 1 gt  dev null   and sets the file descriptor 1 to point to  dev null instead of the stdout  Having done this  Bash then moves rightwards and sees 2 gt  amp 1  This sets the file descriptor 2 to point to the same file as file descriptor 1  and not to file descriptor 1 itself      see this resource on pointers for more info     Since file descriptor 1 points to  dev null  and  file descriptor 2 points to the same file as file descriptor 1  file descriptor 2 now also points to  dev null  Thus both file descriptors point to  dev null  and this is why no output is rendered      To test if you really understand the concept  try to guess the output when we switch the redirection order    echo  stdout   echo  stderror   gt  amp 2   2 gt  amp 1  gt  dev null       stderror   The reasoning here is that evaluating from left to right  Bash sees 2  amp 1  and thus sets the file descriptor 2 to point to the same place as file descriptor 1  ie stdout  It then sets file descriptor 1  remember that   dev null   1  dev null  to point to   dev null  thus deleting everything which would usually be send to to the standard out  Thus all we are left with was that which was not send to stdout in the subshell  the code in the parentheses - i e   stderror     The interesting thing to note there is that even though 1 is just a pointer to the stdout  redirecting pointer 2 to 1 via 2 gt  amp 1 does NOT form a chain of pointers 2 -  1 -  stdout  If it did  as a result of redirecting 1 to  dev null  the code 2 gt  amp 1  gt  dev null would give the pointer chain 2 -  1 -   dev null  and thus the code would generate nothing  in contrast to what we saw above      Finally  I d note that there is a simpler way to do this    From section 3 6 4 here  we see that we can use the operator  amp  gt  to redirect both stdout and stderr  Thus  to redirect both the stderr and stdout output of any command to  dev null  which deletes the output   we simply type    command  amp  gt   dev null  or in case of my example      echo  stdout   echo  stderror   gt  amp 2   amp  gt  dev null     Key takeaways    File descriptors behave like pointers  although file descriptors are not the same as file pointers  Redirecting a file descriptor  a  to a file descriptor  b  which points to file  f   causes file descriptor  a  to point to the same place as file descriptor b - file  f   It DOES NOT form a chain of pointers a -  b -  f Because of the above  order matters  2 gt  amp 1  gt  dev null is     gt  dev null 2 gt  amp 1  One generates output and the other does not      Finally have a look at these great resources   Bash Documentation on Redirection  An Explanation of File Descriptor Tables  Introduction to Pointers

User · Answer

Easiest  way  bash4 only   ls   2 gt  amp - 1 gt  amp -

User · Answer

The following functions can be used to automate the process of toggling outputs beetwen stdout stderr and a logfile      bin bash       set -x        global vars     OUTPUTS REDIRECTED  false      LOGFILE  dev stdout         private  function used by redirect outputs to logfile       function save standard outputs           if     OUTPUTS REDIRECTED      true     then             echo   ERROR     FUNCNAME 0    Cannot save standard outputs because they have been redirected before              exit 1          fi         exec 3 gt  amp 1         exec 4 gt  amp 2          trap restore standard outputs EXIT              Params   1   gt  logfile to write to     function redirect outputs to logfile           if     OUTPUTS REDIRECTED      true     then             echo   ERROR     FUNCNAME 0    Cannot redirect standard outputs because they have been redirected before              exit 1          fi         LOGFILE  1         if   -z   LOGFILE     then             echo   ERROR     FUNCNAME 0    logfile empty   LOGFILE            fi         if     -f  LOGFILE    then             touch  LOGFILE         fi         if     -f  LOGFILE    then             echo   ERROR     FUNCNAME 0    creating logfile   LOGFILE               exit 1         fi          save standard outputs          exec 1 gt  gt   LOGFILE  log  log         exec 2 gt  amp 1         OUTPUTS REDIRECTED  true                private  function used by save standard outputs        function restore standard outputs           if     OUTPUTS REDIRECTED      false     then             echo   ERROR     FUNCNAME 0    Cannot restore standard outputs because they have NOT been redirected              exit 1          fi         exec 1 gt  amp -    closes FD 1  logfile          exec 2 gt  amp -    closes FD 2  logfile          exec 2 gt  amp 4    restore stderr         exec 1 gt  amp 3    restore stdout          OUTPUTS REDIRECTED  false          Example of usage inside script   echo  this goes to stdout  redirect outputs to logfile  tmp one log echo  this goes to logfile  restore standard outputs  echo  this goes to stdout

User · Answer

fernando-fabreti  Adding to what you did I changed the functions slightly and removed the  amp - closing and it worked for me        function saveStandardOutputs         if     OUTPUTS REDIRECTED      false     then         exec 3 gt  amp 1         exec 4 gt  amp 2         trap restoreStandardOutputs EXIT       else           echo   ERROR     FUNCNAME 0    Cannot save standard outputs because they have been redirected before            exit 1        fi          Params   1   gt  logfile to write to   function redirectOutputsToLogfile         if     OUTPUTS REDIRECTED      false     then         LOGFILE  1         if   -z   LOGFILE     then             echo   ERROR     FUNCNAME 0    logfile empty   LOGFILE           fi         if     -f  LOGFILE    then             touch  LOGFILE         fi         if     -f  LOGFILE    then             echo   ERROR     FUNCNAME 0    creating logfile   LOGFILE               exit 1         fi         saveStandardOutputs         exec 1 gt  gt   LOGFILE          exec 2 gt  amp 1         OUTPUTS REDIRECTED  true        else         echo   ERROR     FUNCNAME 0    Cannot redirect standard outputs because they have been redirected before            exit 1        fi       function restoreStandardOutputs         if     OUTPUTS REDIRECTED      true     then       exec 1 gt  amp 3    restore stdout       exec 2 gt  amp 4    restore stderr       OUTPUTS REDIRECTED  false       fi       LOGFILE NAME  tmp one log    OUTPUTS REDIRECTED  false     echo  this goes to stdout    redirectOutputsToLogfile  LOGFILE NAME   echo  this goes to logfile    echo    LOGFILE NAME     restoreStandardOutputs    echo  After restore this goes to stdout

[bash] Redirect stderr and stdout in Bash

Examples related to bash

Examples related to shell

Examples related to stdout

Examples related to io-redirection

Examples related to stderr