redirect COPY of stdout to log file from within bash script itself

Question

I know how to redirect stdout to a file   exec  gt  foo log echo test   this will put the  test  into the foo log file   Now I want to redirect the output into the log file AND keep it on stdout  i e  it can be done trivially from outside the script   script   tee foo log   but I want to do declare it within the script itself  I tried  exec   tee foo log   but it didn t work

User · Accepted Answer

usr bin env bash    Redirect stdout    gt    into a named pipe    gt      running  tee  exec  gt   gt  tee -i logfile txt     Without this  only stdout would be captured - i e  your   log file would not contain any error messages    SEE  and upvote  the answer by Adam Spiers  which keeps STDERR   as a separate stream - I did not want to steal from him by simply   adding his answer to mine  exec 2 gt  amp 1  echo  foo  echo  bar   gt  amp 2   Note that this is bash  not sh  If you invoke the script with sh myscript sh  you will get an error along the lines of syntax error near unexpected token   gt     If you are working with signal traps  you might want to use the tee -i option to avoid disruption of the output if a signal occurs   Thanks to  JamesThomasMoon1979 for the comment      Tools that change their output depending on whether they write to a pipe or a terminal  ls using colors and columnized output  for example  will detect the above construct as meaning that they output to a pipe   There are options to enforce the colorizing   columnizing  e g  ls -C --color always   Note that this will result in the color codes being written to the logfile as well  making it less readable

User · Answer

Can t say I m comfortable with any of the solutions based on exec  I prefer to use tee directly  so I make the script call itself with tee when requested      my script    check tee output           copy  append  stdout and stderr to log file if TEE is unset or true     if    -z  TEE      TEE     true     then          echo  -------------------------------------------   gt  gt  log txt         echo         date   0     gt  gt  log txt         TEE false  0    2 gt  amp 1   tee --append log txt         exit        fi     check tee output     rest of my script   This allows you to do this    your script sh args             tee  TEE true your script sh args    tee  TEE false your script sh args   don t tee export TEE false your script sh args             tee   You can customize this  e g  make tee false the default instead  make TEE hold the log file instead  etc  I guess this solution is similar to jbarlow s  but simpler  maybe mine has limitations that I have not come across yet

User · Answer

Inside your script file  put all of the commands within parentheses  like this     echo start ls -l echo end     tee foo log

User · Answer

Bash 4 has a coproc command which establishes a named pipe to a command and allows you to communicate through it

User · Answer

Neither of these is a perfect solution  but here are a couple things you could try   exec  gt foo log tail -f foo log  amp    rest of your script   or  PIPE tmp fifo mkfifo  PIPE exec  gt  PIPE tee foo log  lt  PIPE  amp    rest of your script rm  PIPE   The second one would leave a pipe file sitting around if something goes wrong with your script  which may or may not be a problem  i e  maybe you could rm it in the parent shell afterwards

User · Answer

The accepted answer does not preserve STDERR as a separate file descriptor   That means    script sh  gt  dev null   will not output bar to the terminal  only to the logfile  and    script sh 2 gt  dev null   will output both foo and bar to the terminal   Clearly that s not the behaviour a normal user is likely to expect   This can be fixed by using two separate tee processes both appending to the same log file      bin bash    See  and upvote  the comment by JamesThomasMoon1979    explaining the use of the -i option to tee  exec  gt    gt  tee -ia foo log  exec 2 gt   gt  tee -ia foo log  gt  amp 2   echo  foo  echo  bar   gt  amp 2    Note that the above does not initially truncate the log file - if you want that behaviour you should add   gt foo log   to the top of the script    The POSIX 1-2008 specification of tee 1  requires that output is unbuffered  i e  not even line-buffered  so in this case it is possible that STDOUT and STDERR could end up on the same line of foo log  however that could also happen on the terminal  so the log file will be a faithful reflection of what could be seen on the terminal  if not an exact mirror of it   If you want the STDOUT lines cleanly separated from the STDERR lines  consider using two log files  possibly with date stamp prefixes on each line to allow chronological reassembly later on

User · Answer

Easy way to make a bash script log to syslog   The script output is available both through  var log syslog and through stderr   syslog will add useful metadata  including timestamps   Add this line at the top   exec  amp  gt   gt  logger -t myscript -s    Alternatively  send the log to a separate file   exec  amp  gt   gt  ts  tee -a  tmp myscript output  gt  amp 2     This requires moreutils  for the ts command  which adds timestamps

User · Answer

Solution for busybox  macOS bash  and non-bash shells  The accepted answer is certainly the best choice for bash   I m working in a Busybox environment without access to bash  and it does not understand the exec  gt   gt  tee log txt  syntax   It also does not do exec  gt  PIPE properly  trying to create an ordinary file with the same name as the named pipe  which fails and hangs   Hopefully this would be useful to someone else who doesn t have bash   Also  for anyone using a named pipe  it is safe to rm  PIPE  because that unlinks the pipe from the VFS  but the processes that use it still maintain a reference count on it until they are finished   Note the use of    is not necessarily safe      bin sh  if     SELF LOGGING      1    then       The parent process will enter this branch and set up logging        Create a named piped for logging the child s output     PIPE tmp fifo     mkfifo  PIPE        Launch the child process with stdout redirected to the named pipe     SELF LOGGING 1 sh  0     gt  PIPE  amp         Save PID of child process     PID           Launch tee in a separate process     tee logfile  lt  PIPE  amp         Unlink  PIPE because the parent process no longer needs it     rm  PIPE            Wait for child process  which is running the rest of this script     wait  PID        Return the error code from the child process     exit    fi    The rest of the script goes here

User · Answer

Using the accepted answer my script kept returning exceptionally early  right after  exec     tee        leaving the rest of my script running in the background  As I couldn t get that solution to work my way I found another solution work around to the problem     Logging setup logfile mylogfile mkfifo   logfile  pipe tee  lt    logfile  pipe  logfile  amp  exec  amp  gt    logfile  pipe rm   logfile  pipe    Rest of my script   This makes output from script go from the process  through the pipe into the sub background process of  tee  that logs everything to disc and to original stdout of the script   Note that  exec  amp    redirects both stdout and stderr  we could redirect them separately if we like  or change to  exec    if we just want stdout    Even thou the pipe is removed from the file system in the beginning of the script it will continue to function until the processes finishes  We just can t reference it using the file name after the rm-line

[bash] redirect COPY of stdout to log file from within bash script itself

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