How to redirect output of an entire shell script within the script itself

Question

Is it possible to redirect all of the output of a Bourne shell script to somewhere  but with shell commands inside the script itself  Redirecting the output of a single command is easy  but I want something more like this     bin sh if     -t 0    then       redirect all of my output to a file here fi    rest of script     Meaning  if the script is run non-interactively  for example  cron   save off the output of everything to a file   If run interactively from a shell  let the output go to stdout as usual  I want to do this for a script normally run by the FreeBSD periodic utility   It s part of the daily run  which I don t normally care to see every day in email  so I don t have it sent   However  if something inside this one particular script fails  that s important to me and I d like to be able to capture and email the output of this one part of the daily jobs  Update  Joshua s answer is spot-on  but I also wanted to save and restore stdout and stderr around the entire script  which is done like this    save stdout and stderr to file    descriptors 3 and 4     then redirect them to  quot foo quot  exec 3 gt  amp 1 4 gt  amp 2  gt foo 2 gt  amp 1           restore stdout and stderr exec 1 gt  amp 3 2 gt  amp 4

User · Answer

Typically we would place one of these at or near the top of the script. Scripts that parse their command lines would do the redirection after parsing.

Send stdout to a file

exec > file

with stderr

exec > file                                                                      
exec 2>&1

append both stdout and stderr to file

exec >> file
exec 2>&1

As Jonathan Leffler mentioned in his comment:

exec has two separate jobs. The first one is to replace the currently executing shell (script) with a new program. The other is changing the I/O redirections in the current shell. This is distinguished by having no argument to exec.

User · Answer

Addressing the question as updated       part of script without redirection               part of script with redirection       gt  file1 2 gt file2      and others as appropriate         residue of script without redirection      The braces           provide a unit of I O redirection   The braces must appear where a command could appear - simplistically  at the start of a line or after a semi-colon    Yes  that can be made more precise  if you want to quibble  let me know    You are right that you can preserve the original stdout and stderr with the redirections you showed  but it is usually simpler for the people who have to maintain the script later to understand what s going on if you scope the redirected code as shown above   The relevant sections of the Bash manual are Grouping Commands and I O Redirection   The relevant sections of the POSIX shell specification are Compound Commands and I O Redirection   Bash has some extra notations  but is otherwise similar to the POSIX shell specification

User · Answer

For saving the original stdout and stderr you can use     exec  fd number  lt  amp 1  exec  fd number  lt  amp 2   For example  the following code will print  walla1  and  walla2  to the log file  a txt    walla3  to stdout   walla4  to stderr      bin bash  exec 5 lt  amp 1 exec 6 lt  amp 2  exec 1 gt    a txt 2 gt  amp 1  echo  walla1  echo  walla2   gt  amp 2 echo  walla3   gt  amp 5 echo  walla4   gt  amp 6

User · Answer

Typically we would place one of these at or near the top of the script. Scripts that parse their command lines would do the redirection after parsing.

Send stdout to a file

exec > file

with stderr

exec > file                                                                      
exec 2>&1

append both stdout and stderr to file

exec >> file
exec 2>&1

As Jonathan Leffler mentioned in his comment:

exec has two separate jobs. The first one is to replace the currently executing shell (script) with a new program. The other is changing the I/O redirections in the current shell. This is distinguished by having no argument to exec.

User · Answer

For saving the original stdout and stderr you can use     exec  fd number  lt  amp 1  exec  fd number  lt  amp 2   For example  the following code will print  walla1  and  walla2  to the log file  a txt    walla3  to stdout   walla4  to stderr      bin bash  exec 5 lt  amp 1 exec 6 lt  amp 2  exec 1 gt    a txt 2 gt  amp 1  echo  walla1  echo  walla2   gt  amp 2 echo  walla3   gt  amp 5 echo  walla4   gt  amp 6

User · Answer

You can make the whole script a function like this   main function       do things here     then at the end of the script have this   if   -z  TERM    then     if not run via terminal  log everything into a log file   main function 2 gt  amp 1  gt  gt   var log my uber script log else     run via terminal  only output to screen   main function fi   Alternatively  you may log everything into logfile each run and still output it to stdout by simply doing     log everything  but also output to stdout main function 2 gt  amp 1   tee -a  var log my uber script log

User · Answer

Typically we would place one of these at or near the top of the script. Scripts that parse their command lines would do the redirection after parsing.

Send stdout to a file

exec > file

with stderr

exec > file                                                                      
exec 2>&1

append both stdout and stderr to file

exec >> file
exec 2>&1

As Jonathan Leffler mentioned in his comment:

exec has two separate jobs. The first one is to replace the currently executing shell (script) with a new program. The other is changing the I/O redirections in the current shell. This is distinguished by having no argument to exec.

User · Answer

Addressing the question as updated       part of script without redirection               part of script with redirection       gt  file1 2 gt file2      and others as appropriate         residue of script without redirection      The braces           provide a unit of I O redirection   The braces must appear where a command could appear - simplistically  at the start of a line or after a semi-colon    Yes  that can be made more precise  if you want to quibble  let me know    You are right that you can preserve the original stdout and stderr with the redirections you showed  but it is usually simpler for the people who have to maintain the script later to understand what s going on if you scope the redirected code as shown above   The relevant sections of the Bash manual are Grouping Commands and I O Redirection   The relevant sections of the POSIX shell specification are Compound Commands and I O Redirection   Bash has some extra notations  but is otherwise similar to the POSIX shell specification

User · Answer

You can make the whole script a function like this   main function       do things here     then at the end of the script have this   if   -z  TERM    then     if not run via terminal  log everything into a log file   main function 2 gt  amp 1  gt  gt   var log my uber script log else     run via terminal  only output to screen   main function fi   Alternatively  you may log everything into logfile each run and still output it to stdout by simply doing     log everything  but also output to stdout main function 2 gt  amp 1   tee -a  var log my uber script log

User · Answer

Addressing the question as updated       part of script without redirection               part of script with redirection       gt  file1 2 gt file2      and others as appropriate         residue of script without redirection      The braces           provide a unit of I O redirection   The braces must appear where a command could appear - simplistically  at the start of a line or after a semi-colon    Yes  that can be made more precise  if you want to quibble  let me know    You are right that you can preserve the original stdout and stderr with the redirections you showed  but it is usually simpler for the people who have to maintain the script later to understand what s going on if you scope the redirected code as shown above   The relevant sections of the Bash manual are Grouping Commands and I O Redirection   The relevant sections of the POSIX shell specification are Compound Commands and I O Redirection   Bash has some extra notations  but is otherwise similar to the POSIX shell specification

User · Answer

You can make the whole script a function like this   main function       do things here     then at the end of the script have this   if   -z  TERM    then     if not run via terminal  log everything into a log file   main function 2 gt  amp 1  gt  gt   var log my uber script log else     run via terminal  only output to screen   main function fi   Alternatively  you may log everything into logfile each run and still output it to stdout by simply doing     log everything  but also output to stdout main function 2 gt  amp 1   tee -a  var log my uber script log

User · Answer

Typically we would place one of these at or near the top of the script. Scripts that parse their command lines would do the redirection after parsing.

Send stdout to a file

exec > file

with stderr

exec > file                                                                      
exec 2>&1

append both stdout and stderr to file

exec >> file
exec 2>&1

As Jonathan Leffler mentioned in his comment:

exec has two separate jobs. The first one is to replace the currently executing shell (script) with a new program. The other is changing the I/O redirections in the current shell. This is distinguished by having no argument to exec.

User · Answer

-t  lt  amp 0      exec  gt  gt  test log

User · Answer

You can make the whole script a function like this   main function       do things here     then at the end of the script have this   if   -z  TERM    then     if not run via terminal  log everything into a log file   main function 2 gt  amp 1  gt  gt   var log my uber script log else     run via terminal  only output to screen   main function fi   Alternatively  you may log everything into logfile each run and still output it to stdout by simply doing     log everything  but also output to stdout main function 2 gt  amp 1   tee -a  var log my uber script log

User · Answer

-t  lt  amp 0      exec  gt  gt  test log

User · Answer

Addressing the question as updated       part of script without redirection               part of script with redirection       gt  file1 2 gt file2      and others as appropriate         residue of script without redirection      The braces           provide a unit of I O redirection   The braces must appear where a command could appear - simplistically  at the start of a line or after a semi-colon    Yes  that can be made more precise  if you want to quibble  let me know    You are right that you can preserve the original stdout and stderr with the redirections you showed  but it is usually simpler for the people who have to maintain the script later to understand what s going on if you scope the redirected code as shown above   The relevant sections of the Bash manual are Grouping Commands and I O Redirection   The relevant sections of the POSIX shell specification are Compound Commands and I O Redirection   Bash has some extra notations  but is otherwise similar to the POSIX shell specification

[bash] How to redirect output of an entire shell script within the script itself?

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