how to redirect output of multiple commands to one file

Question

i have a bash script, that has the following two commands:

ssh host tail -f /some/file | awk ..... > /some/file &  ssh host tail -f /some/file | grep .... > /some/file & 

How can i make the output of both commands be directed into the same file.

Answer 1

Either use 'append' with >> or use braces to encompass the I/O redirections, or (occasionally) use exec:

ssh host tail -f /some/file | awk ..... >  /some/file & ssh host tail -f /some/file | grep .... >> /some/file & 

or:

{ ssh host tail -f /some/file | awk ..... & ssh host tail -f /some/file | grep .... & } > /some/file 

or:

exec > /some/file ssh host tail -f /some/file | awk ..... & ssh host tail -f /some/file | grep .... & 

After the exec, the standard output of the script as a whole goes to /some/file. I seldom use this technique; I usually use the { ...; } technique instead.

Note: You do have to be careful with the braces notation. What I showed will work. Trying to flatten it onto one line requires you to treat the { as if it were a command (followed by a space, for example) and also to treat the } as if it were a command. You must have a command terminator before the } — I used a newline, but an & for background or ; would work too.

Thus:

{ command1;  command2;  } >/some/file { command1 & command2 & } >/some/file 

I also have not addressed the issue of why you have two separate tail -f operations running on a single remote file and why you are not using awk power as a super-grep to handle it all in one — I've only addressed the surface question of how to redirect the I/O of the two commands to one file.

Answer 2

Note you can reduce the number of ssh calls:

{  ssh host tail -f /some/file |       tee >(awk ...) >(grep ...) >/dev/null } > /some/file & 

example:

{ echo foobar | tee >(sed 's/foo/FOO/') >(sed 's/bar/BAR/') > /dev/null; } > outputfile cat outputfile  

fooBAR FOObar