Combine the output of two commands in bash

You can combine two commands by grouping it with { } :

{ command1 & command2; }

so far, you can redirect the group to a file (last ; before } is mandatory), and the space between the open and closing bracket too.

{ command1 & command2; } > new_file

if you want to separate STDOUT and STDERRin two files :

{ command1 & command2; } > STDOUT_file 2> STDERR_file

More generally, it's possible to use either a subshell or command grouping, and redirect the output of the whole group at once.

Code:

( command1 ; command2 ; command3 ) | cat

{ command1 ; command2 ; command3 ; } > outfile.txt

The main difference between the two is that the first one splits of a child process, while the second one operates in the context of the main shell. This can have consequences regarding the setting and use of variables and other environment settings, as well as performance.

Don't forget that the closing bracket in command grouping (and functions) must be separated from the contents by either a semicolon or a newline. This is because "}" is actually a command (keyword) of its own, and must be treated like one.

Most of the solutions so far deal badly with the partial line problem. Assume for a second that the programs are:

cmd1() {
    perl -e 'while(1) { print "a"x3000_000,"\n"}'
}
export -f cmd1
cmd2() {
    perl -e 'while(1) { print "b"x3000_000,"\n"}'
}
export -f cmd2

When running those in parallel you want the output to have full lines of as followed by full lines of bs. What you do not want is as and bs mixing on the same line (tr -s ab replaces repeating as with a single a, so it is easier to see what happens):

# This is bad - half lines are mixed
$ (cmd1 & cmd2 ) | tr -s ab
bababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababa
ababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababab

If you instead use GNU Parallel, you get nice clean full lines with either as or bs but never mixed:

$ parallel --line-buffer ::: cmd1 cmd2 | tr -s ab
a
a
b
b
b
b
a

Newer versions of GNU Parallel even avoids filling up your disk: The above can run forever.