how to redirect output to multiple log files

Question

How to redirect standard output to multiple log files? The following does not work:

some_command 1> output_log_1 output_log_2 2>&1

Answer 1

See man tee:

NAME: tee - read from standard input and write to standard output and files

SYNOPSIS: tee [OPTION]... [FILE]...

Accordingly:

echo test | tee file1 file2 file3

Answer 2

It's an old post but I just found it now...

Instead of redirecting the output to > /dev/null you can redirect it to the last file:

echo "foobarbaz" | tee file1 > file2

Or for appending the output:

echo "foobarbaz" | tee -a file1 >> file2

Answer 3

Let's say your output is generated from a function, cmd() :

cmd() {
    echo hello world!
}

To redirect the output from cmd to two files, but not to the console, you can use:

cmd | tee file1 file2 >/dev/null

This will work for multiple files, given any data source piping to tee:

echo "foobarbaz" | tee file1 file2 file3 file4 > /dev/null

This will also work:

echo $(cmd) | tee file1 file2 >/dev/null

Without the /dev/null redirection, tee will send output to stdout in addition to the files specified.

For example, if this is run from the console, you'll see the output there. Run from a crontab, the output will appear the status message which is mailed to you (also see Gilles' answer here https://unix.stackexchange.com/a/100833/3998).

This worked for me in bash on Ubuntu 12.04, and has been verified in Ubuntu 14.04 using GNU bash 4.3.11(1), so it should work on any recent GNU bash version.

Answer 4

@strugee's answer for zsh is not safe to use. The safe way to do this would be to enclose some_command in brace brackets like:

{some_command} >output_log_1 >output_log_2

and not like this:

some_command >output_log_1 >output_log_2

zsh manual's Multios section explains the reason with an example.

There is a problem when an output multio is attached to an external program. A simple example shows this:

cat file >file1 >file2
cat file1 file2

Here, it is possible that the second ‘cat’ will not display the full contents of file1 and file2 (i.e. the original contents of file repeated twice).

The reason for this is that the multios are spawned after the cat process is forked from the parent shell, so the parent shell does not wait for the multios to finish writing data. This means the command as shown can exit before file1 and file2 are completely written. As a workaround, it is possible to run the cat process as part of a job in the current shell:

{ cat file } >file1 >file2

Here, the {...} job will pause to wait for both files to be written.

So the safe way would be to always enclose the command in braces if one wants to be sure that the copy finishes before they can consume it in following commands.

Answer 5

As @jofel mentioned in a comment under the answer, this can be done natively in zsh:

echo foobar >file1 >file2 >file3

or, with brace expansion:

echo foobar >file{1..3}

Internally this works very similarly to the tee answers provided above. The shell connects the command's stdout to a process that pipes to multiple files; therefore, there isn't any compelling technical advantage to doing it this way (but it does look real good). See the zsh manual for more.

Answer 6

Unable to comment, however, another way to express

echo "foobarbaz" | tee file1 file2 file3 file4 file5 file6 file7 file8 > /dev/null

Could be simplified to this, when dealing with many files.

echo "foobarbaz" | tee file{1..8} > /dev/null

Answer 7

This is the millenial way using JSON, and splitting on newlines. It requires that the producer process in the pipeline writes JSON to stdout and then that gets parsed and multiplexed.

#!/usr/bin/env bash
function fan_out_to_multiple_files() {(
set -eo pipefail;
local last_tidbit=''

local first='true'
local last_line=''

while read line; do

    for i in $(echo "$line" | tr ';' '\n'); do

        last_line="${i}"

        if [[ "$first" == 'true' ]]; then
           first='false';
           i="${last_tidbit}${i}"
           last_tidbit='';
        fi

        echo "i is '$i'"
       ((
         file_path="$(echo "$i" | jq -r '.file_path')"

         if [[ -n "$file_path" ]]; then
              echo "$i" > "$file_path"
          fi

       ) || { echo; }; ) &> /dev/null &

    done;

    # assign
    last_tidbit="$last_line"
    first='false';

  done;

  wait;

)}

you run it like this:

echo -e '{"file_path":"bar123.json"}\n{"file_path":"foo123.json"}' | fan_out_to_multiple_files

this will yield two files on your fs, named foo123.json and bar123.json.

note I write a lot of my bash functions like this:

function has_embedded_subshell {(
   set -eo pipefail; ## now I can set flags without ever effecting current shell
)}