bash_stdout_and_stderr

#!/usr/bin/env bash

# STDOUT stands for Standard Output. It is a stream where general output
# like the regular output of any command goes to. It is, by default,
# routed to the screen so things like printf output to the screen.
# in this example, the output goes to a file.

# to route STDOUT to a file, you can do:
echo "this is standard output" > stdout.txt

echo "the contents of STDOUT were: $( cat ./stdout.txt )"
rm ./stdout.txt

# This also works. The 1 is the default, STDOUT.
echo "this is also standard output" 1> stdout.txt

echo "the contents of STDOUT were: $( cat ./stdout.txt )"
rm ./stdout.txt


echo  # -------------------------------


# STDERR stands for Standard Error. It is a stream where
# error output is piped to. This is, by default, also output
# to the screen but is often rerouted for logging reasons.
# in this example, the output goes to a file.

# to route STDERR to a file, you can do:
crap 2> stderr.txt  # the 2 is required for STDOUT.

echo "the contents of STDERR were: $( cat ./stderr.txt )"
rm ./stderr.txt


echo  # -------------------------------


# Both STDOUT and STDERR can be output to the same file.
# The parenthesis around the commands run them together,
# witholding output until everything within completes.
( echo -n "    crap!    "; crap ) &> both.txt

echo "the contents of STDOUT and STDERR were: $( cat ./both.txt )"
rm ./both.txt


echo  # -------------------------------


# a single > for the redirect clears the contents of the
# output file prior to writing to it. A double > allows
# the output to accumulates in the output file,
echo -e "\n    this is a double '>' to standard output" >> stdout.txt
echo "    this is another double '>' to standard output" >> stdout.txt

echo "the contents of STDOUT were: $( cat ./stdout.txt )"
rm ./stdout.txt


echo  # -------------------------------


# both STDOUT and STDERR can be redirected to separate files.
( echo -n "    crap!    "; crap ) 1> stdout.txt 2> stderr.txt

echo "the contents of STDOUT were: $( cat ./stdout.txt )"
echo "the contents of STDERR were: $( cat ./stderr.txt )"
rm ./stdout.txt
rm ./stderr.txt


echo  # -------------------------------


# to throw away the output altogether, redirect it to '/dev/null',
# another stream that throws away anything directed to it.
echo "crap" 1> /dev/null    # this STDOUT output should go nowhere
crap 2> /dev/null           # this STDERR output should go nowhere.

( echo -n "    crap!    "; crap ) &> /dev/null  # all output should go nowhere.

( echo -n "    crap!    "; crap ) 1> /dev/null 2> stderr.txt  # STDOUT output goes nowhere, but STDERR goes to a file.

( echo -n "    crap!    "; crap ) 1> stdout.txt 2> /dev/null  # STDOUT goes to a file but STDERR goes nowhere.

echo "the contents of STDOUT were: $( cat ./stdout.txt )"
echo "the contents of STDERR were: $( cat ./stderr.txt )"
rm ./stdout.txt
rm ./stderr.txt


echo  # -------------------------------


# commands can be strung together using a pipe, '|'. it'll send the
# output from one command to the next one and so on. 'cat' outputs
# a file's contents, just like a DOS Prompt's type command. When
# no arguments are provided to it, it prints out whatever is piped
# to it (technically referred to as STDIN, Standard Input).
echo "some output" | cat | cat | cat | cat | cat | cat | cat