pipes streams and echo codes

pipes_streams_exit_codes.bash

#!/usr/bin/env bash
#  2>&1 # stderr to stdout
#  1>&2 # stdout to stderr
#  2>   # stderr
#  >    # stdout
# see http://www.tldp.org/LDP/abs/html/io-redirection.html
# exec 3>&1; bundle-audit -v >&3 3>&- | tee reports/bundle-audit.txt 3>&- ; exec 3>&-
# https://stackoverflow.com/questions/1221833/bash-pipe-output-and-capture-exit-status
# <command> | tee out.txt ; test "${PIPESTATUS[0]} -eq 0"
# ( set -o pipefail; command | tee out.txt ); ST=$?
# works with `sh`:
# mkfifo pipe
# tee out.txt < pipe &
# command > pipe
# echo $?
set -o pipefail
gem uninstall -aIx bundler-audit
gem install bundler-audit
bundle-audit check -v | tee reports/bundle-audit.txt

piping_to_subprocesses.sh

#!/usr/bin/env bash
# http://www.tldp.org/LDP/abs/html/io-redirection.html
# fd 0 stdin, 1 stdout, 2 stderr
# 1>&2 # stdout to stderr
# 2>&1 # stderr to stdout
#  |&  # was added to Bash 4 as an abbreviation for 2>&1
# &>   # both stdout and stderr
# 2>   filename  # stderr to a file
# >    filename  # stdout to a file
# : >  filename  # truncate file to zero length or creates zero length (same as touch)
# 0<   filename  # accepts input from a file, e.g. grep search-word <filename
#  <   filename
# [j]<>filename  # open file [rw] and assign fd j. create file if not exist. j is 0 (stdin)if not given.
#  An application of this is writing at a specified place in a file. 
#    echo 1234567890 > File    # Write string to "File".
#     exec 3<> File             # Open "File" and assign fd 3 to it.
#     read -n 4 <&3             # Read only 4 characters.
#     echo -n . >&3             # Write a decimal point there.
#     exec 3>&-                 # Close fd 3.
#     cat File                  # ==> 1234.67890
#     #  Random access, by golly.
# command < input-file > output-file # Multiple instances of input and output redirection and/or pipes can be combined in a single command line.
#   # same as '< input-file command > output-file'   # Although this is non-standard.
# n<&-        Close input file descriptor n.
# 0<&-, <&-   Close stdin.
# n>&-        Close output file descriptor n.
# 1>&-, >&-   Close stdout.
# |  # Pipe. General purpose process and command chaining tool.
     # Similar to ">", but more general in effect.
     # Useful for chaining commands, scripts, files, and programs together.
# Child processes inherit open file descriptors.
# This is why pipes work.
# To prevent an fd from being inherited, close it.
# To learn more, see http://www.tldp.org/LDP/abs/html/ioredirintro.html
#                    http://www.tldp.org/LDP/abs/html/filearchiv.html#DERPM

exec 3>&1                   # Saves current 'value' of stdout in fd 3
  some_command 2>&1         # redirect stderr to stdout
  >&3                       # redirects, by default, fd 1 (stdout) to fd 3^
  3>&- |                    # close fd 3^^, pipe does not inherit it, 
  grep ERROR                # fd 3 is closed for 'grep' (but not 'some_command')
  3>&-                      # close fd 3^ for 'some command'
exec 3>&-                   # Now close fd 3 for the remainder of the script

run_command_and_check_contents_of_content.sh

#!/usr/bin/env bash
npm prune
result=0
output=$(npm install --no-progress 2>&1);
if [ -n "$output" ]; then # only check if non-empty output
  echo $output | grep -v 'requires a peer';
  if [ "$?" -ne "0" ]; then # is non-zero when string is present
    echo "Missing peer dependency: $output"
    result=1
  fi;
fi
exit $result