Best Practices for Writing Bash Scripts

#!/usr/bin/env bash
# Bash3 Boilerplate. Copyright (c) 2014, kvz.io
set -o errexit
set -o pipefail
set -o nounset
# set -o xtrace

# Set magic variables for current file & dir
__dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
__file="${__dir}/$(basename "${BASH_SOURCE[0]}")"
__base="$(basename ${__file} .sh)"
__root="$(cd "$(dirname "${__dir}")" && pwd)" # <-- change this as it depends on your app

arg1="${1:-}"

# Processes a file.
# $1 - the name of the input file
# $2 - the name of the output file
process_file(){
    local -r input_file="$1";  shift
    local -r output_file="$1"; shift
}

declare -r -i port_number=8080
declare -r -a my_array=( apple orange )

my_function() {
    local -r name=apple
}

# using a string to hold a collection
declare -r hosts="host1 host2 host3"
for host in $hosts  # not quoting $hosts here, since we want word splitting
do
    echo "$host"
done

# use an array instead!
declare -r -a host_array=( host1 host2 host3 )
for host in "${host_array[@]}"
do
    echo "$host"
done

main() {
    # print each argument
    for i in "$@"
    do
        echo "$i"
    done
}
# pass all arguments to main
main "$@"

declare -r my_file="/var/tmp/blah"

# instead of dirname, use:
declare -r file_dir="{my_file%/*}"

# instead of basename, use:
declare -r file_base="{my_file##*/}"

# instead of sed 's/blah/hello', use:
declare -r new_file="${my_file/blah/hello}"

# instead of bc <<< "2+2", use:
echo $(( 2+2 ))

# instead of grepping a pattern in a string, use:
[[ $line =~ .*blah$ ]]

# instead of cut -d:, use an array:
IFS=: read -a arr <<< "one:two:three"

# instead of
cat file | command
# use
command < file

# instead of
echo text | command
# use
command <<< text

# for portability, use a heredoc
command << END
text
END

# instead of
grep pattern file | awk '{print $1}'
# use
awk '/pattern/{print $1}'

# instead of
grep pattern file | sed 's/foo/bar/g'
# use
sed -n '/pattern/{s/foo/bar/p}' file

shopt -s nullglob
shopt -s extglob

# get all files with a .yyyymmdd.txt suffix
declare -a dated_files=( *.[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9].txt )

# get all non-zip files
declare -a non_zip_files=( !(*.zip) )

# instead of
find . -type f -mtime +5 | xargs rm -f
# use
find . -type f -mtime +5 -print0 | xargs -0 rm -f

# looping over files
find . -type f -print0 | while IFS= read -r -d $'' filename; do
    echo "$filename"
done

# using temp files
command1 > file1
command2 > file2
diff file1 file2
rm file1 file2

# using process substitution
diff <(command1) <(command2)

# don't use exit status
grep -q pattern file
if (( $? == 0 ))
then
    echo "pattern was found"
fi

# use the command as the condition
if grep -q pattern file
then
    echo "pattern was found"
fi

echo "An error message" >&2