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#!/usr/bin/env bash
# First line of the script is the shebang which tells the system how to execute
# the script: https://en.wikipedia.org/wiki/Shebang_(Unix)
# As you already figured, comments start with #. Shebang is also a comment.

# Simple hello world example:
echo "Hello world!" # => Hello world!

# Each command starts on a new line, or after a semicolon:
echo "This is the first command"; echo "This is the second command"
# => This is the first command
# => This is the second command

# Declaring a variable looks like this:
variable="Some string"

# But not like this:
variable = "Some string" # => returns error "variable: command not found"
# Bash will decide that `variable` is a command it must execute and give an error
# because it can't be found.

# Nor like this:
variable= "Some string" # => returns error: "Some string: command not found"
# Bash will decide that "Some string" is a command it must execute and give an
# error because it can't be found. In this case the "variable=" part is seen
# as a variable assignment valid only for the scope of the "Some string"
# command.

# Using the variable:
echo "$variable" # => Some string
echo '$variable' # => $variable
# When you use a variable itself, assign it, export it, or else, you write
# its name without $. If you want to use the variable's value, you should use $.
# Note that ' (single quote) won't expand variables!
# You can write variables without surrounding double quotes but it's not
# recommended due to how Bash handles variables with spaces in them.

# Parameter expansion ${...}:
echo "${variable}" # => Some string
# This is a simple usage of parameter expansion such as two examples above.
# Parameter expansion gets a value from a variable.
# It "expands" or prints the value.
# During the expansion time the value or parameter can be modified.
# Below are other modifications that add onto this expansion.

# String substitution in variables:
echo "${variable/Some/A}" # => A string
# This will substitute the first occurrence of "Some" with "A".
# Prepend a / before the search string to substitute every instance of it.
variable="Some string. Some character"
echo "${variable//Some/A}" # => A string. A character
# This will substitute every occurrence of "Some" with "A".

# Substring from a variable:
length=7
echo "${variable:0:length}" # => Some st
# This will return only the first 7 characters of the value
echo "${variable: -5}" # => tring
# This will return the last 5 characters (note the space before -5).
# The space before minus is mandatory here.

# String length:
echo "${#variable}" # => 11

# Indirect expansion:
other_variable="variable"
echo ${!other_variable} # => Some string
# This will expand the value of `other_variable`.

# The default value for variable:
echo "${foo:-"DefaultValueIfFooIsMissingOrEmpty"}"
# => DefaultValueIfFooIsMissingOrEmpty
# This works for null (foo=) and empty string (foo=""); zero (foo=0) returns 0.
# Note that it only returns default value and doesn't change variable value.

# Declare an array with 6 elements:
array=(one two three four five six)
# Print the first element:
echo "${array[0]}" # => "one"
# Print all elements:
echo "${array[@]}" # => "one two three four five six"
# Print the number of elements:
echo "${#array[@]}" # => "6"
# Print the number of characters in third element
echo "${#array[2]}" # => "5"
# Print 2 elements starting from fourth:
echo "${array[@]:3:2}" # => "four five"
# Print all elements each of them on new line.
for item in "${array[@]}"; do
    echo "$item"
done

# Built-in variables:
# There are some useful built-in variables, like:
echo "Last program's return value: $?"
echo "Script's PID: $$"
echo "Number of arguments passed to script: $#"
echo "All arguments passed to script: $@"
echo "Script's arguments separated into different variables: $1 $2..."
echo "Current script file: $BASH_SOURCE"
echo "Current function name: $FUNCNAME"
echo "Current line number: $LINENO"
echo "A random number: $RANDOM"             # random int between 0–32767
echo "Seconds since shell started: $SECONDS"
echo "Unix epoch time: $EPOCHSECONDS"       # Bash 5+

# $IFS, Internal Field Separator, controls word splitting (default: space/tab/newline)
IFS=',' read -ra fields <<< "a,b,c"
echo "${fields[1]}" # => b

# $PIPESTATUS, array of exit codes from the last pipeline
false | true | false
echo "${PIPESTATUS[@]}" # => 1 0 1

# Brace Expansion {...}
# used to generate arbitrary strings:
echo {1..10} # => 1 2 3 4 5 6 7 8 9 10
echo {a..z} # => a b c d e f g h i j k l m n o p q r s t u v w x y z
# This will output the range from the start value to the end value.
# Note that you can't use variables here:
from=1
to=10
echo {$from..$to} # => {$from..$to}

# Now that we know how to echo and use variables,
# let's learn some of the other basics of Bash!

# Our current directory is available through the command `pwd`.
# `pwd` stands for "print working directory".
# We can also use the built-in variable `$PWD`.
# Observe that the following are equivalent:
echo "I'm in $(pwd)" # execs `pwd` and interpolates output
echo "I'm in $PWD" # interpolates the variable

# If you get too much output in your terminal, or from a script, the command
# `clear` clears your screen:
clear
# Ctrl-L also works for clearing output.

# Reading a value from input:
echo "What's your name?"
read name
# Note that we didn't need to declare a new variable.
echo "Hello, $name!"

# We have the usual if structure.
# Condition is true if the value of $name is not equal to the current user's login username:
if [[ "$name" != "$USER" ]]; then
    echo "Your name isn't your username"
else
    echo "Your name is your username"
fi

# To use && and || with if statements, you need multiple pairs of square brackets:
read age
if [[ "$name" == "Steve" ]] && [[ "$age" -eq 15 ]]; then
    echo "This will run if $name is Steve AND $age is 15."
fi

if [[ "$name" == "Daniya" ]] || [[ "$name" == "Zach" ]]; then
    echo "This will run if $name is Daniya OR Zach."
fi

# To check if a string is empty or not set use -z and -n to check if it is NOT empty
if [[ -z "$name" ]]; then
    echo "Name is unset"
fi

# There are other comparison operators for numbers listed below:
# -ne - not equal
# -lt - less than
# -gt - greater than
# -le - less than or equal to
# -ge - greater than or equal to

# There is also the `=~` operator, which tests a string against the Regex pattern:
email=[email protected]
if [[ "$email" =~ [a-z]+@[a-z]{2,}\.(com|net|org) ]]
then
    echo "Valid email!"
fi

# There is also conditional execution
echo "Always executed" || echo "Only executed if first command fails"
# => Always executed
echo "Always executed" && echo "Only executed if first command does NOT fail"
# => Always executed
# => Only executed if first command does NOT fail

# A single ampersand & after a command runs it in the background. A background command's
# output is printed to the terminal, but it cannot read from the input.
sleep 30 &
# List background jobs
jobs # => [1]+  Running                 sleep 30 &
# Bring the background job to the foreground
fg
# Ctrl-C to kill the process, or Ctrl-Z to pause it
# Resume a background process after it has been paused with Ctrl-Z
bg
# Kill job number 2
kill %2
# %1, %2, etc. can be used for fg and bg as well

# Redefine command `ping` as alias to send only 5 packets
alias ping='ping -c 5'
# Escape the alias and use command with this name instead
\ping localhost #\ping 192.168.1.1 
# Print all aliases
alias -p

# Expressions are denoted with the following format:
echo $(( 10 + 5 )) # => 15

# Unlike other programming languages, bash is a shell so it works in the context
# of a current directory. You can list files and directories in the current
# directory with the ls command:
ls # Lists the files and subdirectories contained in the current directory

# This command has options that control its execution:
ls -l # Lists every file and directory on a separate line
ls -t # Sorts the directory contents by last-modified date (descending)
ls -R # Recursively `ls` this directory and all of its subdirectories

# Results (stdout) of the previous command can be passed as input (stdin) to the next command
# using a pipe |. Commands chained in this way are called a "pipeline", and are run concurrently.
# The `grep` command filters the input with provided patterns.
# That's how we can list .txt files in the current directory:
ls -l | grep "\.txt"

# Use `cat` to print files to stdout:
cat file.txt

# We can also read the file using `cat`:
Contents=$(cat file.txt)
# "\n" prints a new line character
# "-e" to interpret the newline escape characters as escape characters
echo -e "START OF FILE\n$Contents\nEND OF FILE"
# => START OF FILE
# => [contents of file.txt]
# => END OF FILE

# Use `cp` to copy files or directories from one place to another.
# `cp` creates NEW versions of the sources,
# so editing the copy won't affect the original (and vice versa).
# Note that it will overwrite the destination if it already exists.
cp srcFile.txt clone.txt
cp -r srcDirectory/ dst/ # recursively copy

# Look into `rsync` or `sftp` if you plan on exchanging files between computers.
# `rsync` is the modern standard for fast, resumable, delta-synced transfers.
# `sftp` is more interactive (like an FTP session over SSH).
# Note: `scp` still works but is deprecated by OpenSSH in favor of `sftp`.

# Use `mv` to move files or directories from one place to another.
# `mv` is similar to `cp`, but it deletes the source.
# `mv` is also useful for renaming files!
mv s0urc3.txt dst.txt # sorry, l33t hackers...

# Since bash works in the context of a current directory, you might want to
# run your command in some other directory. We have cd for changing location:
cd ~    # change to home directory
cd      # also goes to home directory
cd ..   # go up one directory
        # (^^say, from /home/username/Downloads to /home/username)
cd /home/username/Documents   # change to specified directory
cd ~/Documents/..    # now in home directory (if ~/Documents exists)
cd -    # change to last directory
# => /home/username/Documents

# Use subshells to work across directories
(echo "First, I'm here: $PWD") && (cd someDir; echo "Then, I'm here: $PWD")
pwd # still in first directory

# Use `mkdir` to create new directories.
mkdir myNewDir
# The `-p` flag causes new intermediate directories to be created as necessary.
mkdir -p myNewDir/with/intermediate/directories
# if the intermediate directories didn't already exist, running the above
# command without the `-p` flag would return an error

# You can redirect command input and output (stdin, stdout, and stderr)
# using "redirection operators". Unlike a pipe, which passes output to a command,
# a redirection operator has a command's input come from a file or stream, or
# sends its output to a file or stream.

# Read from stdin until ^EOF$ and overwrite hello.py with the lines
# between "EOF" (which are called a "here document"):
cat > hello.py << EOF
#!/usr/bin/env python
import sys
print("#stdout", file=sys.stdout)
print("#stderr", file=sys.stderr)
for line in sys.stdin:
    print(line, file=sys.stdout)
EOF
# Variables will be expanded if the first "EOF" is not quoted

# Use <<- to strip leading tabs (useful inside indented functions/if blocks):
if true; then
	cat <<- EOF
	This text can be indented with tabs.
	The leading tabs will be stripped from the output.
	EOF
fi

# Run the hello.py Python script with various stdin, stdout, and
# stderr redirections:
python hello.py < "input.in" # pass input.in as input to the script

python hello.py > "output.out" # redirect output from the script to output.out

python hello.py 2> "error.err" # redirect error output to error.err

python hello.py > "output-and-error.log" 2>&1
# redirect both output and errors to output-and-error.log
# &1 means file descriptor 1 (stdout), so 2>&1 redirects stderr (2) to the current
# destination of stdout (1), which has been redirected to output-and-error.log.

python hello.py > /dev/null 2>&1
# redirect all output and errors to the black hole, /dev/null, i.e., no output

# The output error will overwrite the file if it exists,
# if you want to append instead, use ">>":
python hello.py >> "output.out" 2>> "error.err"

# Overwrite output.out, append to error.err, and count lines:
info bash 'Basic Shell Features' 'Redirections' > output.out 2>> error.err
wc -l output.out error.err

# Run a command and print its file descriptor (e.g. /dev/fd/123)
# see: man fd
echo <(echo "#helloworld")

# Overwrite output.out with "#helloworld":
cat > output.out <(echo "#helloworld")
echo "#helloworld" > output.out
echo "#helloworld" | cat > output.out
echo "#helloworld" | tee output.out >/dev/null

# Cleanup temporary files verbosely (add '-i' for interactive)
# WARNING: `rm` commands cannot be undone
rm -v output.out error.err output-and-error.log
rm -r tempDir/ # recursively delete
# You can install the `trash-cli` Python package to have `trash`
# which puts files in the system trash and doesn't delete them directly
# see https://pypi.org/project/trash-cli/ if you want to be careful

# Commands can be substituted within other commands using $( ):
# The following command displays the number of files and directories in the
# current directory.
echo "There are $(ls | wc -l) items here."

# The same can be done using backticks `` but they can't be nested -
# the preferred way is to use $( ).
echo "There are `ls | wc -l` items here."

# Bash uses a `case` statement that works similarly to switch in Java and C++:
case "$Variable" in
    # List patterns for the conditions you want to meet
    0) echo "There is a zero.";;
    1) echo "There is a one.";;
    *) echo "It is not null.";;  # match everything
esac

# `for` loops iterate for as many arguments given:
# The contents of $Variable is printed three times.
for Variable in {1..3}
do
    echo "$Variable"
done
# => 1
# => 2
# => 3


# Or write it the "traditional for loop" way:
for ((a=1; a <= 3; a++))
do
    echo $a
done
# => 1
# => 2
# => 3

# They can also be used to act on files..
# This will run the command `cat` on file1 and file2
for Variable in file1 file2
do
    cat "$Variable"
done

# ..or the output from a command
# This will `cat` the output from `ls`.
for Output in $(ls)
do
    cat "$Output"
done

# Bash can also accept patterns, like this to `cat`
# all the Markdown files in current directory
for Output in ./*.markdown
do
    cat "$Output"
done

# while loop:
while [ true ]
do
    echo "loop body here..."
    break
done
# => loop body here...

count=0
while [[ "count" -lt 3 ]]; do
    echo "count is $count"
    ((count++))
done
# => count is 0
# => count is 1
# => count is 2

# same with "until loop", runs while the condition is FALSE (opposite of while):
count=0
until [[ "$count" -ge 3 ]]; do
    echo "count is $count"
    (( count++ ))
done
# => count is 0
# => count is 1
# => count is 2

# You can also define functions
# Definition:
function foo ()
{
    echo "Arguments work just like script arguments: $@"
    echo "And: $1 $2..."
    echo "This is a function"
    returnValue=0    # Variable values can be returned
    return $returnValue
}
# Call the function `foo` with two arguments, arg1 and arg2:
foo arg1 arg2
# => Arguments work just like script arguments: arg1 arg2
# => And: arg1 arg2...
# => This is a function
# Return values can be obtained with $?
resultValue=$?
# More than 9 arguments are also possible by using braces, e.g. ${10}, ${11}, ...

# or simply
bar ()
{
    echo "Another way to declare functions!"
    return 0
}
# Call the function `bar` with no arguments:
bar # => Another way to declare functions!

# Calling your function
foo "My name is" $Name

# `local` makes a variable scoped to a function (otherwise it's global):
greet() {
    local greeting="Hi"
    echo "$greeting, $1!"
}
greet "World"  # => Hi, World!
echo "$greeting" # => (empty, variable was local to the function)

# `readonly` prevents a variable from being changed:
readonly PI=3.14159
# PI=3 # => error: PI: readonly variable
# note: Avoid using readonly in the first place if the value might need to change.

# `declare` lets you set variable attributes:
declare -i num=42       # integer, arithmetic is automatic
num+=10
echo "$num"             # => 52  (not "4210")

declare -l lower="HELLO"
echo "$lower"           # => hello  (forced lowercase)

declare -u upper="hello"
echo "$upper"           # => HELLO  (forced uppercase)

declare -r constant="can't change me"  # same as readonly

# Namerefs (Bash 4.3+), a variable that references another variable by name:
declare -n ref=variable
echo "$ref"  # => Some string (the value of $variable)
ref="New value"
echo "$variable"  # => New value (the original variable was changed)

# mapfile / readarray, read lines into an indexed array:
mapfile -t lines < file.txt
echo "${lines[0]}"   # first line
echo "${#lines[@]}"  # total number of lines
# -t strips trailing newlines from each element

# There are a lot of useful commands you should learn:
# prints last 10 lines of file.txt
tail -n 10 file.txt

# prints first 10 lines of file.txt
head -n 10 file.txt

# print file.txt's lines in sorted order
sort file.txt

# report or omit repeated lines, with -d it reports them
uniq -d file.txt

# prints only the first column before the ',' character
cut -d ',' -f 1 file.txt

# replaces every occurrence of 'okay' with 'great' in file.txt
# (regex compatible)
sed -i 's/okay/great/g' file.txt
# be aware that this -i flag means that file.txt will be changed
# -i or --in-place erase the input file (use --in-place=.backup to keep a back-up)

# print to stdout all lines of file.txt which match some regex
# The example prints lines which begin with "foo" and end in "bar"
grep "^foo.*bar$" file.txt

# pass the option "-c" to instead print the number of lines matching the regex
grep -c "^foo.*bar$" file.txt

# Other useful options are:
grep -r "^foo.*bar$" someDir/ # recursively `grep`
grep -n "^foo.*bar$" file.txt # give line numbers
grep -rI "^foo.*bar$" someDir/ # recursively `grep`, but ignore binary files

# perform the same initial search, but filter out the lines containing "baz"
grep "^foo.*bar$" file.txt | grep -v "baz"

# if you literally want to search for the string,
# and not the regex, use `grep -F` (or the deprecated `fgrep`)
grep -F "foobar" file.txt

# The `trap` command allows you to execute a command whenever your script
# receives a signal. Here, `trap` will execute `rm` if it receives any of the
# three listed signals.
trap "rm $TEMP_FILE; exit" SIGHUP SIGINT SIGTERM

# `sudo` is used to perform commands as the superuser
# usually it will ask interactively the password of superuser
NAME1=$(whoami)
NAME2=$(sudo whoami)
echo "Was $NAME1, then became more powerful $NAME2"

# Read Bash shell built-ins documentation with the bash `help` built-in:
help
help help
help for
help return
help source
help .

# Read Bash manpage documentation with `man`
apropos bash
man 1 bash
man bash

# Read info documentation with `info` (`?` for help)
apropos info | grep '^info.*('
man info
info info
info 5 info

# Read bash info documentation:
info bash
info bash 'Bash Features'
info bash 6
info --apropos bash


# SAFER SCRIPTING WITH SET OPTIONS


# Exit immediately if a command fails
set -e

# Treat unset variables as errors
set -u

# If any command in a pipeline fails, the whole pipeline fails
# (without this, only the last command's exit code matters)
set -o pipefail

# You'll often see all three combined at the top of robust scripts:
set -euo pipefail

# To undo any of them, use +
set +e  # disable exit-on-error

# Print each command before executing it, useful for debugging
set -x
echo "this will be printed with a + prefix before running"
set +x  # turn it off again


# FILE TEST OPERATORS

file="example.txt"

if [[ -e "$file" ]]; then echo "exists"; fi
if [[ -f "$file" ]]; then echo "is a regular file"; fi
if [[ -d "$file" ]]; then echo "is a directory"; fi
if [[ -r "$file" ]]; then echo "is readable"; fi
if [[ -w "$file" ]]; then echo "is writable"; fi
if [[ -x "$file" ]]; then echo "is executable"; fi
if [[ -s "$file" ]]; then echo "is non-empty"; fi
if [[ -L "$file" ]]; then echo "is a symbolic link"; fi

# Compare two files
if [[ "file1.txt" -nt "file2.txt" ]]; then echo "file1 is newer"; fi
if [[ "file1.txt" -ot "file2.txt" ]]; then echo "file1 is older"; fi


# ADVANCED STRING MANIPULATION

var="Hello, World!"

# Uppercase / lowercase (Bash 4+)
echo "${var^^}"   # => HELLO, WORLD!
echo "${var,,}"   # => hello, world!
echo "${var^}"    # => Hello, World! (capitalize first letter only)

# Remove prefix (shortest match)
path="/usr/local/bin/script.sh"
echo "${path#/usr/}"         # => local/bin/script.sh

# Remove prefix (longest match)
echo "${path##*/}"           # => script.sh  (like `basename`)

# Remove suffix (shortest match)
echo "${path%.sh}"           # => /usr/local/bin/script

# Remove suffix (longest match)
echo "${path%%/*}"           # => (empty, longest prefix from the left)

# Practical example, get directory from a path (like `dirname`):
echo "${path%/*}"            # => /usr/local/bin

# Replace first occurrence
echo "${var/World/Bash}"     # => Hello, Bash!

# Replace all occurrences
sentence="one two one three one"
echo "${sentence//one/1}"    # => 1 two 1 three 1

# Slice: ${var:offset:length}
echo "${var:7:5}"            # => World

# String length
echo "${#var}"               # => 13

# HERE STRINGS

# Feed a string directly to a command's stdin with <<<
# (a lighter alternative to echo "..." | command)
grep "World" <<< "Hello, World!"
# => Hello, World!

# Useful with read to parse a string into variables
read -r first second <<< "foo bar"
echo "$first"   # => foo
echo "$second"  # => bar

# ASSOCIATIVE ARRAYS (dictionaries)

declare -A person

person["name"]="Alice"
person["age"]=30
person["city"]="Zurich"

echo "${person["name"]}"   # => Alice

# Iterate over keys
for key in "${!person[@]}"; do
    echo "$key = ${person[$key]}"
done

# Check if a key exists
if [[ -v person["name"] ]]; then
    echo "name key exists"
fi

# Delete a key
unset 'person["city"]'

# All keys / all values
echo "${!person[@]}"  # => name age
echo "${person[@]}"   # => Alice 30

# ARITHMETIC

a=5
b=3

echo $(( a + b ))   # => 8
echo $(( a ** b ))  # => 125  (exponentiation)
echo $(( a % b ))   # => 2    (modulo)

# Use as a conditional, (( )) returns true if non-zero
if (( a > b )); then
    echo "$a is greater"
fi

# Increment / decrement
(( a++ ))
(( b -= 2 ))

# `let` is an older alternative
let result=a*b
echo "$result"

# Bash only does integers. For floating point, use bc:
echo "scale=4; 10 / 3" | bc   # => 3.3333
echo "scale=2; sqrt(2)" | bc -l  # => 1.41

# PRINTF (more reliable than echo)

# echo has inconsistent behavior across systems (-e, -n flags vary).
# printf is portable and predictable.

printf "Hello, %s!\n" "World"               # => Hello, World!
printf "Pi is approximately %.4f\n" 3.14159  # => Pi is approximately 3.1416
printf "%05d\n" 42                           # => 00042  (zero-padded)
printf "%-10s | %s\n" "left" "right"         # left-aligned column

# printf does not add a newline by default, you control it with \n


# ENVIRONMENT VARIABLES

# export makes a variable available to child processes
export MY_VAR="hello"

# View all environment variables
env
printenv

# View a single one
printenv HOME

# Unset a variable
unset MY_VAR

# Temporarily set an env variable for a single command only
MY_VAR="temp" some_command   # MY_VAR is not changed in the current shell



# SOURCE / DOT OPERATOR


# `source` (or `.`) runs a script in the CURRENT shell, changes affect your session.
# Unlike `bash script.sh` which runs in a subshell and changes are lost.
source ~/.bashrc
. ~/.bashrc  # identical, dot is the POSIX version

# Common use: load shared functions or config from another file
source ./lib/helpers.sh

# PARSING ARGUMENTS WITH GETOPTS

# getopts is the standard built-in for handling flags like -v, -f filename
# A colon after a letter means it expects an argument

usage() {
    echo "Usage: $0 [-v] [-f filename]"
    exit 1
}

verbose=false
filename=""

while getopts ":vf:" opt; do
    case $opt in
        v) verbose=true ;;
        f) filename="$OPTARG" ;;
        :) echo "Option -$OPTARG requires an argument."; usage ;;
        \?) echo "Invalid option: -$OPTARG"; usage ;;
    esac
done

# Shift past the processed options so $1, $2... refer to remaining args
shift $(( OPTIND - 1 ))

$verbose && echo "Verbose mode on"
[[ -n "$filename" ]] && echo "File: $filename"

# SELECT, INTERACTIVE MENUS

echo "Pick a shell:"
select shell in bash zsh fish quit; do
    case $shell in
        quit) break ;;
        "") echo "Invalid choice" ;;
        *) echo "You picked $shell"; break ;;
    esac
done

# PROCESS SUBSTITUTION

# <(command) treats the output of a command as if it were a file.
# Useful when a command requires a filename, not stdin.

# Compare output of two commands without creating temp files:
diff <(ls dir1/) <(ls dir2/)

# Loop over command output without a subshell (variables persist after the loop)
while IFS= read -r line; do
    echo "Got: $line"
done < <(ls -1)

# ROBUST ERROR HANDLING WITH TRAP

# Clean up temp files no matter how the script exits
TEMP_FILE=$(mktemp)

cleanup() {
    echo "Cleaning up..."
    rm -f "$TEMP_FILE"
}

# ERR fires on any error, EXIT fires when the script ends (for any reason)
trap cleanup EXIT
trap 'echo "Error on line $LINENO"' ERR

# Trap specific signals
trap 'echo "Interrupted!"; exit 1' SIGINT SIGTERM

# SHELL OPTIONS WITH SHOPT

# Enable case-insensitive globbing
shopt -s nocaseglob

# Make ** match files recursively (Bash 4+)
shopt -s globstar
for file in **/*.sh; do
    echo "$file"
done

# nullglob: if a glob matches nothing, expand to nothing instead of the literal pattern
shopt -s nullglob
for f in ./*.nonexistent; do
    echo "$f"  # this loop body never runs if there are no matches
done
shopt -u nullglob  # turn it off again

# extglob: enables extended pattern matching operators
shopt -s extglob
# ?(pattern) , zero or one match
# *(pattern) , zero or more matches
# +(pattern) , one or more matches
# @(pattern) , exactly one match
# !(pattern) , anything that does NOT match
files=(report_2023.txt report_2024.txt notes.txt)
for f in "${files[@]}"; do
    case "$f" in
        !(report_*)) echo "Not a report: $f" ;;
    esac
done

# EXEC

# exec replaces the current shell process with a new command.
# No new process is forked, the shell IS the command from that point on.
# Anything after exec in the script won't run.

# Common use: redirect all script output to a log file from within the script itself
exec > script.log 2>&1
echo "This goes to script.log, not the terminal"

# Or replace the shell with another program entirely
# exec /bin/sh   # uncomment to actually do this


# COPROCESSES

# coproc runs a command in the background with two-way pipes connected to it.
# You can write to its stdin and read from its stdout.
coproc myproc { while read -r line; do echo "echo: $line"; done; }

echo "hello" >&"${myproc[1]}"    # write to the coproc's stdin
read -r reply <&"${myproc[0]}"   # read from the coproc's stdout
echo "$reply"                    # => echo: hello

# Kill the coproc when done
kill "$myproc_PID" 2>/dev/null

# XARGS

# xargs builds and runs commands from stdin, great for piping to commands
# that don't accept stdin directly

# Delete all .tmp files found by find
find . -name "*.tmp" | xargs rm -f

# -I {} lets you place the argument wherever you want in the command
find . -name "*.txt" | xargs -I {} cp {} /backup/

# Run at most 4 parallel jobs with -P
find . -name "*.log" | xargs -P 4 -I {} gzip {}

# Safely handle filenames with spaces using null delimiter
find . -name "*.txt" -print0 | xargs -0 rm -f

# AWK, FIELD PROCESSING

# awk processes text line by line, splitting each into fields ($1, $2, ...)
# Default field separator is whitespace

echo "Alice 30 New York" | awk '{ print $1, "is", $2, "years old" }'
# => Alice is 30 years old

# Use a custom delimiter with -F
echo "name,age,city" | awk -F',' '{ print $3 }'
# => city

# Print lines where the second field is greater than 25
awk -F',' '$2 > 25 { print $1 }' people.csv

# Sum a column
awk -F',' '{ sum += $2 } END { print "Total:", sum }' data.csv

# Print total line count (like wc -l)
awk 'END { print NR }' file.txt


# SED, STREAM EDITING (beyond the basics)

# Substitute with flags:
sed 's/foo/bar/'        file.txt   # first occurrence per line
sed 's/foo/bar/g'       file.txt   # all occurrences
sed 's/foo/bar/2'       file.txt   # second occurrence only
sed 's/foo/bar/gi'      file.txt   # case-insensitive, all occurrences

# Delete lines matching a pattern
sed '/^#/d' file.txt    # remove comment lines

# Print only matching lines (like grep)
sed -n '/pattern/p' file.txt

# Edit in place and keep a backup
sed -i.bak 's/old/new/g' file.txt  # original saved as file.txt.bak

# Address ranges, act only on lines 5 to 10
sed '5,10s/foo/bar/g' file.txt

# Insert a line before/after a match
sed '/pattern/i\New line before' file.txt
sed '/pattern/a\New line after'  file.txt

# MISC SEFUL PATTERNS

# Assign a default only if variable is unset or empty (and persist it)
: "${NAME:=DefaultName}"
echo "$NAME"  # => DefaultName if NAME was previously unset

# Exit with an error message if a required variable is unset
: "${REQUIRED_VAR:?'REQUIRED_VAR must be set'}"

# Check if a command exists before using it
if command -v jq &>/dev/null; then
    echo "jq is installed"
fi

# Time how long a command takes
time sleep 1

# Quickly make a backup copy of a file using brace expansion
cp config.yaml{,.bak}   # expands to: cp config.yaml config.yaml.bak

# Create a whole project directory tree in one line
mkdir -p project/{src,tests,docs,scripts}

# Read a file line by line safely
# (IFS= preserves whitespace; || handles missing trailing newline)
while IFS= read -r line || [[ -n "$line" ]]; do
    echo "Line: $line"
done < file.txt

# Fail loudly if a command errors, with a message, without relying on set -e
some_command || { echo "some_command failed"; exit 1; }

# Capture a multiline string into a variable with a here-doc
# (Quoting 'EOF' prevents variable expansion inside the block)
read -r -d '' BLOCK << 'EOF'
This is
a multiline
string
EOF
echo "$BLOCK"

# THE WAIT BUILT-IN

# `wait` pauses until background jobs finish
sleep 2 &
pid=$!
wait "$pid"   # wait for a specific PID
echo "Process $pid finished with exit code $?"

# wait -n (Bash 4.3+), wait for ANY one background job to finish
sleep 1 &
sleep 3 &
wait -n
echo "First job done"


# BASH 5.3+ FEATURES

# 1. Forkless command substitution, runs in the current shell, no subshell fork.
#    ${ command; } captures stdout like $(), but without forking:
result=${ echo "no fork"; }
echo "$result"  # => no fork

#    ${| command; } runs in the current shell and expects the result in REPLY:
${| REPLY="computed value"; }
echo "$REPLY"   # => computed value
# This is much faster in tight loops since it avoids fork+pipe overhead.

# 2. GLOBSORT, control how pathname expansion results are sorted.
#    Options: name, size, blocks, mtime, atime, ctime, numeric, none
#    Prefix with - for descending order.
GLOBSORT=mtime           # sort globs by modification time (ascending)
ls *.txt
GLOBSORT=-size            # sort by size, largest first
GLOBSORT=none             # no sorting (filesystem order)

# 3. BASH_MONOSECONDS, monotonic clock, not affected by system time changes.
#    Useful for measuring elapsed time reliably.
start=$BASH_MONOSECONDS
sleep 1
elapsed=$(( BASH_MONOSECONDS - start ))
echo "Elapsed: ${elapsed}s"

# 4. EPOCHSECONDS and EPOCHREALTIME (Bash 5.0+, but worth knowing).
echo "$EPOCHSECONDS"     # => 1747500000  (Unix timestamp, integer)
echo "$EPOCHREALTIME"    # => 1747500000.123456  (microsecond precision)

# 5. source -p PATH, specify where to look for the sourced file
#    instead of using $PATH.
source -p /opt/scripts helper.sh

# 6. read -E, enables Readline tab completion during interactive read.
# read -E -p "Enter a filename: " fname

# 7. New loadable builtins:
#    kv      , generate associative arrays from key-value data
#    strptime, convert date strings to Unix timestamps
#    fltexpr , floating-point arithmetic (like `let` but for floats)