add LearnBash.sh for reference

LearnBash.sh

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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 $2..."
+
+# Brace Expansion {...}
+# used to generate arbitrary strings:
+echo {1..10} # => 1 2 3 4 5 6 7 8 9 10
+echo {a..z} # => a b c d e f g h i j k l m n o p q r s t u v w x y z
+# This will output the range from the start value to the end value.
+# Note that you can't use variables here:
+from=1
+to=10
+echo {$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=me@example.com
+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 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 `scp` or `sftp` if you plan on exchanging files between computers.
+# `scp` behaves very similarly to `cp`.
+# `sftp` is more interactive.
+
+# 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
+from __future__ import print_function
+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
+
+# 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...
+
+# 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" $Name
+
+# There are a lot of useful commands you should learn:
+# prints last 10 lines of file.txt
+tail -n 10 file.txt
+
+# prints first 10 lines of file.txt
+head -n 10 file.txt
+
+# print file.txt's lines in sorted order
+sort file.txt
+
+# report or omit repeated lines, with -d it reports them
+uniq -d file.txt
+
+# prints only the first column before the ',' character
+cut -d ',' -f 1 file.txt
+
+# replaces every occurrence of 'okay' with 'great' in file.txt
+# (regex compatible)
+sed -i 's/okay/great/g' file.txt
+# be aware that this -i flag means that file.txt will be changed
+# -i or --in-place erase the input file (use --in-place=.backup to keep a back-up)
+
+# print to stdout all lines of file.txt which match some regex
+# The example prints lines which begin with "foo" and end in "bar"
+grep "^foo.*bar$" 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 `fgrep` (or `grep -F`)
+fgrep "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