diff --git a/LearnBash.sh b/LearnBash.sh new file mode 100644 index 0000000..58b5135 --- /dev/null +++ b/LearnBash.sh @@ -0,0 +1,498 @@ +#!/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