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+---
+category: tool
+tool: bash
+contributors:
+    - ["Max Yankov", "https://github.com/golergka"]
+    - ["Darren Lin", "https://github.com/CogBear"]
+    - ["Alexandre Medeiros", "http://alemedeiros.sdf.org"]
+    - ["Denis Arh", "https://github.com/darh"]
+filename: LearnBash.sh
+---
+
+Bash is a name of the unix shell, which was also distributed as the shell for the GNU operating system and as default shell on Linux and Mac OS X.
+Nearly all examples below can be a part of a shell script or executed directly in the shell.
+
+[Read more here.](http://www.gnu.org/software/bash/manual/bashref.html)
+
+```bash
+#!/bin/bash
+# First line of the script is shebang which tells the system how to execute
+# the script: http://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!
+
+# Each command starts on a new line, or after semicolon:
+echo 'This is the first line'; echo 'This is the second line'
+
+# Declaring a variable looks like this:
+VARIABLE="Some string"
+
+# But not like this:
+VARIABLE = "Some string"
+# Bash will decide that VARIABLE is a command it must execute and give an error
+# because it couldn't be found.
+
+# Using the variable:
+echo $VARIABLE
+echo "$VARIABLE"
+echo '$VARIABLE'
+# When you use the variable itself — assign it, export it, or else — you write
+# its name without $. If you want to use variable's value, you should use $.
+# Note that ' (single quote) won't expand the variables!
+
+# String substitution in variables
+echo ${VARIABLE/Some/A}
+# This will substitute the first occurance of "Some" with "A"
+
+# Substring from a variable
+echo ${VARIABLE:0:7}
+# This will return only the first 7 characters of the value
+
+# Default value for variable
+echo ${FOO:-"DefaultValueIfFOOIsMissingOrEmpty"}
+# This works for null (FOO=), empty string (FOO=""), zero (FOO=0) returns 0
+
+# Builtin variables:
+# There are some useful builtin variables, like
+echo "Last program return value: $?"
+echo "Script's PID: $$"
+echo "Number of arguments: $#"
+echo "Scripts arguments: $@"
+echo "Scripts arguments seperated in different variables: $1 $2..."
+
+# 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:
+# use 'man test' for more info about conditionals
+if [ $NAME -ne $USER ]
+then
+    echo "Your name is your username"
+else
+    echo "Your name isn't your username"
+fi
+
+# There is also conditional execution
+echo "Always executed" || echo "Only executed if first command fails"
+echo "Always executed" && echo "Only executed if first command does NOT fail"
+
+# Expressions are denoted with the following format:
+echo $(( 10 + 5 ))
+
+# Unlike other programming languages, bash is a shell — so it works in a context
+# of current directory. You can list files and directories in the current
+# directory with the ls command:
+ls
+
+# These commands have options that control their execution:
+ls -l # Lists every file and directory on a separate line
+
+# Results of the previous command can be passed to the next command as input.
+# grep command filters the input with provided patterns. That's how we can list
+# .txt files in the current directory:
+ls -l | grep "\.txt"
+
+# You can also redirect a command, input and error output.
+python2 hello.py < "input.in"
+python2 hello.py > "output.out"
+python2 hello.py 2> "error.err"
+# The output error will overwrite the file if it exists, if you want to
+# concatenate them, use ">>" instead.
+
+# 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.";;
+esac
+
+# for loops iterate for as many arguments given:
+# The contents of var $VARIABLE is printed three times.
+for VARIABLE in {1..3}
+do
+    echo "$VARIABLE"
+done
+
+# while loop:
+while [true]
+do
+    echo "loop body here..."
+    break
+done
+
+# You can also define functions
+# Definition:
+function foo ()
+{
+    echo "Arguments work just like script arguments: $@"
+    echo "And: $1 $2..."
+    echo "This is a function"
+    return 0
+}
+
+# or simply
+bar ()
+{
+    echo "Another way to declare functions!"
+    return 0
+}
+
+# Calling your function
+foo "My name is" $NAME
+
+# There are a lot of useful commands you should learn:
+tail -n 10 file.txt
+# prints last 10 lines of file.txt
+head -n 10 file.txt
+# prints first 10 lines of file.txt
+sort file.txt
+# sort file.txt's lines
+uniq -d file.txt
+# report or omit repeated lines, with -d it reports them
+cut -d ',' -f 1 file.txt
+# prints only the first column before the ',' character
+```