bash

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