bash

bash manpage
more examples: 100 shell script examples / bash example

[bash tutorial](http://linuxconfig.org/bash-scripting-tutorial)

#!/bin/bash
#Variables
STRING="HELLO WORLD!!!"
echo $STRING 

OF=myhome_directory_$(date +%Y%m%d).tar.gz
tar -czf $OF /home/linuxconfig 

#Global vs. Local variables
#Define bash global variable
#This variable is global and can be used anywhere in this bash script
VAR="global variable"
function bash {
#Define bash local variable
#This variable is local to bash function only
local VAR="local variable"
echo $VAR
}
echo $VAR
bash
# Note the bash global variable did not change
# "local" is bash reserved word
echo $VAR

#Passing arguments to the bash script
# use predefined variables to access passed arguments
#echo arguments to the shell
echo $1 $2 $3 ' -> echo $1 $2 $3'

# We can also store arguments from bash command line in special array
args=("$@")
#echo arguments to the shell
echo ${args[0]} ${args[1]} ${args[2]} ' -> args=("$@"); echo ${args[0]} ${args[1]} ${args[2]}'

#use $@ to print out all arguments at once
echo $@ ' -> echo $@'

# use $# variable to print out
# number of arguments passed to the bash script
echo Number of arguments passed: $# ' -> echo Number of arguments passed: $#' 

#Executing shell commands with bash
# use backticks " ` ` " to execute shell command
echo `uname -o`
# executing bash command without backticks
echo uname -o 

#Reading User Input
echo -e "Hi, please type the word: \c "
read  word
echo "The word you entered is: $word"
echo -e "Can you please enter two words? "
read word1 word2
echo "Here is your input: \"$word1\" \"$word2\""
echo -e "How do you feel about bash scripting? "
# read command now stores a reply into the default build-in variable $REPLY
read
echo "You said $REPLY, I'm glad to hear that! "
echo -e "What are your favorite colours ? "
# -a makes read command to read into an array
read -a colours
echo "My favorite colours are also ${colours[0]}, ${colours[1]} and ${colours[2]}:-)" 

#Bash Trap Command
# bash trap command
trap bashtrap INT
# bash clear screen command
clear;
# bash trap function is executed when CTRL-C is pressed:
# bash prints message => Executing bash trap subrutine !
bashtrap()
{
    echo "CTRL+C Detected !...executing bash trap !"
}
# for loop from 1/10 to 10/10
for a in `seq 1 10`; do
    echo "$a/10 to Exit." 
    sleep 1;
done
echo "Exit Bash Trap Example!!!" 

#Arrays
#Declare simple bash array
#Declare array with 4 elements
ARRAY=( 'Debian Linux' 'Redhat Linux' Ubuntu Linux )
# get number of elements in the array
ELEMENTS=${#ARRAY[@]}

# echo each element in array 
# for loop
for (( i=0;i<$ELEMENTS;i++)); do
    echo ${ARRAY[${i}]}
done 

#Read file into bash array
# Declare array
declare -a ARRAY
# Link filedescriptor 10 with stdin
exec 10<&0
# stdin replaced with a file supplied as a first argument
exec < $1
let count=0

while read LINE; do

    ARRAY[$count]=$LINE
    ((count++))
done

echo Number of elements: ${#ARRAY[@]}
# echo array's content
echo ${ARRAY[@]}
# restore stdin from filedescriptor 10
# and close filedescriptor 10
exec 0<&10 10<&-


#Simple Bash if/else statement
directory="./BashScripting"

# bash check if directory exists
if [ -d $directory ]; then
    echo "Directory exists"
else 
    echo "Directory does not exists"
fi 

#Nested if/else
# Declare variable choice and assign value 4
choice=4
# Print to stdout
 echo "1. Bash"
 echo "2. Scripting"
 echo "3. Tutorial"
 echo -n "Please choose a word [1,2 or 3]? "
# Loop while the variable choice is equal 4
# bash while loop
while [ $choice -eq 4 ]; do

# read user input
read choice
# bash nested if/else
if [ $choice -eq 1 ] ; then

        echo "You have chosen word: Bash"

else                   

        if [ $choice -eq 2 ] ; then
                 echo "You have chosen word: Scripting"
        else

                if [ $choice -eq 3 ] ; then
                        echo "You have chosen word: Tutorial"
                else
                        echo "Please make a choice between 1-3 !"
                        echo "1. Bash"
                        echo "2. Scripting"
                        echo "3. Tutorial"
                        echo -n "Please choose a word [1,2 or 3]? "
                        choice=4
                fi   
        fi
fi
done 


#Arithmetic Comparisons
-lt <
-gt >
-le <=
-ge >=
-eq ==
-ne !=
# declare integers
NUM1=2
NUM2=2
if [ $NUM1 -eq $NUM2 ]; then
    echo "Both Values are equal"
else 
    echo "Values are NOT equal"
fi 

# declare integers
NUM1=2
NUM2=1
if [ $NUM1 -eq $NUM2 ]; then
    echo "Both Values are equal"
else 
    echo "Values are NOT equal"
fi 

# declare integers
NUM1=2
NUM2=1
if   [ $NUM1 -eq $NUM2 ]; then
    echo "Both Values are equal"
elif [ $NUM1 -gt $NUM2 ]; then
    echo "NUM1 is greater then NUM2"
else 
    echo "NUM2 is greater then NUM1"
fi 

#String Comparisons
=   equal
!=  not equal
<   less then
>   greater then
-n s1   string s1 is not empty
-z s1   string s1 is empty

#Declare string S1
S1="Bash"
#Declare string S2
S2="Scripting"
if [ $S1 = $S2 ]; then
    echo "Both Strings are equal"
else 
    echo "Strings are NOT equal"
fi 

#Declare string S1
S1="Bash"
#Declare string S2
S2="Bash"
if [ $S1 = $S2 ]; then
    echo "Both Strings are equal"
else 
    echo "Strings are NOT equal"
fi 

#Bash File Testing

-b filename Block special file
-c filename Special character file
-d directoryname    Check for directory existence
-e filename Check for file existence
-f filename Check for regular file existence not a directory
-G filename Check if file exists and is owned by effective group ID.
-g filename true if file exists and is set-group-id.
-k filename Sticky bit
-L filename Symbolic link
-O filename True if file exists and is owned by the effective user id.
-r filename Check if file is a readable
-S filename Check if file is socket
-s filename Check if file is nonzero size
-u filename Check if file set-ser-id bit is set
-w filename Check if file is writable
-x filename Check if file is executable

file="./file"
if [ -e $file ]; then
    echo "File exists"
else 
    echo "File does not exists"
fi 

while [ ! -e myfile ]; do
# Sleep until file does exists/is created
sleep 1
done 

#Loops 
#Bash for loop
# bash for loop
for f in $( ls /var/ ); do
    echo $f
done 


#from command line
for f in $( ls /var/ ); do echo $f; done 

#Bash while loop
COUNT=6
# bash while loop
while [ $COUNT -gt 0 ]; do
    echo Value of count is: $COUNT
    let COUNT=COUNT-1
done 

#Bash until loop
COUNT=0
# bash until loop
until [ $COUNT -gt 5 ]; do
        echo Value of count is: $COUNT
        let COUNT=COUNT+1
done 

#Control bash loop with
# This bash script will locate and replace spaces
# in the filenames
DIR="."
# Controlling a loop with bash read command by redirecting STDOUT as
# a STDIN to while loop
# find will not truncate filenames containing spaces
find $DIR -type f | while read file; do
# using POSIX class [:space:] to find space in the filename
if [[ "$file" = *[[:space:]]* ]]; then
# substitute space with "_" character and consequently rename the file
mv "$file" `echo $file | tr ' ' '_'`
fi;
# end of while loop
done 


#Bash Functions
# BASH FUNCTIONS CAN BE DECLARED IN ANY ORDER
function function_B {
        echo Function B.
}
function function_A {
        echo $1
}
function function_D {
        echo Function D.
}
function function_C {
        echo $1
}
# FUNCTION CALLS
# Pass parameter to function A
function_A "Function A."
function_B
# Pass parameter to function C
function_C "Function C."
function_D 

#Bash Select
PS3='Choose one word: ' 

# bash select
select word in "linux" "bash" "scripting" "tutorial" 
do
  echo "The word you have selected is: $word"
# Break, otherwise endless loop
  break  
done

exit 0 


#Case statement conditional
echo "What is your preferred programming / scripting language"
echo "1) bash"
echo "2) perl"
echo "3) phyton"
echo "4) c++"
echo "5) I do not know !"
read case;
#simple case bash structure
# note in this case $case is variable and does not have to
# be named case this is just an example
case $case in
    1) echo "You selected bash";;
    2) echo "You selected perl";;
    3) echo "You selected phyton";;
    4) echo "You selected c++";;
    5) exit
esac 


#Bash quotes and quotations
# Escaping Meta characters
#Declare bash string variable
BASH_VAR="Bash Script"

# echo variable BASH_VAR
echo $BASH_VAR

#when meta character such us "$" is escaped with "\" it will be read literally
echo \$BASH_VAR 

# backslash has also special meaning and it can be suppressed with yet another "\"
echo "\\" 

#Single quotes
#Declare bash string variable
 BASH_VAR="Bash Script"

 # echo variable BASH_VAR
 echo $BASH_VAR

 # meta characters special meaning in bash is suppressed when  using single quotes 
 echo '$BASH_VAR  "$BASH_VAR"' 

#Double Quotes
#Declare bash string variable
BASH_VAR="Bash Script"

# echo variable BASH_VAR
echo $BASH_VAR

# meta characters and its special meaning in bash is 
# suppressed when using double quotes except "$", "\" and "`"

echo "It's $BASH_VAR  and \"$BASH_VAR\" using backticks: `date`" 

#Bash quoting with ANSI-C style
\a  alert (bell)    \b  backspace
\e  an escape character \f  form feed
\n  newline \r  carriage return
\t  horizontal tab  \v  vertical tab
\\  backslash   \`  single quote
\nnn    octal value of characters ( see [http://www.asciitable.com/ ASCII table] )  \xnn    hexadecimal value of characters ( see [http://www.asciitable.com/ ASCII table] )

#Arithmetic Operations
let RESULT1=$1+$2
echo $1+$2=$RESULT1 ' -> # let RESULT1=$1+$2'
declare -i RESULT2
RESULT2=$1+$2
echo $1+$2=$RESULT2 ' -> # declare -i RESULT2; RESULT2=$1+$2'
echo $1+$2=$(($1 + $2)) ' -> # $(($1 + $2))' 

#Bash Arithmetics
echo '### let ###'
# bash addition
let ADDITION=3+5
echo "3 + 5 =" $ADDITION

# bash subtraction
let SUBTRACTION=7-8
echo "7 - 8 =" $SUBTRACTION 

# bash multiplication
let MULTIPLICATION=5*8
echo "5 * 8 =" $MULTIPLICATION

# bash division
let DIVISION=4/2
echo "4 / 2 =" $DIVISION

# bash modulus
let MODULUS=9%4
echo "9 % 4 =" $MODULUS

# bash power of two
let POWEROFTWO=2**2
echo "2 ^ 2 =" $POWEROFTWO


echo '### Bash Arithmetic Expansion ###'
# There are two formats for arithmetic expansion: $[ expression ] 
# and $(( expression #)) its your choice which you use

echo 4 + 5 = $((4 + 5))
echo 7 - 7 = $[ 7 - 7 ]
echo 4 x 6 = $((3 * 2))
echo 6 / 3 = $((6 / 3))
echo 8 % 7 = $((8 % 7))
echo 2 ^ 8 = $[ 2 ** 8 ]


echo '### Declare ###'

echo -e "Please enter two numbers \c"
# read user input
read num1 num2
declare -i result
result=$num1+$num2
echo "Result is:$result "

# bash convert binary number 10001
result=2#10001
echo $result

# bash convert octal number 16
result=8#16
echo $result

# bash convert hex number 0xE6A
result=16#E6A
echo $result 

#Round floating point number
# get floating point number
floating_point_number=3.3446
echo $floating_point_number
# round floating point number with bash
for bash_rounded_number in $(printf %.0f $floating_point_number); do
echo "Rounded number with bash:" $bash_rounded_number
done 

#Bash floating point calculations
# Simple linux bash calculator 
echo "Enter input:" 
read userinput
echo "Result with 2 digits after decimal point:"
echo "scale=2; ${userinput}" | bc 
echo "Result with 10 digits after decimal point:"
echo "scale=10; ${userinput}" | bc 
echo "Result as rounded integer:"
echo $userinput | bc 

#Redirections
#STDOUT from bash script to STDERR
echo "Redirect this STDOUT to STDERR" 1>&2 
#STDERR from bash script to STDOUT
cat $1 2>&1 
#stdout to screen
ls file1 
#stdout to file
ls file1 > STDOUT