Monday, August 3, 2009









This
bash script tutorial assumes no previous knowledge of bash scripting.As
you will soon discover in this quick comprehensive bash scripting
guide, learning the bash shell scripting is very easy task. Lets begin
this bash scripting tutorial with a simple "Hello World" script. Let's
start with Learning the bash Shell: Unix Shell Programming


1. Hello World Bash
Shell Script






First you need to find out where is your bash interpreter
located. Enter the following into your command line:



$ which bash 


bash interpreter location: /bin/bash


Open up you favorite text editor and a create file called
hello_world.sh. Insert the following lines to a file:


NOTE:Every bash shell script in this tutorial starts with shebang:"#!"
which is not read as a comment. First line is also a place where you
put your interpreter which is in this case: /bin/bash.


Here is our first bash shell script example:



#!/bin/bash
# declare STRING variable
STRING="Hello World"
#print variable on a screen
echo $STRING


















Navigate to a directory where your hello_world.sh is located
and make the file executable:



$ chmod +x hello_world.sh 


Make bash shell script executable


Now you are ready to execute your first bash script:



./hello_world.sh 


Example of simple bash shell script

2. Simple Backup
bash shell script



#!/bin/bash
tar -czf myhome_directory.tar.gz /home/linuxconfig












Simple Backup bash script

3. Variables


In this example we declare simple bash variable and print it on the
screen ( stdout ) with echo command.
#!/bin/bash
STRING="HELLO WORLD!!!"
echo $STRING

Bash string Variables in bash script


Your backup script and variables:


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

Bash backup Script with bash Variables

3.1. Global vs. Local
variables


#!/bin/bash
#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

Global vs. Local Bash variables in bash script

4. Passing
arguments to the bash script


#!/bin/bash
# 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: $#'

/arguments.sh Bash Scripting Tutorial 

Passing arguments to the bash script

5. Executing
shell commands with bash


#!/bin/bash
# use backticks " ` ` " to execute shell command
echo `uname -o`
# executing bash command without backticks
echo uname -o

Executing shell commands with bash

6. Reading User Input


#!/bin/bash

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]}:-)"

Reading User Input with bash

7. Bash Trap Command










#!/bin/bash
# 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!!!"





8. Arrays


8.1. Declare simple
bash array


#!/bin/bash
#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

Declare simple bash array

8.2. Read file into
bash array



#!/bin/bash
#Declare array
declare -a ARRAY
#Open file for reading to array
exec 10
let count=0

while read LINE <&10; do

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

echo Number of elements: ${#ARRAY[@]}
# echo array's content
echo ${ARRAY[@]}
# close file
exec 10>&-


Read file into bash array

9. Bash if / else /
fi statements


9.1. Simple Bash
if/else statement


Please note the spacing inside the [ and ] brackets! Without the
spaces, it won't work!
#!/bin/bash
directory="./BashScripting"

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

Bash if else fi statement

9.2. Nested if/else


#!/bin/bash

# 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

Nested Bash if else statement

10. Bash Comparisons


10.1. Arithmetic
Comparisons





























-lt <
-gt >
-le <=
-ge >=
-eq ==
-ne !=

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

Bash Arithmetic Comparisons
#!/bin/bash
# declare integers
NUM1=2
NUM2=1
if [ $NUM1 -eq $NUM2 ]; then
echo "Both Values are equal"
else
echo "Values are NOT equal"
fi

Bash Arithmetic Comparisons - values are NOT equal
#!/bin/bash
# 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

Bash Arithmetic Comparisons - greater then

10.2. String Comparisons





























= equal
!= not equal
< less then
> greater then
-n s1string s1 is not empty
-z s1string s1 is empty

#!/bin/bash
#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

Bash String Comparisons - values are NOT equal
#!/bin/bash
#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 interpreter location: /bin/bash

11. Bash File Testing





































































-b filenameBlock special file
-c filenameSpecial character file
-d directorynameCheck for directory existence
-e filenameCheck for file existence
-f filenameCheck for regular file existence not a
directory
-G filenameCheck if file exists and is owned by
effective group ID.
-g filenametrue if file exists and is set-group-id.
-k filenameSticky bit
-L filenameSymbolic link
-O filenameTrue if file exists and is owned by the
effective user id.
-r filenameCheck if file is a readable
-S filenameCheck if file is socket
-s filenameCheck if file is nonzero size
-u filenameCheck if file set-ser-id bit is set
-w filenameCheck if file is writable
-x filenameCheck if file is executable

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

Bash File Testing - File does not exist Bash File Testing - File exists


Similarly
for example we can use while loop to check if file does not exists.
This script will sleep until file does exists. Note bash negator "!"
which negates the -e option.


#!/bin/bash

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

12. Loops


12.1. Bash for loop


#!/bin/bash

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

Running for loop from bash shell command line:
$ for f in $( ls /var/ ); do echo $f; done 

Bash for loop

12.2. Bash while loop


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

Bash while loop

12.3. Bash until loop


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

Bash until loop

12.4. Control bash
loop with


Here is a example of while loop controlled by standard input. Until the
redirection chain from STDOUT to STDIN to the read command exists the
while loop continues.
#!/bin/bash
# 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 script to replace spaces in the filenames with _

13. Bash Functions


!/bin/bash
# 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 Functions

14. Bash Select


#!/bin/bash

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

Bash Select

15. Case statement
conditional


#!/bin/bash
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 case statement conditiona

16. Bash quotes and
quotations


Quotations and quotes are important part of bash and bash scripting.
Here are some bash quotes and quotations basics.

16.1. Escaping Meta
characters


Before we start with quotes and quotations we should know something
about escaping meta characters. Escaping will suppress a special
meaning of meta characters and therefore meta characters will be read
by bash literally. To do this we need to use backslash "\" character.
Example:
#!/bin/bash

#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 "\\"

escaping meta characters in bash

16.2. Single quotes


Single quotes in bash will suppress special meaning of every meta
characters. Therefore meta characters will be read literally. It is not
possible to use another single quote within two single quotes not even
if the single quote is escaped by backslash.
#!/bin/bash

#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"'

Using single quotes in bash

16.3. Double Quotes


Double quotes in bash will suppress special meaning of every meta
characters except "$", "\" and "`". Any other meta characters will be
read literally. It is also possible to use single quote within double
quotes. If we need to use double quotes within double quotes bash can
read them literally when escaping them with "\". Example:
#!/bin/bash

#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`"

Using double quotes in bash

16.4. Bash
quoting with ANSI-C style


There is also another type of quoting and that is ANSI-C. In this type
of quoting characters escaped with "\" will gain special meaning
according to the ANSI-C standard.







































/aalert (bell)/bbackspace
/ean escape character/fform feed
/nnewline/rcarriage return
/thorizontal tab/vvertical tab
\\backslash\`single quote
\nnnoctal value of characters ( see
[http://www.asciitable.com/ ASCII table] )
\xnnhexadecimal value of characters ( see
[http://www.asciitable.com/ ASCII table] )

The syntax fo ansi-c bash quoting is: $'' . Here is an example:
#!/bin/bash

# as a example we have used \n as a new line, \x40 is hex value for @
# and \56 is octal value for .
echo $'web: www.linuxconfig.org\nemail: web\x40linuxconfig\56org'

quoting in bash with ansi-c stype

17. Arithmetic Operations


17.1. Bash
Addition Calculator Example




#!/bin/bash

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 Addition Calculator

17.2. Bash Arithmetics


#!/bin/bash

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

Bash Arithmetic Operations

17.3. Round
floating point number


#!/bin/bash
# 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

Round floating point number with bash

17.4. Bash
floating point calculations


#!/bin/bash
# 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

Bash floating point calculations

18. Redirections


18.1. STDOUT
from bash script to STDERR


#!/bin/bash

echo "Redirect this STDOUT to STDERR" 1>&2

To proof that STDOUT is redirected to STDERR we can redirect script's
output to file:

STDOUT from bash script to STDERR

18.2. STDERR
from bash script to STDOUT


#!/bin/bash

cat $1 2>&1

To proof that STDERR is redirected to STDOUT we can redirect script's
output to file:

STDERR from bash script to STDOUT

18.3. stdout to screen


The simple way to redirect a standard output ( stdout ) is to simply
use any command, because by default stdout is automatically redirected
to screen.
cat /proc/partitions 

bash stdout to screen

18.4. stdout to file


Here we use ">" to redirect stdout to a file "partitions.txt".
cat /proc/partitions > partitions.txt 

bash stdout to file

18.5. stderr to file


In this example you will redirect the standard error ( stderr ) to a
file and stdout to a default screen.
grep -r hda6 * . 2> stderr.txt 

bash stderr to file

18.6. stdout to stderr


In this case the output of a command will be written to the same
descriptor as a stderr.
grep -r hda6 * . 1>&2 stderr.txt 

bash stdout to stderr

18.7. stderr to stdout


In this case the stderr of a command will be written to the same
descriptor as a stdout.
grep -r hda6 * . 2>&1 stderr.txt 

bash stderr to stdout

18.8. stderr and
stdout to file


grep -r hda6 * . &> stderr_and_stdout.txt 

stderr and stdout to file

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