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 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 Now you are ready to execute your first bash script: ./hello_world.sh 2. Simple Backup bash shell script #!/bin/bash tar -czf myhome_directory.tar.gz /home/linuxconfig 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 $STRING Your backup script and variables: #!/bin/bash OF=myhome_directory_$(date +%Y%m%d).tar.gz tar -czf $OF /home/linuxconfig 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 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 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 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 ${colours[2]}:-)" 7. Bash Trap Command #!/bin/bash # bash trap command trap bashtrap INT # bash clear screen command clear; # bash trap function is executed when CTRL-C is pressed: # bash prints message => Executing bash trap subrutine ! bashtrap() { echo "CTRL+C Detected !...executing bash trap !" } # for loop from 1/10 to 10/10 for a in `seq 1 10`; do echo "$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 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>&- 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 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 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 #!/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 #!/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 10.2. String Comparisons = equal != not equal < less then > greater then -n s1 string s1 is not empty -z s1 string 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 #!/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 11. 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 #!/bin/bash file="./file" if [ -e $file ]; then echo "File exists" else echo "File does not exists" fi 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 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 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 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 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 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 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 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 "\\" 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 "$BASH_VAR"' 16.3. Double Quotes Double quotes in bash will suppress special meaning of every meta characters except "$", "\" 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`" 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. /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] ) 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' 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))' 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 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 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 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: 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: 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 18.4. stdout to file Here we use ">" to redirect stdout to a file "partitions.txt". cat /proc/partitions > partitions.txt 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 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 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 18.8. stderr and stdout to file grep -r hda6 * . &> stderr_and_stdout.txt