This document provides an overview and introduction to BASH scripting and REGEX basics. It includes an agenda that covers BASH introduction, basics, more advanced topics, and an introduction to REGEX. For BASH, it discusses topics like shell, variables, functions, commands, decision making, loops, and more. It provides examples and exercises for many of these BASH scripting concepts. It also briefly discusses the Shellshock bug in the context of understanding shells.

1. BASH Scripting and REGEX basics
FredLUG
How to demystify BASH and REGEX
Peter Larsen
Sr. Solutions Architect, Red HatSeptember 27, 2014

2. Agenda
 Introduction to BASH
 Basics – with exercises
 A little more advanced
 Introduction to REGEX
 Understanding the Shellshock bug

3. BASH Introduction
 Shell – what is it and who cares?
 Current directory, umask
 Exit codes
 Functions
 Built in commands
 Environment Variables
 Traps, Options and Aliases
 sh, ksh, csh and the rest of the family
 How does it work?
 Command Expansion
 Command Execution

4. Command Expansion
 Variable assignments and redirections are saved for later
processing
 Words are expanded. In the result, the first word is taken
to be the command and the rest are arguments
 Redirections are executed
 Text after = are expanded/substituded

5. Command Execution
 If no / in command shell searches for command
 If function, calls function
 If shell built-in, execute
 Search $PATH
 To execute
 Create subshell
 Run command in subshell
 If not found, return exit code 127

6. BASH - Basics
 Hello World
 Variables
 Passing arguments to a script
 Arrays
 Basic Operations
 Basic String Operations
 Decision Making
 Loops
 Shell Functions

7. Hello World
 Anything after # is ignored
 Blank lines are ignored
 Start all bash scripts with #!/bin/bash
 To execute a bash script, set execution bit or run:
bash[scriptname]
 Always provide path to script or place it in $HOME/bin

8. Hello World -Example
 Create a script that writes „Hello, World!“ on the screen
 The „print“ command in bash is called echo – it's an
internal bash command.
#!/bin/bash
echo "Hello, World!"

9. Variables
 Variables are created when assigned
 Syntax: VAR=VALUE
 Note: No spaces before/after the =
 Case sensitive – one word, can contain _ but not other special characters
 Read variables by adding $ in front of it or use ${var}
 Use to escape special characters like $
 Preserve white-spaces with “ “
 Assign results of commands to variables with ` (back-ticks) or $(command)

10. Variables – Examples
PRICE_PER_APPLE=5
MyFirstLetters=ABC
greeting='Hello world!'
PRICE_PER_APPLE=5
echo "The price of an Apple today is: $HK
$PRICE_PER_APPLE"
MyFirstLetters=ABC
echo "The first 10 letters in the alphabet are:
${MyFirstLetters}DEFGHIJ"
greeting='Hello world!'
echo $greeting now with spaces:
"$greeting"
FILELIST=`ls`
FileWithTimeStamp=/tmp/my-dir/file_$(/bin/date +%Y-%m-
%d).txt

11. Variables - Exercise
 Create 3 variables in the sample code:
 String (BIRTHDATE) – contain the text “Aug 11 1967”
 Integer (PRESENTS) – contain the number 10
 Complex (BIRTHDAY) – contain the weekday of $BIRTHDATE
(Friday)
 Hint: use the 'date' command to get the weekday from a
date
date -d “$date1” +%A
#!/bin/bash
# Change this code
BIRTHDATE=None
Presents=None
BIRTHDAY=None
# Testing code - do not change it
if [ "$BIRTHDATE" == "Aug 11 1967" ] ; then
echo BIRTHDATE is correct, it is $BIRTHDATE
else
echo "BIRTHDATE is incorrect - please retry"
fi
if [ $Presents == 10 ] ; then
echo I have received $Presents presents
else
echo "Presents is incorrect - please retry"
fi
if [ "$BIRTHDAY" == "Friday" ] ; then
echo I was born on a $BIRTHDAY
else
echo "BIRTHDAY is incorrect - please retry"
fi

12. Variables - Solution
 BIRTHDATE="Sep 27 2014"
 Presents=10
 BIRTHDAY=$(date -d "$BIRTHDATE" +%A)
#!/bin/bash
# Change this code
BIRTHDATE="Aug 11 1967"
Presents=10
BIRTHDAY=$(date -d "$BIRTHDATE" +%A)
# Testing code - do not change it
if [ "$BIRTHDATE" == "Sep 27 2014" ] ; then
echo BIRTHDATE is correct, it is $BIRTHDATE
else
echo "BIRTHDATE is incorrect - please retry"
fi
if [ $Presents == 10 ] ; then
echo I have received $Presents presents
else
echo "Presents is incorrect - please retry"
fi
if [ "$BIRTHDAY" == "Friday" ] ; then
echo I was born on a $BIRTHDAY
else
echo "BIRTHDAY is incorrect - please retry"
[fredlug@fredlug class]$ bash ./var-solution.sh
BIRTHDATE is correct, it is Aug 11 1967
I have received 10 presents
I was born on a Friday

13. Passing arguments to a script
 Arguments are passed to a script when it's run
 Arguments are given after the command line with spaces between them
 Refer to arguments inside the script with:
 $1 first argument
 $2 second argument
 Etc.
 $0 is the script name
 $# number of arguments
 $@ all parameters space delimited

14. Arguments - Examples
 ./my_shopping.sh apple 5 banana 8 “Fruit Basket” 15
 $ echo $3 →banana
 $ echo “A $5 costs just $6” →A Fruit Basket costs just 15
 $ echo $# →6

15. Arrays
 Several values in the same variable name
 Created with space separated values in ( )
 Total array values: ${#arrayname[@]}
 Use ${array[index]} to refer to values
 Note index numbers start at 0 (not 1).

16. Array - Examples
 my_array=( apple banana “Fruit Basket” orange )
 new_array[2]=apricot
 $ echo ${#my_array[@]} →4
 $ echo ${my_array[3]} →orange
 $ my_array[4]=”carrot”
 $ echo ${#my_array[@]} →5
 $ echo ${my_array[${#my_array[@]}-1]} →carrot

17. Array - Exercise
 Create a bash script
 Define array NAMES with 3 entries: John, Eric and Jessica
 Define array NUMBERS with 3 entries: 1, 2, 3
 Define variable NumberOfNames containing the number of
names in the NAMES array using $# special variable
 Define variable second_name that contains the second name in
the NAMES array
 Print the content of NumberOfNames and second_name
#!/bin/bash
NAMES=( John Eric Jessica )
# write your code here
NUMBERS=(1 2 3)
NumberOfNames=${#NAMES[@]}
second_name=${NAMES[1]}
echo NumberofNames is:
$NumberOfNames
echo second_name is: $second_name
[fredlug@fredlug class]$ bash
./array.sh
NumberofNames is: 3

18. Basic Operations
 Use $((expression))
 Addition: a + b
 Subtraction: a – b
 Multiplication: a * b
 Division: a / b
 Modulo: a % b (integer remainder of a divided with b)
 Exponentitation: a ** b (a to the power of b)

19. Basic Operations - Exercise
 Given
 COST_PINEAPPLE=50
 COST_BANANA=4
 COST_WATERMELON=23
 COST_BASKET=1
 Calculate TOTAL of a fruit basket containing 1 pinapple, 2 bananas
and 3 watermelons
 Print the content of TOTAL
#!/bin/bash
COST_PINEAPPLE=50
COST_BANANA=4
COST_WATERMELON=23
COST_BASKET=1
TOTAL=$(( $COST_BASKET +
( $COST_PINEAPPLE * 1 )
+ ( $COST_BANANA * 2 )
+ ( $COST_WATERMELON * 3 )
))
echo Total is: $TOTAL
$ bash ./operations.sh
Total is: 128

20. Basic String Operations
 STRING=”this is a string”
 String length: ${#STRING} →16
 Numerical position of character:
expr index $STRING “a” →9
 Substring: ${STRING:$POS:$LEN)
 POS=1, LEN=3 →his
 ${STRING:12} →ring # from pos and to the end of var
 Substring replacement: ${STRING[@]/string/text} →this is a text
 Substring replace ALL: ${STRING[@]//is/xx}→thxx xx a string
 Delete all occurrences: ${STRING[@]// a /}→this is string
 Replace first occurrence: ${STRING[@]/#this/that/}
 Replace last occurrence: ${STRING[@]/%string/text}

21. Strings - Exercise
 Given BUFFET="Life is like a snowball. The important thing is
finding wet snow and a really long hill."
 Create ISAY variable with the following changes:
 First occurence of 'snow' with 'foot'
 Delete second occurence of snow
 Replace 'finding' with 'getting'
 Delete all characters following 'wet'
 Print ISAY
#!/bin/bash
BUFFET="Life is like a snowball.
The important thing is finding wet snow and
a really long hill."
ISAY="$BUFFET"
ISAY=${ISAY[@]/snow/foot}
echo First: $ISAY
ISAY=${ISAY[@]/snow/}
echo Second: $ISAY
ISAY=${ISAY[@]/finding/getting}
echo Third: $ISAY
POS=`expr index "$ISAY" "w"`
ISAY=${ISAY:0:POS+3}
$ bash ./string2.sh
First: Life is like a football. The important thing is finding wet snow and a really long hill.
Second: Life is like a football. The important thing is finding wet and a really long hill.
Third: Life is like a football. The important thing is getting wet and a really long hill.
Fourth: Life is like a football. The important thing is getting wet

22. Decision Making
 If [ expression ]; then
code the true part
else
code the false part
fi
 Else can be replace with elif if followed by another if
 Case $variable in
“condition1”)
command ...
;;
“condition2”)
command ...
;;
esac
mycase=1
case $mycase in
1) echo "You selected bash";;
2) echo "You selected perl";;
3) echo "You selected
python";;
4) echo "You selected c++";;
5) exit
esac

23. Expressions
 Can be combined with ! (not), && (and) and || (or)
 Conditional expressions should use [[ ]] (double)
 Nummeric Comparisons
 $a -lt $b $a < $b
 $a -gt $b $a > $b
 $a -le $b $a <= $b
 $a -ge $b $a >= $b
 $a -eq $b $a == $b
 $a -ne $b $a != $b
 String Comparisons
 “$a” = “$b” or “$a” == “$b”
 “$a” != “$b”
 -z “$a” a is empty

24. Decision making - Exercise
 Change variables to make expressions true
#!/bin/bash
# change these variables
NUMBER=10
APPLES=12
KING=GEORGE
# modify above variables
# to make all decisions below TRUE
if [ $NUMBER -gt 15 ] ; then
echo 1
fi
if [ $NUMBER -eq $APPLES ] ; then
echo 2
fi
if [[ ($APPLES -eq 12) || ($KING = "LUIS") ]] ; then
echo 3
fi
if [[ $(($NUMBER + $APPLES)) -le 32 ]] ; then
echo 4
NUMBER=1
6
APPLES=1
6
KING=LUI
S

25. Loops
 “for” loop
for arg in [list]
do
command(s) ....
done
 “while” loop
while [ condition ]
do
command(s) ...
done
 “until” loop
until [ condition ]
do
command(s) ...
done
 “break” - skip iteration
 “continue” - do next loop
now

26. Loop Examples
# loop on array member
NAMES=(Joe Jenny Sara Tony)
for N in ${NAMES[@]} ; do
echo My name is $N
done
# loop on command output results
for f in $( ls *.sh /etc/localtime ) ;
do
echo "File is: $f"
done
COUNT=4
while [ $COUNT -gt 0 ]; do
echo Value of count is:
$COUNT
COUNT=$(($COUNT - 1))
done
COUNT=1
until [ $COUNT -gt 5 ]; do
echo Value of count is:
$COUNT
COUNT=$(($COUNT + 1))
done
# Prints out 0,1,2,3,4
COUNT=0
while [ $COUNT -ge 0 ]; do
echo Value of COUNT is: $COUNT
COUNT=$((COUNT+1))
if [ $COUNT -ge 5 ] ; then
break
fi
done
# Prints out only odd numbers - 1,3,5,7,9
COUNT=0
while [ $COUNT -lt 10 ]; do
COUNT=$((COUNT+1))
# Check if COUNT is even
if [ $(($COUNT % 2)) = 0 ] ; then
continue
fi
echo $COUNT
done

27. Loop Exercise
 NUMBERS=(951 402 984 651 360 69 408 319 601 485
980 507 725 547 544 615 83 165 141 501 263)
 Print all even numbers in order of array
 Do not print anything after 547
#!/bin/bash
NUMBERS=(951 402 984 651 360 69 408 319 601 485
980 507 725 547 544 615 83 165 141 501 263)
for num in ${NUMBERS[@]}
do
if [ $num == 547 ]; then
break
fi
MOD=$(( $num % 2 ))
if [ $MOD == 0 ]; then
echo $num
fi
done
$ bash
./loops.sh
402
984
360
408
980

28. Shell Functions
 Sub-routine that implements set of commands and
operations.
 Can take parameters
 Useful for repeated tasks
 function_name {
command ....
}
function function_B {
echo Function B.
}
function function_A {
echo $1
}
function adder {
echo $(($1 + $2))
}
# FUNCTION CALLS
# Pass parameter to function A
function_A "Function A." # Function
A.
function_B # Function B.
# Pass two parameters to function adder
adder 12 56 # 68

29. Functions - Exersize
 Write a function ENGLISH_CALC which process the
following:
 ENGLISH_CALC 3 plus 5
 ENGLISH_CALC 5 minus 1
 ENGLISH_CALC 4 times 6
 The function prints the results as 3 + 5 = 8, 5 – 1 = 4 etc.
#!/bin/bash
function ENGLISH_CALC {
NUM1=$1 ; OPTXT=$2 ; NUM2=$3
case $OPTXT in
plus) OP='+' ;;
minus) OP='-' ;;
times) OP='*' ;;
*)
echo Bad operator $OPTXT
;;
esac
echo $NUM1 "$OP" $NUM2 = $(($NUM1 $OP
$NUM2))
}
ENGLISH_CALC 3 plus 5
ENGLISH_CALC 5 minus 1
ENGLISH_CALC 4 times 6

30. BASH – Advanced
 Special Variables
 Bash trap command
 File testing
 There's a lot more features – this is not comprehensive
 $man bash is your friend

31. Special Variables
* $* = “$1 $2 $3 ......” $ Process ID of shell
@ $@ “$1” “$2” “$3” ..... ! Process ID of most recent
background process
# Number of parameters 0 Name of shell or program being
executed
? Exit status _ Aboslute path of shell or command
- Current option flags (shopt)

32. Bash trap command
 “trap” executes a script automatically when a signal is
received
 $ trap program sigspec
 List all signals with “trap -l”
 Great for catching a HUP or INT to clean up temporary
files etc before exiting

33. File testing
 Used as a condition to set actions based on file attributes
 Exists, readable, writable etc.
 File1 older/newer than File2
 Commonly used in if statements [ ] [[ ]] etc.

34. File Testing options
 -f regular file exists
 -d directory exists
 -h symbolic link exists
 -r file is readable
 -w file is writable
 file1 -nt file2: file1 newer
than file2
 file1 -ot file3: file1 older
than file2
 file1 -ef file2: file1 and file2
refers to same inode

35. Regular Expressions
 Characters/Strings
 Character Classes and Bracket Expressions
 Anchoring
 Backslash and special expressions
 Repetition
 Concatenation, Alternation, Precedence

36. Demo file
 Create a file resolv.conf with the following contents
 Make sure grep is aliased to:
grep –color=auto

 ; generated by /sbin/dhclient-script ^$[](){}-
?*.+:_
search brq.com mylab.brq.com lab.eng.brq.com world.com
nameserver 12.14.255.7
nameserver 14.14.255.6

37. Characters/Strings
 Simple Character strings are matched as you would expect

38. Character Classes and Bracket Expressions
 [ ] is a set of characters that matches. A string matches if it
matches any of the characters in the set. A ^ inside the [ ]
means do not patch
 Predefined sets like [[:alnum:]] [[:digit:]] exists to make
writing easier
 Decimal point (.) matches any single character

39. Examples

40. Anchoring
 Locks the search pattern to a specific position
 ^ beginning of line
 $ end of line

41. Backslash and special expressions
 Backslashes can prefix special
functions
 < = Start of word
 > = End of word
 b = beginning of word
 B = not b
 w = word
 W = not word

42. Repetition
 * repeats 0 or more times
 + repeats 1 or more times
 ? repeats 0 or 1 time
 {5} repeats 5 times
 {2,3} repeats 2 or 3 times

43. Concatenation, Alternation, Precedence
 Concatenation: sequence of characters (literal/special)
 Alternation: Separate different patterns with |
 Precedence: Parentheses, Repetition, Concatenation,
Alternation
 Use ( ) to group things together for later reference

44. Concatenation/Alteration example

45. #shellshock – the famous BASH bug
 So what is it? CVE-2014-6271
 Try this at your command prompt:
x='() { :;}; echo vulnerable' bash -c "echo test"
 Does it print vulnerable? If so, you need to update your
BASH right away.
 $ rpm -q bash
Should report version 4.2.45-5.4 – if not “yum update” now.

46. #shellshock - how
 A little known “hack” allows functions to be treated as
variables
 $ function foo { echo "hi mom"; }
$ export -f foo
$ bash -c 'foo' # Spawn nested shell, call 'foo'
hi mom
 Great Blog:http://lcamtuf.blogspot.com/2014/09/quick-
notes-about-bash-bug-its-impact.html#comment-form

47. #shellshock – how continued
 $ foo='() { echo "hi mom"; }' bash -c 'foo'
hi mom
 Let's break it down
foo='() { echo “hi mom”;}'
“magic property” () {
is executed before the command is run
execute bash running foo
 Since env variables are used by httpd, dhcpd and other daemons, it potentially
allows them to run code by simply setting a value in a variable.

48. Thanks for coming
Questions?