1. BASH Scripting
Intro and Variables

2. Objectives Introduce Unix script writing including:
introducing variables
locality
basic I/O
Introduce script execution
Introduce the .bash_profile file

3. Overview Executing Variables I/O creating/assignment accessing
list variables (arrays)
exporting
read only
eliminating
predefined
I/O
read
echo (write)

4. Example of First Script
-bash-3.2$cat helloworld # show contents of script echo Hello World!! # display Hello World!! on screen -bash-3.2$. helloworld # execute script (note the period space (. ) prior to the script name) Hello World!!

5. Steps for scripts vi sfile requires execute permission sfile
chmodsfile
requires path
PATH+=:dir
. sfile
source sfile
bash sfile
sh sfile
./sfile
sfile

6. Executing a Script Path set3 command chmod1 separate shell2 . scriptNo
source script
bash script
Yes
sh script
./script
script
1 – requires execute mode
2 – runs in separate shell. Variables set have no affect outside of script even with export but exported variables are exported to sub scripts
3 – requires PATH variable to be set to point to current directory (.)

7. Shell Positional Parameters
Parameters are sent by entering values when the shell is called.
Variable
Result $0
name of called script
$1 $2 … $9
parameter 1, 2,… 9 $#
number of parameters
values of all parameters using:
“1” “2”, etc (multiple parameters) $*
“1 2 etc” (a single parameter) $?
return code $$
Process ID (PID)

8. Shell Positional Parameters
Assuming a script called params was called using the parameters a b c:
params a b c
Variable
Result $0
params
$1 $2
a b $# 3
a b c $? $* $$
13061

9. set Parameters may be set within a script using the set command
Example: set w x y z will set parameters 1 through 4 to “w” “x” “y” and “z”, even if they were set to values when the shell was called.
If fewer variables were set than were called the “extra” variables are lost
Example:
if a script called paramsset is called using:
paramsset a b c d
and inside paramsset is the command:
set x y z
x, y, and z are the only values available (“d” is lost)

10. Variables creating assignment must begin with a character (a-z,A-Z)
may contain alphanumeric characters (a-z,A-Z,0-9) and the underscore character(_)
case sensitive, thus Ron is different than ron
assignment
var=val NOTE: no spaces surrounding =
var=“space delimited vals”

11. Viewing Variables Set
Show all variables set: set

12. Variables accessing $var - shows value of variable
${var}othertext – ensures that var is the substitution variable (not varothertext)
${var-val} - shows value of var if defined otherwise uses val (if val includes spaces, it must be enclosed in “; if val is a command must be inclosed in `)

13. Variable Examples Description Command
create a variable using a value or the output from a command
myvar=“Fri Oct 19 11:31:16 EDT 2007” or
myvar=`date`
shows variable
echo $myvar
echo ${myvar-`date`}
clear variable
myvar=
shows a blank line
delete variable
unset myvar
shows current date (myvar no longer exists)
Please note the ` is called a grave accent and is NOT a single quote. It is located above the tab key, on the same key as the tilde ~. Do not confuse this with the single quote (next to the Enter key).

14. More Variable Substitution
${var:opval} op
Purpose
Description =
Initializing var to val if it is not initialized
if var is not set, sets var to the word val +
Test for existence of a variable
If var exists and is not null, return val else return null ?
Catching and terminating scripts with undefined or unitialized variables
if var has been set return its value, else val message displayed script terminates

15. Variable substitution examples
Variables set
Substitution
Result
val=Ron val2=Sue
val:=$val2
Ron
val:+$val2
Sue
val:?$val2
val= val2=Sue
i.e. val has no value
val:=val2
val:+val2
val:?val2
variablesub: line 22:
val: Sue
val2=Sue
i.e. val never set

16. Variable pattern matching # and %
If the pattern matches the beginning of the variable's value, delete the shortest part that matches and return the rest.
${variable##pattern}
If the pattern matches the beginning of the variable's value, delete the longest part that matches and return the rest.
${variable%pattern}
If the pattern matches the end of the variable's value, delete the shortest part that matches and return the rest.
${variable%%pattern}
If the pattern matches the end of the variable's value, delete the longest part that matches and return the rest.

17. Variable pattern matching # and %
Pattern-matching operators are useful for stripping off components of pathnames, such as directory prefixes and filename suffixes.
An easy way to remember the usage is to think of how we write percents. For example 75% the number (#) 75 precedes and the percent sign (%) follows
Doubling the symbol (## or %% indicates repeated deletions until pattern is found.

18. # and % Examples
An example that shows how all of the operators may be used. Assume that the variable path has the value:
/home/student/project/Class.Name.java
Expression
Result
Comments
$path
/home/student/project/Class.Name.java
${path##/*/}
Class.Name.java
deletes all directories until file dot (.) is found
${path#/*/}
student/project/Class.Name.java
deletes first directory only
${path%%.*}
/home/student/project/Class
deletes all dot (.) filenames from right
${path%.*}
/home/student/project/Class.Name
deletes first occurrence from right

19. Variables
Exporting: Variables are considered “local” to the script, that is, a variable used in a particular script has no affect outside of that particular script. The export command will allow the variable to be used in a child script. A child script is a script that has been started from another script (the parent). It is important to note that exporting only goes from parent to child not from child to parent.

20. Variable exporting ParentScript: ChildScript: Script Command Result
name1=Ron
name2=Sue
export name2
sh ChildScript
echo $name2
ChildScript:
echo $name1
echo $name2
name2=Bob
Script
Command
Result
Reason
ChildScript
echo $name1
blank
name1 is local to ParentScript and not exported
echo $name2
Sue
name2 is defined and exported from ParentScript
ParentScript
name2 cannot be transferred to ParentScript from ChildScript so ParentScript’s name2 is unchanged

21. readonly Variables
declaring a value readonly makes it a constant, for example:
name=Ron
readonly name
echo $name
name=Sue
results in the following error:
readonly: line 4: name: readonly variable

22. eliminating Variables
var= simply removes the value (sets it to null) of the variable NOT the variable itself
unset var removes the variable altogether

23. predefined Variables (selected)
Description
HOME
home directory
IFS
Internal field separator
PATH
Directories to search for commands and scripts
CDPATH
Directories to use with the cd command
PS1
Prompt to be used
PS2
Prompt to be used if the command is continued on multiple lines
PS3
Prompt string for select statements

24. predefined Variables (selected)
Description
MAIL
file that receives your mail
MAILCHECK
Interval used by the shell to check for mail (default 600 sec)
SHELL
TERM TZ

25. PS1, PS2 variables (selected*)
Value
Description \a
ASCII bell (the beep)
\A, t, \T
Current Time in HH:MM (24 hours), HH:MM:SS (24 and 12 hours)
\D{format}
Date in format (see
\h, \H
hostname, complete hostname \s
Name of shell \u
Username \v
Version \w
Current directory
Example: PS1=‘\s-\v$’ shows -bash-3.2$, which is the current default.
*For a complete list of variables, see pages in Unix in a Nutshell

26. I/O
read : allow the user to enter data into a variable from standard input (keyboard)
echo : displays contents onto standard output (display)

27. .bash_profile A user may have a hidden file called .bash_profile
This file is really a script that is automatically called when one logs in to the machine.
.bash_profile is responsible for setting up the Unix environment specifically for you. The Windows/DOS equivalents for this was autoexec.bat and config.sys
A key variable in the .bash_profile is the PATH variable. This variable provides Unix with a search path for locating files

28. What to do if you don’t have a .bash_profile file.
Search in your home directory for the .bash_profile file using: ls –a .bash_profile
If it does not exist, copy the /etc/profile file to your home directory renaming it .bash_profile
This file is extremely important, so be sure to save a copy of it, prior to altering it, in order to restore it, should things go wrong.

29. Modifying the PATH variable
To modify the PATH variable to add a directory to the existing PATH use the following commands:
PATH+=“:newpath”
In Unix the colon (:) is a separator between different paths. In Windows/DOS the separator is a semi-colon (;).
Be extremely careful in modifying the PATH, as many commands will no longer work if the PATH is corrupted.

30. I/O (read) format: read [options] var1[ var2…] Options (selected)
-p ‘string’ : prompt with a string (no need to use an echo for prompting)
-s : hide input, good for password entries
-t seconds : Wait for input specified number of seconds, if time expires do not set variables

31. I/O (echo) format: echo [options] [strings] Options:
-e : Enable escape character interpretation (see next slide)
-n : Do not append a newline to the output (useful for concatenating strings or when prompting for a read and read to take place on same line as prompt
-E : disable interpretation of escape characters

32. Prompt and Read Examples
-bash-3.2$cat helloworld #read name from keyboard read -p 'Enter name: ' name # print out hello world echo Hello $name!! -bash-3.2$. helloworld Enter name: Sue Hello Sue!!
-bash-3.2$cat helloworld #prompt for name echo Enter Name: #read name from keyboard read name # print out hello world echo Hello $name!! -bash-3.2$. helloworld Enter Name: Soumyaroop Hello Soumyaroop!!

33. Escape Characters
the escape character, a backslash (\), allows a different interpretation for the following selected characters:
\a : Audible alert
\b : backspace
\c : continue on same line (same as –n option)
\e : escape character
\f : form feed
\n : newline
\r : carriage return (on some systems \r\n are both required for a new line)
\t : horizontal tab

34. Review Introduced Unix script writing including:
introducing variables
locality
basic I/O
Introduced script execution
Introduced the .bash_profile file

35. BASH Scripting
Intro and Variables