Bourne%20Shell%20Programming

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Bourne%20Shell%20Programming

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Title: Bourne%20Shell%20Programming

1
Bourne Shell Programming

Web Sources
Bourne Shell Tutorials
http//steve-parker.org/sh/sh.shtml
http//www.geocities.com/gregl/htm/bourne_shell_t
utorial.htm

2
Bourne Shell Programming Topics Covered

Shell variables
Using Quotes
Arithmetic On Shell
Passing Arguments
Testing conditions
Branching
if-else, if-elif, case
Looping
while, for, until
break and continue

Using options using getopts
Reading Data
Environment and subshells
Parameter substitution
Using set
I/O redirection
Shell archives
The eval, trap, wait commands

3
Part 1Some Regular Expression Characters

. (dot) any character
- beginning of line
- end of line
- zero or more occurences of previous regular
expression
chars any character in the set of chars
chars any character not in chars.
\min,max\ at least min occurences and at most
max occurences of the previous regular
expression.

4
Shell Scripts

Bourne Shell/Korn Shell
Invoking a shell script
shell_script_file or sh -options
shell_script_file
the script file must have execute-permission.
Shell Variables
mydir/usr/jsmith/bin
count assign a null value to the variable
echo mydir display the contents of mvdir
x
echo x substitutes the names of the files in
the directory
name of a command, options and arguments can be
stored inside variables
commandls
option-l
filenamenamesFile
command option filename shell performs the
variable substitution before it
executes the command.

2
5
Quotes

The Double Quote
The special characters, , back quotes () and
back slashes (\) are not ignored.
Example
x
echo x filenames are substituted
echo x x is displayed
echo x is displayed, variable substitution
is done inside the double quotes, no file name
substitution is done and is passed to the
shell.

The Single Quote
The white-space characters enclosed between the
single quotes are preserved by the shell.
The special characters are ignored.
Example
filename/usr/jsmith/bin/prog1
echo filename
echo filename
echo ltgt ()

6
Quotes

The Back Quote
purpose is to tell the shell to execute the
enclosed command and to insert the standard
output from the command at that point on the
command line.
Example
echo The date and time is date
echo There are who wc -l users logged on
filelistls
echo filelist (what is the output)
mail sort -u names lt memo
-u option removes the duplicate
entries from the file

The Back Slash
Is same as putting single quotes around a single
character.
Quotes the single character that immediately
follows it.
X
echo \x x is displayed
Is interpreted inside the double quotes.
Use backslash inside the double quotes to remove
the meaning of characters that otherwise would be
interpreted.
Examples
echo \x x is displayed
echo The value of x is \x\
The value of x is 5 is displayed

7
Arithmetic On Shell

A variable is just considered a string of
characters.
Example
x1
xx1
echo x will display 11
A unix program expr evaluates an expression given
to it on the command line.
Each operator and operand given to expr must be a
separate argument. The operators, , -, , /,
are recognized.
Example
i1
iexpr i 1
Evaluates only integer arithmetic expressions.
awk may be used for floating point calculations.
expr 10 2 what is the problem with this?

8
Passing Arguments

Number of arguments passed to the program
from the command line.
references all the arguments
Example
cat showArgs
echo arguments passed.
echo they are
showArgs a b c d 1 output -
showArgs a b c d 1 output -
showArgs cat names output -
showArgs x output -
cat lookup
grep 1 phonebook
lookup Mary Jones
What is the result?

9
Positional Parameters

Positional parameters
set shell script arguments. e.g.
my_script a b xy 1 2 3
positional parameters have the values
0 -- my_script
1 -- a
2 -- b
3 -- xy
4 -- 1 2 3
- references all the variables passed as
arguments

10
The shift command

shift
left-shifts the positional parameters.
If more than 9 arguments are supplied, arguments
10 and up cannot be referenced.
use shift to access these arguments.
shift assigns value in 2 into 1, 3 into 2
etc.
The number of arguments () gets decremented by
one on each shift.

cat testshift
echo
shift
echo
shift
echo
cat testshift 1 2 3 4 5 6 7 8 9 10
What is the output?

11
Testing Conditions

if statement allows to test a condition and
branch on the exit status of the condition
tested.
An exit status of 0 indicates the program
executed successfully.
An exit status of non-zero indicates a failure.
? contains the exit status of the last command
executed.
Operators for integer comparisons
eq (equal), -ge (greater than or equal), -gt
(greater than), le (less than or equal), -lt
(less than) and ne (not equal)
Operators for string comparisons
, !, -n (string is not null) and z (string is
null)
File operators
-d file file is a directory
-f file file is an ordinary file
-r file file is readable by the process
-s file is of non-zero length

12
Testing Conditions

Examples
user1
who grep user gt /dev/null
- the exit status of the last command in the
pipe line is returned.

13
The test command

The test command is used to test one or more
conditions in an if statement.
yABC
test "y" ABC
echo ? displays 0
x
test -n x checks if x is not null
echo ? displays 1
test -z x checks if string is null
echo ?
xABC
"x" ABC same as using test
! "x" ABC
x5
-a for logical and -o for logical or
"x" -ge 0 -a "x" -lt 10
-f file1 -a -r file1

14
Branching

cat isUserOn
checks if a user is logged on
User1
if who grep user what is the problem with
matching a username in the output of
who?
then
echo user is logged on
fi
if -ne 1 checking for the correct
number of arguments
then
echo Incorrect number of args
exit 1 terminates the program with the exit
status
fi
if NAME John Doe -a BAL -gt
5000then echo okelse echo not okfi

15
Using case

case allows a comparison of a single character
against other values and execute a command if a
match is found.
cat ctype
xA
case "x The value in x is compared with each
of the cases
until a match is found. When a match is
found, the
commands up to the double colons are
executed.
in
0-9 ) echo digit
A-Z ) echo uppercase
a-z ) echo lowercase
) echo special character
esac
Exercise Can you rewrite the script passing the
value to be tested as an argument?

16
The and constructs

command1 command2
if the exit status of command 1 is 0 then command
2 is executed.
Example
EDITOR -z "EDITOR" EDITOR/bin/ed
echo "EDITOR"
command1 command2
If the exit status of command 1 is not zero, then
command 2 is executed.
Example
grep name phonebook echo Couldnt find
name

17
Debugging with a -x option

sh -x ctype A
1 -eq 1
xA
echo A
wc -c
num2
2 -ne 1
echo Enter a single character
Enter a single character
exit 1

Trace the execution of any program by typing in
sh -x followed by the name of the program and its
arguments.
Starts up a new shell to execute the program with
the debug option.
Commands are printed at the terminal as they are
executed preceded by a sign.

18
Looping

The for loop is executed for as many words as are
included after in
for var in listofwords do commandsdone
for i in 1 2 3
do
echo i
done
for file in substitutes all the files in the
directory
do
processCmd file
done

19
The for loop

for var uses all the arguments given to the
program on the command line
do
command
command
done
for file in Replaces 1, 2 as 1, 2 etc
do xwc -l file echo There are echo x cut
-f1 -d lines in filedone
for file in _at_ Replaces 1, 2 as 1, 2
etc. Should be included in double quotes
do echo file
done

20
Looping

while command
do
command1
command2
done
command1 and 2 are executed until command returns
a nonzero exit status
Print command line arguments
while -ne 0
do
echo 1
shift
done

until command
do
command1
command2
done
command1 and command2 are executed as long as
command returns a non-zero exit status.
until who grep user
do
sleep 60
done

21
Break and continue

break to break out of a loop.
break n to break out of n inner most loops
for file
do
variable error can be set to a value
count1
while count -le 5
do
process file
if -n error contains a value
then
break 2
fi
countexpr count 1
done
done

continue the remaining commands in the loop are
skipped.
for file
do
if ! -f file
then
echo file not found
continue
fi
cat file
done

22
The getopts command

The built-in command, getopts processes the
command line arguments.
Format getopts options variable
Used in designing programs which take options.
Example
isLoggedOn mary
isLoggedOn -m mary
isLoggedOn -m -t 120 mary
isLoggedOn -t 120 -m mary
The getopts command is designed to be executed
inside a loop.

11
23
An example using getopts
24
An example using getopts (cont)
25
Reading Data

read variables shell reads a line from stdin
and assigns the first word to the first variable,
second word to the second variable etc.If there
are more words than variables, the excess words
are assigned to the last variable.
Can redirect the input from a file.
Examples
read x y reads the input from the stdin
read x y lt names reads the first line in
names file and assigns the first word into x and
the rest into y
Example
while read val1 val2
do
totalexpr val1 val2
echo total
done

Copying a file using mycopy
check if two arguments are supplied. if not exit
with an error message and usage
fromFile"1"
toFile"2"
check if the destination is a directory
if -d "toFile" then
extract the file name from the source
file name and
concatenate it to the destination
directory
toFiletoFile/basename fromFile
fi
check if a file with the destination file name
exists
if -f "toFile" then
echo "The file toFile already exists. Do
you want to overwrite it?"
read answer
if "answer" ! "yes" then
exit 0

27
SubShells

cat shelltest
x200
echo x
shelltest output is 200. The program shelltest
executed in a
subshell.
x100
echo x output is 100
A subshell runs in its own environment with its
own local variables.
A subshell has no knowledge of the local
variables in the parent shell, therefore cannot
change the value of a variable in the parent
shell.
The command export is used to make the variables
in a shell known to all subshells executed from
that point.

cat one
echo x x y y
one x y are displayed since x and y have
null values
x100
y200
export y
one y200 is displayed
Once a variable is exported, it remains exported
to all the subshells that are subsequently
executed.
Exported variables and their values are copied
into a subshells environment, where they may be
accessed. Any changes made to them have no effect
on the variables in the parent shell.
A variable can be exported before or after it is
assigned a value.

29
Subshells

cat one
echo "x x"
echo "y y"
cat two
x1
y2
z3
export z
three
cat three
echo "x x"
echo "y y"
echo "z z"

one What is the output
y100
export y
one What is the output
two output
x
y what is the output?
z 3 why?
three
output
x
y 100
z Why?

30
Some Environment Variables

PS1- stores the command prompt
PS2 stores the secondary command prompt
HOME Stores the home directory
PATH contains a list of directories which the
shell searches whenever you type in the name of
the program to be executed.
The PATH specifies the directories to be searched
for programs to be executed and not for any other
types of files.
echo PATH displays output which may look
like this/bin/usr/bin
CDPATH

31
The set command

Is used to set various shell options.
Is used to reassign the positional parameters.
Set with no arguments will give a list of all the
variables that exist in your environment.
HOME, PATH, SHELL are some of the variables that
are displayed.
Set can be used to reassign the positional
parameters can be used to parse the data from a
file or the terminal.
Example
cat testSet
set x y z
echo 1 2 3
echo
echo
testSet
x y z
3
x y z

Examples
cat countWords
read line reads a line of input from the
terminal
set line each word in the input line is
assigned
to a positional parameter
echo No. of words in the input line
A modified version of countWords
read line reads a line of input from the
terminal
set --line each word in the input line is
assigned
to a positional parameter. With the
option, an input line like -1 2 will not
cause problems.
echo

33
The eval command

eval command-line shell scans the command line
twice.
Examples
pipe
ls pipe wc l
will generate the errors - not found wc not
found l not found
The reason Pipes and IO redirection is done
before variable substitution
Fix
eval ls pipe wc l will work as expected
The first time, the shell scans the line and
substitutes as the value of the pipe. Then the
command line is rescanned at which point the pipe
is recognized.
Exercise
How do you print the last argument given to a
program?

34
In-line Input Redirection

Command ltlt endingword The shell will use the
lines from the stdin as the input to the command
until a line with endingword
Example
wc -l ltltTheEnd
gt Hi
gt This is a test
gt TheEnd
2

35
Shell Archives

One or more related shell programs can be put
into a single file and shipped to a username with
a mail command.
The shipped file can be unpacked by running the
shell on it.
Example
An archive file called shellProgs is created
using the shell script, shellArchive The file
shellProgs contains two shell scripts lookup
and countWords.
The file shellProgs is mailed to a username.
The user unpacks the file shellProgs by running
the shell on it resulting in the files, lookup
and countWords

cat shellProgs
echo "Extracting script lookup"
cat gtlookup ltlt\END_OF_DATA
name1
grep "name" PHONEBOOK
if ? -ne 0
then
echo "Name is not in the phone book"
fi
END_OF_DATA
echo "Extracting script countWords"
cat gtcountWords ltlt\END_OF_DATA
read line
set line
echo "NoOfWOrds "
END_OF_DATA

cat shellArchive
Creates a shell archive from a set of files.
The filenames are given as
command line arguments
For file
do
echo
echo echo extracting script file
echo cat gtfile ltlt\END_OF_DATA
cat file
echo END_OF_DATA
done

38
Grouping Commands

The semicolon allows users to string commands
together on a single line as if one command was
being issued. Each command within the semicolon
separated list is executed by the shell in
succession.
Example
cd /user/jsmith make
The parentheses, when grouped around a command,
cause the command to be executed in a sub-shell.
Example
cd ls change to the home directory and list
the contents
cprogs shellprogs contents of the home
directory
(cd cprogs ls ) ls
a.c b.c x.c contents of cprogs
cprogs shellprogs contents of home directory

39
Grouping commands

Examples
linehi
(linebye) execute it in a subshell
echo line hi
linebye execute it in the current
shell
echo line bye

40
Parameter Substitution

Parameter substitution is a method of providing a
default value for a variable in the event that it
is currently null.
The construct uses a combination of braces
delimiting a variable and its default. The
variable and default value are separated by a
keyword.
The keyword serves as a condition for when to
assign the value to the variable. Supposing a
script tries to de-reference a null variable, a
good programmer can avoid catastrophic errors by
using parameter substitution

Examples
parameter - Substitute the value of
parameter.
fileprog1
cp file filea intention is to copy prog1 into
prog1a. Will not work since filea will be
treated as a variable
cp file filea

parameter-value - Substitute the value of
parameter if it is not null otherwise, use
value.
result1
echo "result is result-2"
result is 1
result
echo "result is result-2 the value can be
a backquoted command
result is 2
echo "result"

parametervalue Substitute the value of
parameter if it is not null otherwise, use value
and also assign value to parameter.
Example
result
echo "result is result2"
result is 2
echo "result"
2

parameter?value Substitute the value of
parameter if it is not null otherwise, write
value to standard error and exit. If value is
omitted, then write "parameter parameter null or
not set" instead.
Example
result
echo "result is result?"
sh result Parameter null or not set.
echo "result is result? result set now

parametervalue Substitute value if parameter
is not null otherwise, substitute nothing.
Example
tracingT
echo "tracing is tracingon"
tracing is on

46
exec

The exec built-in command spawns its arguments
into a new process and runs it in place of the
currently running process.
Example
if "MYSCRIPT" "" then
exit
else
if -x MYSCRIPT then
echo "Executing MYSCRIPT"
exec MYSCRIPT
fi
fi

47
Synchronizing tasks

Since UNIX offers multitasking, commands can be
sent to the background so that they are executed
when the kernel finds time.
After a command is sent to the background, the
shell frees itself to handle other commands.
To background a command, the user appends an
ampersand, , to it.
Example
find / -name junkfile.tar print 2gt/dev/null
1 10876 process id is printed by the shell
date handle another command
Wed Jun 21 084658 PDT 2000

48
wait

Sometimes it may be necessary to synchronize a
script with an asynchronous process, ie. a
backgrounded job.
Wait is a built-in command for such
synchronization.
Wait takes an optional process number as its
argument and pauses the shell's execution until
the specified process had terminated.
If only one process has been sent to the
background, then wait can be issued without an
argument. It will automatically wait for the job
to complete. If more than one job is running in
the background
Wait ! waits for the last process that is sent
to the background
process identifiers can be stored in variables
and passed to wait to specify the job to be
waited for.

49
wait

Examples
find / -name junkfile.tar print 2gt/dev/null
1 10876 process id is printed by the shell
Wait 10876
processCmd1
pid1!
processCmd2
pid2!
wait pid1
wait pid2

50
trap

The pressing of DELETE key at the terminal, when
a program is in execution, sends a signal to the
executing program.
Using the trap command, the program can specify
the action to be taken on receiving the signal.
Usage trap commands signals, where commands are
the actions to be taken on receiving the signals.
Some Commonly used signal numbers
0 Exit from Shell
1 Hangup
2 Interrupt (eg Delete key)
15 Software termination (sent by kill, for
example)

51
Example

Example
!/bin/sh
i1
JUNKjunkfile
trap rm JUNKexit 2
while i -le 100
Do
remove the file when interrupt is
received
echo i gtgt JUNK
iexpr i 1
done

52
More On I/O

The shell permits the combination of the basic
I/O constructs with a file descriptor.
A file descriptor (also known as a file handle)
is a non-negative digit that points at a file.
The file descriptors for stdin, stdout, and
stderr are 0, 1, and 2, respectively. Any of
these may be redirected to a file or to each
other. In other words, it is quite possible to
send the output from stdin and stderr to the same
file. This is quite useful when a user would
rather check a script's results after it has
completed processing.
Examples
command 2gt file redirects the standard error
from any command
cd JUNK 2gtgtout the directory JUNK does not
exist
cat out
sh JUNK not found.
cd JUNK gtgtout 2gt1 Change directory to JUNK,
redirect stdout to the file out, and then
redirect stderr to the same file that stdout
uses."
cat out
sh JUNK not found.

53
Functions

The Bourne shell allows the grouping of commands
into a reusable instruction set called a
function.
Functions have two parts a label and a body. The
label names the function and is used to invoke
the function.
Some rules in declaring a function
The function label must be unique it cannot be
the same as any other variable or other function.
An empty set of parentheses must always follow
the function label. They instruct the shell that
a function is being defined.
Braces, , must be used to delimit the
function's body.
A function must be defined before it can be used
in a script.

54
Functions

cat fun_example1
!/bin/sh
setEnvironment ()
ROOTPWD
LIBROOT/proglib
BINROOT/bin
echo "Trying to print environment..."
echo ROOT LIB BIN
invoking the function setEnvironment
echo "Trying to print environment again..."
echo ROOT LIB BIN

fun_example1
output
Trying to print environment...
Trying to print environment again...
/proglib /bin

56
Functions with arguments

cat fun_example2
!/bin/sh
setEnvironment ()
ROOT1
LIBROOT/proglib
BINROOT/bin
echo "Trying to print environment..."
echo ROOT LIB BIN
invoking setEnvironment with an argument
setEnvironment ./demos
echo "Trying to print environment again..."
echo ROOT LIB BIN

fun_example2
output
Trying to print environment...
Trying to print environment again...
./demos ./demos/proglib ./demos/bin

58
Returning a value

Functions return the exit status of the last
command executed.
it uses the status of the last command issued.
A script controls its exit status by issuing an
exit with a non-negative value. On the other
hand, functions do not use exit because it is
designed to terminate the shell. Instead,
functions use return.
The return command stops execution of a function
returning program control to the point in the
script where the function was called. Script
execution continues from where the function was
invoked. The format of return follows return n
where n is any non-negative integer. Providing a
return value is optional just as providing a
status code to exit is optional. If no code is
given, return defaults to the value returned by
the last command executed.

59
Functions returning a value

cat fun_example3
!/bin/sh
isADir()
if ! -d 1
then
return 1
fi
isADir demos
echo ?
isADir fun2
echo ?

60
Functions

cat fun_example4
!/bin/sh
isADir()
if ! -d 1
then
return 1
fi
Functions can be embedded in a test
if "isADir HOME"
then
echo "Yes HOME is a directory"
fi

61
Changes made in functions

Changes made to variables persist beyond
execution of the function.
Cat fun_example5
!/bin/sh
change()
ONE"Hi"
TWO"NewVal"
ONE"Bye"
echo "ONE ONE"
echo "TWO TWO"
change
echo "ONE ONE"
echo "TWO TWO"

fun_example5
output
changesInfun
ONE Bye
TWO
ONE Hi
TWO NewVal

63
Including functions in multiple scripts

cat counterUser
!/bin/sh
execute the file printData in the
current shell. This will cause any functions
defined inside the file printData to be read in
and defined In the current shell.
. printData
x0
while x -le 10
do
calling function printData
printArg x
xexpr x 1
done
echo "Done"

cat printData
!/bin/sh
printArg ()
echo 1
echo

64
Functions

Functions are a very useful tool to allow
scripters to organize actions into logical
blocks. It is easier to think of a program a
series of steps to perform and then to expand
those steps into functions that perform the
necessary actions. This is much better than
trying to list every single command in order.
In addition, functions can improve a script's
performance. Rather than employ functions, you
might consider grouping logical blocks into
subscripts which the main script uses. This
technique will work just fine, but the program's
execution time will take a hit. When a script
calls a subscript, the shell must find the
subscript on disk, open it, read it into memory,
and then execute it. This process happens every
time a subscript is called even though the
subscript may have been used previously.
Functions are read once into memory as soon as
they are declared. They have the advantage of one
time read for multiple execution.

65
Some Environment Variables

PS1- stores the command prompt
PS2 stores the secondary command prompt
HOME Stores the home directory
PATH contains a list of directories which the
shell searches whenever you type in the name of
the program to be executed.
The PATH specifies the directories to be searched
for programs to be executed and not for any other
types of files.
echo PATH displays output which may look
like this/bin/usr/bin
CDPATH

66
The .profile file

At the end of the login sequence, the login shell
executes two special files - /etc/profile and
.profile in your home directory.
/etc/profile set up by the SysAdmin and does
the tasks that the administrator wants to execute
when you log in (eg checking if you have mail)
.profile (you will get a default .profile when
you get your account) gets automatically gets
executed when you log in. You can include any
commands that you want executed whenever you
login.
Example
PATH/bin/usr/bin/HOME/bin
CDPATHHOMEHOME/progs
PS1gt
PS2?
export PATH CDPATH PS1 PS2