Transcript Shell Programming

Shell Programming
• 1. Understanding Unix shell programming
language:
• A. It has features of high-level languages.
• B. Convenient to do the programming.
• C. It is a built-in programming language.
• D. The price we have to pay --- speed.
Shell Script
• The shell program files are called shellprocedures, shell script or simply script.
• Example:
• $ cat s1
• # s1
• # display # of users currently logged in
• #
• who | wc –l (end of the script)
• #---remark or comment in the Unix shells.
More about echo Command
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The echo command special characters:
\b ---- A backspace
\c ---- Inhibit the default next line (return
key) at the end of the output string.
\n ---- A carriage return and a form feed.
\r ---- A carriage return without a linefeed.
\bt ---- A tab character.
\0n ---- A zero followed by 1-, 2-, or 3-digits
octal number representing the ASCII code of a
character.
Examples
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$ cat BOX1
echo “The following is the output of $0 script
echo “Total number of command line arguments: $# “
echo “The first parameter : $1 “
echo “ The second parameter : $2 “
echo “ This is the list of all the parameters : $* “
$We run the script first by typing BOX1 [Enter]
• Then we run the script by typing BOX1 is empty [Enter]
and pay attention to the differences.
The command line parameters
• The shell can read up to 10 command line
parameters (also called arguments) from the
command line into the special variables
(also called positional variables, or
parameters).
• The special variables are numbered in
sequence from 0 to 9 (no number 10)
• i.e., $0, $1, $2, … $9.
The shell positional variables
• $0 --- Contains the name of the script, as typed on the
command line
• $1, $2, … $9 --- contains the first through ninth
command line parameters.
• $# --- Contains the number of command line
parameters.
• $@ --- Contains all command line parameters: $1
$2 ….. $9.
• $? --- Contains the exit status of the last command.
• $* --- Contains all command line parameters: $1 $2…
$9
• $$ --- Contain the PID number of the executing process.
(week11 BOX1 , BOX2)
Assigning Values
• 1. $ set One Two Three [Enter]
• $echo $1 $2 $3 [Enter]
• 2. $set `date` [Enter]
• $echo $1 $2 $3 [Enter]
• 3. $echo $1 $2 $3 $4 $5 $6 [Enter]
Terminating Programs: The exit
• Exit will end the program and when a
number is included, the exit code will be
the number you assigned. Otherwise, a 0
exit code will reflect a successful exit and
a non zero will mean a failed script or
command.
• Use echo $? To display the last exit code.
Conditions and Tests
• The if-then construct
• The if-then-else Construct
• The if-then-elif Construct
Structure of the if-then construct
• if [condition]
• then
• commands
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• last – command
• fi
Rules of Conditions and Tests
• 1. The if statement ends with the reserved
word fi (if typed backward).
• 2. The indentation is not necessary, but it
certainly makes the code more readable.
• 3. If the condition is true, then all the
commands between the then and fi, what is
called the body of the if, are executed. If the
condition is false, the body of the if is
skipped, and the line after fi is executed.
if – then – else Construct
• if [condition]
• then
• true – commands
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• last – true – command
else
false – commands
…
last – false – command
fi
Control Structures
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case.. in ..esac
case expression in
pattern { |pattern })
list
::
esac
Example
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$ cat simple
# Sample
# A simple script to test the read command
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echo "Give me a long sentence:\c"
read Word1 Word2 Rest
echo " $Word1 \n $Word2 \n $Rest"
echo " End of my act“
echo `cal 2007`
(week11 simple2)
Example
• # The largest of three
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• # This program accepts three numbers and shows the largest of
them
• echo "Enter three numbers and I will show you the largest of
them>> "
• read num1 num2 num3
• if test "$num1" -gt "$num2" -a "$num1" -gt "$num3"
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echo "The largest number is: $num1"
• elif test "$num2" -gt "$num1" -a “$num2” -gt "$num3"
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echo "The largest number is: $num2"
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echo "The largest number is: $num3"
• fi
• exit 0 (week11 Large3)
True or false: The test command
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# test command example
echo "Are you okay?"
echo "input Y for yes or N for no:\c"
read answer
if test "$answer" = Y
then
echo "Glad to hear that"
else
echo "Go home or the hospital!"
fi (week11 test1 and test2)
Logical “and” “or” “not”
• -a ---- Logical AND
• -o ---- Logical OR
• ! ---- Logical NOT
Test command numeric test operators
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-eq --- number1 –eq number2
Is number1 equal number2?
-ne ---gt ---ge ---lt ---le ---
String Values Testing
• Test command can also be used in string
coma rations. However, different set of
operators will be used.
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$ STRING5 [Enter]
$ test –z “$STRING5” [Enter]
Use echo $? To see the testing result.
0--- True
Non-zero number --- False
Test Operator for string
• = ---string1 = string2 ;
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Does string1 match string2?
• != ---• -n ---•
Does string contain characters(nonzero
length?)
• -z ---- Is string an empty string (zero length)
Test file characteristics
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-r ---test -r fileName ;
Does file exist,and is it readable?
-w ---s --Does file exist, and a nonzero length?
-f --Does file exist, but is not a directory file?
-d --Does file exist, and is it a directory file?
Example
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$FILE=xyz
#File testing script
FILE=xyz
if test -r "$FILE"
then
echo "READABLE"
elif test -w "$FILE"
then
echo "WRITEABLE"
else
echo "Read and Write Access Denied"
fi
(week11 testFile)
More Examples
• HI
• looping