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CS CS-- 344344 - - Unix OperatingUnix Operating
System Fundamentals System Fundamentals
Lecture 10
Introduction to Shell Programming
Based on slides created by
Dr. Bangalore for the
Spring 2005 offering of
the course
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Shell: Turn Interpretation OffShell: Turn Interpretation Off
Shell interprets characters entered at the
command prompt
To turn off this interpretation we must use
Backslash โ
Double quotes โ โ โ Single quotes โ โ โ
What happens when the following commands are
used
echo โ$HOMEโ โ$HOMEโ $HOME echo My files are: โ*โ echo โMy files are: *โ echo โMy files are: *โ
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UABEffect of Quoting on Special Characters Effect of Quoting on Special Characters
newline NO NO NO ! YES NO NO
spaces NO NO NO
` (back quote) YES NO NO
โ (double quote) - NO NO
โ (single quote) NO - NO
$ YES NO NO
; NO NO NO
NO NO NO
| NO NO NO
< NO NO NO
{ } NO NO NO
[ ] NO NO NO
Character Inside โ โ Inside โ โ After \
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SummarySummary
For the bash shell
The backslash character and single quotes turn off interpretation for all special characters The double quotes turn off interpretation of all special characters except $, `, and \ (\ - only if the next character is interpreted)
For the C shell
The backslash character turns off interpretation for all special characters The single quotes turn off interpretation of all special characters except the! The double quotes turn off interpretation of all special characters except $, `, and!
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Miscellaneous Miscellaneous
Passing Special Characters to Utilities
grep treats $ character as end of line (e.g., grep ';$' MyClass.java grep ';$' MyClass.java โ look for lines ending with ;)
Single and double quotes can be mixed in
commands (only outer quotes have any effect)
echo โ โ$USERโ โ echo โ โ$USERโ โ echo โ$USERโ?โ $USER โ โ$USERโโ echo โ$USER \โ โDate is \โ date \โ $USER *\โ
Interpreting special characters in variable names
$ aa=t* $ echo $aa '$aa' "$aa" t.c t1.c temp.java tmp typescript $aa t*
$ aa=โtโ $ echo $aa '$aa' "$aa" t.c t1.c temp.java tmp typescript $aa t
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โโechoechoโโ^ commandcommand
Using โ โ and โ โ with echoecho
$ echo ' $ echo 'HelloHello $USER$USER' Hello $USER $ echo "Hello $ echo "Hello $USER$USER"" Hello afgane
When the shell interprets โโ it stops interpretation
until a matching โโ is found
To eliminate new line with echoecho use
echo echo โ โn n echo echo (^) โ โnn โโEnter your choice:Enter your choice: โโ; echo; echo (^) โโ1, 2, 31, 2, 3โโ
Shell Programming Shell Programming
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IntroductionIntroduction
Used for storing commonly used sequences of commands
So, a series of shell commands can be stored in a regular text file for later
execution
Before running a script, need to give it
execute permissions with chmod utility
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Determining shell for a Script Determining shell for a Script
When script is run, the system determines which shell the script was written for and then executes the shell using the shell as stdin The system knows which shell to use to execute the script by examining the first line of the script: If the first line is just a #, the script is interpreted by the shell from which it is executed If the first line is of the form #! pathName, then the executable program pathName is used to interpret the script If neither if the two rules apply, then the script is interpreted by a Bourne shell.
$ cat myScript.sh
Print todayโs date
echo โn The date today is date
$ cat myBashScript.sh #! /bin/bash
Print todayโs date
echo โn The date today is date
$ cat myDefaultScript.sh
Print todayโs date
echo โn The date today is date
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Passing Arguments to a ScriptPassing Arguments to a Script
Similar to passing command-line arguments in
other programming languages, we can pass
arguments to a shell script
Example: ./myscript arg1 arg2./myscript arg1 arg
Shell interprets the arguments are follows:
$1 โ argument 1 $2 โ argument 2 and so on $0 โ the name of the current script $* โ all arguments $# โ no. of arguments $$ โ process id (PID) of process running the script
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Example: script with complex arguments Example: script with complex arguments
$ cat > cmdscript.sh echo 'The name of the script is: ' $ echo 'No. of command-line arguments = ' $# echo 'First three arguments are: ' $1 $2 $ echo 'Complete argument list is:' $* echo 'PID of this process = ' $$ $ chmod + cmdscript.sh
$ ./ cmdscript.sh โHello Worldโ โ$USERโ โ$USERโ _The name of the script is: ./cmdscript.sh No. of command-line arguments = 3 First three arguments are: Hello World afgane $USER Complete argument list is: Hello World afgane $USER* PID of this process = 5828_ $ ./ cmdscript.sh a b c The name of the script is: ./cmdscript.sh No. of command-line arguments = 3 First three arguments are: a b c Complete argument list is: a b c PID of this process = 5831
