1. Shell 编程 陈伟

2. Index The shell of Linux Bourne Shell Programming
find.and.regular.expression
text manipulation
Shell 编程

3. The shell of Linux Bourne shell (sh), C shell (csh), Korn shell (ksh),
TC shell (tcsh),
Bourne Again shell (bash)
Shell 编程

4. Difference between programming and scripting languages
Programming languages are generally a lot more powerful and a lot faster than scripting languages. Programming languages generally start from source code and are compiled into an executable. This executable is not easily ported into different operating systems.
A scripting language also starts from source code, but is not compiled into an executable. Rather, an interpreter reads the instructions in the source file and executes each instruction. Interpreted programs are generally slower than compiled programs. The main advantage is that you can easily port the source file to any operating system.
Shell 编程

5. The first bash program
We must know how to use a text editor. There are two major text editors in Linux:
vi, emacs (or xemacs).
So fire up a text editor; for example:
$ vi &
and type the following inside it:
#!/bin/bash echo “Hello World”
The first line tells Linux to use the bash interpreter to run this script. We call it hello.sh. Then, make the script executable:
$ chmod 700 hello.sh
$ ls –l
-rwx hello.sh
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6. The first bash program To execute the program: $ hello.sh
-bash: hello.sh: command not found
The home directory (where the command hello.sh is located)
is not in the variable PATH
echo $PATH
:bin:/usr/bin:…
We must specify the path of hello.sh
$/home/srinaldi/Scripts/hello.sh
$./hello.sh
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7. The second bash program
We write a program that copies all files into a directory, and then deletes the directory along with its contents. This can be done with the following commands:
$ mkdir trash $ cp * trash $ rm -rf trash
$ mkdir trash
Instead of having to type all that interactively on the shell, write a shell program instead:
$ cat trash
#!/bin/bash # this script deletes some files
cp * trash rm -rf trash mkdir trash
echo “Deleted all files!”
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8. Variables
We can use variables as in any programming languages. Their values are always stored as strings, but there are mathematical operators in the shell language that will convert variables to numbers for calculations.
We have no need to declare a variable, just assigning a value to its reference will create it.
Example
#!/bin/bash
STR=“Hello World!”
echo $STR
Line 2 creates a variable called STR and assigns the string "Hello World!" to it. Then the value of this variable is retrieved by putting the '$' in at the beginning.
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9. Warning !
The shell programming language does not type-cast its variables. This means that a variable can hold number data or character data.
count=0
count=Sunday
Switching the TYPE of a variable can lead to confusion for the writer of the script or someone trying to modify it, so it is recommended to use a variable for only a single TYPE of data in a script.
\ is the bash escape character and it preserves the literal value of the next character that follows.
$ ls \*
ls: *: No such file or directory
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10. Single and Double Quote
When assigning character data containing spaces or special characters, the data must be enclosed in either single or double quotes.
Using double quotes (partial quoting) to show a string of characters will allow any variables in the quotes to be resolved
$ var=“test string”
$ newvar=“Value of var is $var”
$ echo $newvar
Value of var is test string
Using single quotes (full quoting) to show a string of characters will not allow variable resolution
$ var=’test string’
$ newvar=’Value of var is $var’
Value of var is $var
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11. Examples mistake Not resolved $ pippo= pluto $ pippo =pluto $ ls [Pp]*
$ var=“’(]\\{}\$\””
$ echo $var
$ echo “$var”
$ echo ‘$var’
$ echo \z # z
$ echo \\z # \z
$ echo ‘\\z’ # \\z
mistake
Not resolved
Shell 编程

12. Examples $ pippo=cat $ echo “comando = \” $pippo \” “ comando =“ cat ”
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13. The export command
The export command puts a variable into the environment so it will be accessible to child processes. For instance:
$ x=hello
$ bash # Run a child shell.
$ echo $x # Nothing in x.
$ exit # Return to parent.
$ export x
$ bash
$ echo $x
hello # It's there.
If the child modifies x, it will not modify the parent’s original value. Verify this by changing x in the following way:
$ x=ciao
$ exit
hello
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14. Environmental Variables
There are two types of variables:
Local variables
Environmental variables
Environmental variables are set by the system and can usually be found by using the env command. Environmental variables hold special values. For instance,
$ echo $SHELL
/bin/bash
$ echo $PATH
/usr/X11R6/bin:/usr/local/bin:/bin:/usr/bin
Environmental variables are defined in /etc/profile, /etc/profile.d/ and ~/.bash_profile. These files are the initialization files and they are read when bash shell is invoked. When a login shell exits, bash reads ~/.bash_logout
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15. Environmental Variables
HOME: The default argument (home directory) for cd.
PATH: The search path for commands. It is a colon-separated list of directories that are searched when you type a command.
Usually, we type in the commands in the following way:
$ ./trash.sh
By setting PATH=$PATH:. our working directory is included in the search path for commands, and we simply type:
$ trash.sh
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16. Environmental Variables
LOGNAME: contains the user name
HOSTNAME: contains the computer name.
MACHTYPE: system harware
PS1: sequence of characters shown before the prompt
\t hour
\d date
\w current directory
\W last part of the current directory
\u user name
\$ prompt character
Example
rinaldi]$ PS1=‘ciao \u *’
ciao rinaldi* _
UID: contains the id number of the user (cannot be changed).
SHLVL: contains the shell level
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17. Exit
The exit command may be used to terminate a script. It can also return a value, which is available to the script’s parent process.
When a script ends with
exit nnn
nnn=0-255 the exit status is nnn
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18. Read command
The read command allows you to prompt for input and store it in a variable.
Example (read.sh)
#!/bin/bash
echo -n “Enter name of file to delete: ”
read file
echo “Type 'y' to remove it, 'n' to change your mind ... ”
rm -i $file
echo "That was YOUR decision!"
Line 3 creates a variable called file and assigns the input from keyboard to it. Then the value of this variable is retrieved by putting the '$' in at its beginning.
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19. Read command Options Example read –s (does not echo input)
read –nN (accepts only N characters of input)
read –p “message” (prompts message)
read –tT (accepts input for T seconds)
Example
$ read –s –n1 -p “Yes (Y) or not (N)?” answer
Yes (Y) or not (N) ? Y
$ echo $answer Y
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20. Command Substitution
The backquote “`” is different from the single quote “´”. It is used for command substitution: `command`
$ LIST=`ls`
$ echo $LIST
hello.sh read.sh
PS1=“`pwd`>”
/home/rinaldi/didattica/>
We can perform the command substitution by means of $(command)
$ LIST=$(ls)
rm $( find / -name “*.tmp” )
ls $( pwd )
ls $( echo /bin )
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21. Example
$ a=`echo Hello` $ echo $a $ echo ‘$a’ $ b=`ls /home` $ echo $b $ echo $a $b $ echo “$a $b”
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22. Arithmetic Operators + plus - minus * multiplication / division
** exponentiation
% modulo
Example
$ a=(5+2)*3
$ echo $a
$ b=2**3
$ echo $a+$b
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23. Arithmetic Evaluation
The let statement can be used to do mathematical functions:
$ let X=10+2*7
$ echo $X 24
$ let Y=X+2*4
$ echo $Y 32
An arithmetic expression can be evaluated by $[expression] or $((expression))
$ echo $((123+20))
143
$ VALORE=$[123+20]
$ echo $[123*$VALORE]
1430
$ echo $[2**3]
$ echo $[8%3]
Not necessary to use $X to refer to the value of X
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24. Arithmetic Evaluation
Example (operations.sh)
#!/bin/bash echo -n “Enter the first number: ”; read x
echo -n “Enter the second number: ”; read y
add=$(($x + $y))
sub=$(($x - $y)) mul=$(($x * $y)) div=$(($x / $y))
mod=$(($x % $y))
# print out the answers: echo “Sum: $add” echo “Difference: $sub” echo “Product: $mul”
echo “Quotient: $div” echo “Remainder: $mod”
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25. Conditional Statements
Conditionals let we decide whether to perform an action or not, this decision is taken by evaluating an expression. The most basic form is:
if [expression];
then
statements
elif [expression];
else fi
the elif (else if) and else sections are optional
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26. Expressions
An expression can be: String comparison, Numeric comparison, File operators and Logical operators and it is represented by [expression]:
String Comparisons:
= compare if two strings are equal
!= compare if two strings are not equal
-n evaluate if string length is greater than zero
-z evaluate if string length is equal to zero
Examples:
[ s1 = s2 ] (true if s1 same as s2, else false)
[ s1 != s2 ] (true if s1 not same as s2, else false)
[ s1 ] (true if s1 is not empty, else false)
[ -n s1 ] (true if s1 has a length greater then 0, else false)
[ -z s2 ] (true if s2 has a length of 0, otherwise false)
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27. Expressions Number Comparisons: Examples:
-eq compare if two numbers are equal
-ge compare if one number is greater than or equal to a number
-le compare if one number is less than or equal to a number
-ne compare if two numbers are not equal
-gt compare if one number is greater than another number
-lt compare if one number is less than another number
Examples:
[ n1 -eq n2 ] (true if n1 same as n2, else false)
[ n1 -ge n2 ] (true if n1greater then or equal to n2, else false)
[ n1 -le n2 ] (true if n1 less then or equal to n2, else false)
[ n1 -ne n2 ] (true if n1 is not same as n2, else false)
[ n1 -gt n2 ] (true if n1 greater then n2, else false)
[ n1 -lt n2 ] (true if n1 less then n2, else false)
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28. echo -n “Enter your login name: " read name if [ “$name” = “$USER” ];
#!/bin/bash # if0.sh
echo -n “Enter your login name: "
read name
if [ “$name” = “$USER” ];
then
echo “Hello, $name. How are you today ?”
else
echo “You are not $USER, so who are you ?” fi
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29. #!/bin/bash # if1.sh
echo -n “Enter a number 1 < x < 10: "
read num
if [ “$num” -lt 10 ]; then
if [ “$num” -gt 1 ]; then
echo “$num*$num=$(($num*$num))”
else
echo “Wrong insertion !” fi

30. Expressions Files operators: Examples:
-d check if path given is a directory
-f check if path given is a file
-s check if path given is a symbolic link
-e check if file name exists
-s check if a file has a length greater than 0
-r check if read permission is set for file or directory
-w check if write permission is set for a file or directory
-x check if execute permission is set for a file or directory
Examples:
[ -d fname ] (true if fname is a directory, otherwise false)
[ -f fname ] (true if fname is a file, otherwise false)
[ -e fname ] (true if fname exists, otherwise false)
[ -s fname ] (true if fname length is greater then 0, else false)
[ -r fname ] (true if fname has the read permission, else false)
[ -w fname ] (true if fname has the write permission, else false)
[ -x fname ] (true if fname has the execute permission, else false)
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31. Example
#!/bin/bash if [ -f /etc/fstab ];
then cp /etc/fstab .
echo “Done.”
else echo “This file does not exist.”
exit 1 fi
Exercise. Write a shell script which accepts a file name <pippo>:
The script checks if file exists, and creates a directory Backup
If file exists, copies the file to the same name + .bak (if the .bak file already exists ask if you want to replace it).
If the file does not exist then exits with the message “The file <pippo> does not exist!!!”
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32. Solution
#!/bin/bash if [ ! –d ./Backup ] then mkdir ./Backup fi read –p “insert the name of a file” pippo if [ -f $pippo ] then cp $pippo ./Backup/$pippo.bak else echo “The file $pippo does not exist!!!”
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33. Expressions Logical operators: ! negate (NOT) a logical expression
-a logically AND two logical expressions
-o logically OR two logical expressions
#!/bin/bash # if3.sh
echo -n “Enter a number 1 < x < 10:”
read num
if [ “$num” -gt 1 –a “$num” -lt 10 ];
then
echo “$num*$num=$(($num*$num))”
else
echo “Wrong insertion !” fi
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34. Expressions Logical operators:
&& logically AND two logical expressions
|| logically OR two logical expressions
#!/bin/bash # if4.sh
echo -n "Enter a number 1 < x < 10: "
read num
if [ “$number” -gt 1 ] && [ “$number” -lt 10 ];
then
echo “$num*$num=$(($num*$num))”
else
echo “Wrong insertion !” fi
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35. Example 1 $ cat iftrue.sh # iftrue.sh #!/bin/bash
echo “Enter a path: ”; read x
if cd $x 2> /dev/null then
echo “I am in $x and it contains”; ls
else
echo “The directory $x does not exist”;
exit 1 fi
$ ./iftrue.sh
Enter a path: /home
srinaldi afrosini riccardo …
Enter a path: pippo
The directory pippo does not exist
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36. Shell Parameters Special parameters
Positional parameters are assigned from the shell’s argument when it is invoked. Positional parameter “N” may be referenced as “${N}”, or as “$N” when “N” consists of a single digit.
Special parameters
$# is the number of parameters passed
$0 returns the name of the shell script running as well as its location in the filesystem
$* gives a single word containing all the parameters passed to the script
gives an array of words containing all the parameters passed to the script
$ cat sparameters.sh ( sparameters.sh )
#!/bin/bash
echo “$#; $0; $1; $2; $*;
$ sparameters.sh alba chiara
2; ./sparameters.sh; alba; chiara; alba chiara; alba chiara
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37. Trash $ cat trash.sh ( trash.sh ) #!/bin/bash if [ $# -eq 1 ]; then
if [ ! –d “$HOME/trash” ];
mkdir “$HOME/trash” fi
mv $1 “$HOME/trash”
else
echo “Use: $0 filename”
exit 1
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38. Case Statement
Used to execute statements based on specific values. Often used in place of an if statement if there are a large number of conditions.
Value used can be an expression
each set of statements must be ended by a pair of semicolons;
a *) is used to accept any value not matched with list of values
case $var in
val1)
statements;;
val2) *)
esac
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39. Example #!/bin/bash ( case.sh )
echo -n “Enter a number 1 < x < 10: ”
read x
case $x in
1) echo “Value of x is 1.”;;
2) echo “Value of x is 2.”;;
3) echo “Value of x is 3.”;;
4) echo “Value of x is 4.”;;
5) echo “Value of x is 5.”;;
6) echo “Value of x is 6.”;;
7) echo “Value of x is 7.”;;
8) echo “Value of x is 8.”;;
9) echo “Value of x is 9.”;;
0 | 10) echo “wrong number.”;;
*) echo “Unrecognized value.”;;
esac
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40. Iteration Statements
The for structure is used when you are looping through a range of variables.
for var in list do
statements
done
statements are executed with var set to each value in the list.
#!/bin/bash
let sum=0
for num in
let “sum = $sum + $num”
echo $sum
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41. Iteration Statements: <list>
#!/bin/bash for x in paper pencil pen; do
echo “The value of variable x is: $x”
sleep 1
done
# The value of variable x is paper
# The value of variable x is pencil
# The value of variable x is pen
#!/bin/bash for x in “paper A4” “pencil STADTLER” “pen BIC”; do
# The value of variable x is paper A4
# The value of variable x is pencil STADTLER
# The value of variable x is pen BIC
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42. Iteration Statements: <list>
#!/bin/bash lista=“antonio
michele
paolo
luca”
for x in $lista do
echo “The value of variable x is: $x”
sleep 1
done
# The value of variable x is antonio
# The value of variable x is michele
# The value of variable x is paolo
# The value of variable x is luca
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43. Iteration Statements: <list>
#!/bin/bash
for x in * do
ls -l “$x”
sleep 1
done
# Lists all files in current directory
for x in /bin
# Lists all files in /bin
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44. Iteration Statements: <list>
#!/bin/bash
read –p “Insert the name of a directory” directory
echo "symbolic links in directory \“ $directory \“ "
for file in $( find $directory -type l ) # -type l = symbolic links do
echo "$file"
done | sort # Otherwise file list is unsorted
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45. Iteration Statements: <list>
if the list part is left off, var is set to each parameter passed to the script ( $1, $2, $3,…)
$ cat for1.sh ( for1.sh )
#!/bin/bash for x do
echo “The value of variable x is: $x”
sleep 1
done
$ for1.sh alba chiara
The value of variable x is: alba
The value of variable x is: chiara
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46. Example 1
#!/bin/bash ( old.sh ) # Move the command line arg files to old directory. if [ $# -eq 0 ] #check for command line arguments then echo “Usage: $0 file …” exit 1 fi if [ ! –d “$HOME/old” ] mkdir “$HOME/old” echo The following files will be saved in the old directory: echo $* for p in $* #loop through all command line arguments do mv $p “$HOME/old/” chmod 400 “$HOME/old/$p” done ls -l “$HOME/old”
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47. Example 2
#!/bin/bash ( args.sh ) # Invoke this script with several arguments: “one two three“ if [ ! -n “$1” ]; then echo “Usage: $0 arg1 arg2 ..." ; exit 1 fi echo ; index=1 ; echo “Listing args with \”\$*\”:” for arg in “$*” ; do echo “Arg $index = $arg” let “index+=1” # increase variable index by one done echo “Entire arg list seen as single word.” echo “Listing args with for arg in ; do let “index+=1” echo “Arg list seen as separate words.” ; exit 0
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48. Operations on vabiables
……. let “index += 5” #increment index by 5 …… += #increment variable -= # decrement variable *= # multiply variable /= # divide variable
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49. Using Arrays with Loops
In the bash shell, we may use arrays. The simplest way to create one is using one of the two subscripts:
pet[0]=dog
pet[1]=cat
pet[2]=fish
pet[4]=apple
pet=( dog cat fish apple )
We may have up to 1024 elements. To extract a value, type ${arrayname[i]}
$ echo ${pet[0]}
dog
$ echo ${pet[2]}
fish
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50. Arrays To extract all the elements, use an asterisk as:
echo ${arraynames[*]}
To see how many elements are in the array:
echo ${#arraynames[*]}
We can combine arrays with loops using a for loop:
for x in ${arrayname[*]} do
echo ${arrayname[$x]}
done
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51. A C-like for loop An alternative form of the for structure is
for (( EXPR1 ; EXPR2 ; EXPR3 )) do
statements
done
First, the arithmetic expression EXPR1 is evaluated. EXPR2 is then evaluated repeatedly until it evaluates to 0. Each time EXPR2 is evaluates to a non-zero value, statements are executed and EXPR3 is evaluated.
$ cat for2.sh
#!/bin/bash echo –n “Enter a number: ”; read x
let sum=0
for (( i=1 ; $i<$x ; i=$i+1 )) ; do
let “sum = $sum + $i”
echo “the sum of the first $x numbers is: $sum”
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52. Debugging
Bash provides two options which will give useful information for debugging
-v : displays each line of the script as typed before execution
-x : displays each line before execution (abbreviated)
Usage: #!/bin/bash –v, or #!/bin/bash –x
$ cat for3.sh
#!/bin/bash –x
echo –n “Enter a number: ”; read x
let sum=0
for (( i=1 ; $i<$x ; i=$i+1 )) ; do
let “sum = $sum + $i”
done
echo “the sum of the first $x numbers is: $sum”
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53. Debugging
$ for3.sh + echo –n ‘Enter a number: ’ Enter a number: + read x 3 + let sum=0 + (( i=0 )) + (( 0<=3 )) + let ‘sum = 0 + 0’ + (( i=0+1 )) + (( 1<=3 )) + let ‘sum = 0 + 1’ + (( i=1+1 )) + (( 2<=3 )) + let ‘sum = 1 + 2’ + (( i=2+1 )) + (( 3<=3 )) + let ‘sum = 3 + 3’ + (( i=3+1 )) + (( 4<=3 )) + echo ‘the sum of the first 3 numbers is: 6’ the sum of the first 3 numbers is: 6
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54. Where is the error? A buggy script #!/bin/bash -x # ex74.sh a=37
if [$a -gt 27 ]
then echo $a
fi exit
Output from script:
+a=37
+’[37’ -gt 37 ‘]’
+ ./ex74.sh: [37: command not found ….
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55. While Statements
The while structure is a looping structure. Used to execute a set of commands while a specified condition is true. The loop terminates as soon as the condition becomes false. If condition never becomes false, loop will never exit.
while expression do
statements
done
$ cat while.sh ( while.sh )
#!/bin/bash echo –n “Enter a number: ”; read x
let sum=0; let i=1
while [ $i –le $x ]; do
let “sum = $sum + $i”
i=$i+1
echo “the sum of the first $x numbers is: $sum”
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56. Menu #!/bin/bash # menu.sh clear ; loop=y while [ “$loop” = y ] ; do
echo “Menu”; echo “====”
echo “D: print the date”
echo “W: print the users who are currently log on.”
echo “P: print the working directory”
echo “Q: quit.”
echo
read –s choice
case $choice in
D | d) date ;;
W | w) who ;;
P | p) pwd ;;
Q | q) loop=n ;;
*) echo “Illegal choice.” ;;
esac
done
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57. Find a Pattern and Edit
$ cat grep_edit.sh #!/bin/bash # grep_edit.sh # Edit argument files $2 ..., that contain pattern $1 if [ $# -le 1 ] then echo “Usage: $0 pattern file …” ; exit 1 else pattern=$1 # Save original $1 shift # shift the positional parameter to the left by 1 while [ $# -gt 0 ] # New $1 is first filename do grep “$pattern” $1 > /dev/null if [ $? -eq 0 ] ; then # If grep found pattern vi $1 # then vi the file fi shift done $ grep_edit.sh while ~
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58. Continue Statements
The continue command causes a jump to the next iteration of the loop, skipping all the remaining commands in that particular loop cycle.
#!/bin/bash
LIMIT=19
echo
echo “Printing Numbers 1 through 20 (but not 3 and 11)”
a=0
while [ $a -le “$LIMIT” ]; do
a=$(($a+1))
if [ “$a” -eq 3 ] || [ “$a” -eq 11 ]
then
continue fi
echo -n “$a ”
done
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59. Break Statements
The break command terminates the loop (breaks out of it).
#!/bin/bash
LIMIT=19
echo “Printing Numbers 1 through 20, but something happens after 2 … ”
a=0
while [ $a -le “$LIMIT” ]; do
a=$(($a+1))
if [ “$a” -gt 2 ]
then
break fi
echo -n “$a ”
done
echo; echo; echo
exit 0
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60. Until Statements
The until structure is very similar to the while structure. The until structure loops until the condition is true. So basically it is “until this condition is true, do this”.
until [expression] do
statements
done
$ cat countdown.sh
#!/bin/bash #countdown.sh #echo “Enter a number: ”; read x
echo ; echo Count Down
until [ “$x” -le 0 ]; do
echo $x
x=$(($x –1))
sleep 1
echo ; echo GO !
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61. Manipulating Strings
Bash supports a surprising number of string manipulation operations. Unfortunately, these tools lack a unified focus.
${#string} gives the string length
${string:position} extracts sub-string from $string at $position
${string:position:length} Extracts $length characters of sub-string from $string at $position
Example
$ st=
$ echo ${#st} 10
$ echo ${st:6}
6789
$ echo ${st:6:2} 67
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62. Parameter Substitution
Manipulating and/or expanding variables
${parameter-default}, If parameter not set, use default.
$ echo ${username-`whoami`}
rinaldi
$ username=simone
simone
${parameter=default}, If parameter not set, set it to default.
$ echo ${username=`whoami`}
$ echo $username
${parameter+value}, If parameter set, use value, else use null string.
$ echo ${username+andrea}
andrea
$ echo ${pippo+andrea}
# null string
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63. Parameter Substitution
${parameter?msg}, If parameter set, use it, else print msg
$ value=${total?’total is not set’}
total: total is not set
$ total=10
$ echo $value 10
Example:
#!/bin/bash
OUTFILE=symlinks.list
directory=${1-`pwd`}
for file in “$( find $directory -type l )” do
echo “$file”
done | sort >> “$HOME/$OUTFILE”
exit 0
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64. Advanced operations on strings
${string#substring}, strips the shortest match of substring from the front of string.
pippo=abbcaabccbcabcdbcdaba
echo ${pippo#a*c}
# aabccbcabcdbcdaba
echo ${pippo##a*c} # strips the longest match
# daba
${string%substring}, strips the shortest match of substring from the front of string.
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65. Advanced operations on strings
${string/substring/replacement},strips the first match of substring in string with replacement.
pippo=abbcaabccbcabcdbcdabab
echo ${pippo/ca/11}
# abb11abccbcabcdbcdabab
echo ${pippo//ca/11}
# abb11abccb11bcdbcdabab # replaces all matches
echo ${pippo/[ab]?c/000}
# a000aabccbcabcdbcdabab
echo ${pippo/c*a/\!}
# abbc!b
echo ${pippo//b?/00}
# a00caa00c00a00d00da00b
Shell 编程

66. Functions
Functions make scripts easier to maintain. Basically it breaks up the program into smaller pieces. A function performs an action defined by you, and it can return a value if you wish.
#!/bin/bash hello() { echo “You are in function hello()” }
echo “Calling function hello()…” hello echo “You are now out of function hello()”
In the above, we called the hello() function by name by using the line: hello . When this line is executed, bash searches the script for the line hello(). It finds it right at the top, and executes its contents.
Shell 编程

67. Functions
#!/bin/bash function check() { if [ -e "/home/$1" ] then return 0 else return 1 fi } echo “Enter the name of the file: ” ; read x if check $x echo “$x exists !” echo “$x does not exists !” fi.
Shell 编程

68. Greatest Common Divisor
#!/bin/bash # gcd.sh: greatest common divisor # Uses Euclid's algorithm # The "greatest common divisor" (gcd) of two integers is the largest integer # that will divide both, leaving no remainder. # Euclid's algorithm uses successive division. # In each pass, dividend <--- divisor, divisor <--- remainder # until remainder = 0. The gcd = dividend, on the final pass. ARGS=2 E_BADARGS=65 if [ $# -ne "$ARGS" ] then echo "Usage: `basename $0` first-number second-number" exit $E_BADARGS fi
Shell 编程

69. Greatest common divisor
gcd () { dividend=$1 divisor=$2 remainder=1 until [ "$remainder" -eq 0 ] do let "remainder = $dividend % $divisor" dividend=$divisor divisor=$remainder done } # Last $dividend is the gcd. gcd $1 $2 echo; echo "GCD of $1 and $2 = $dividend“ exit 0
Shell 编程

70. Script 1: Picking a random card from a deck
#!/bin/bash # Count how many elements. Suites=“Clubs Diamonds Hearts Spades” Denominations=“ Jack Queen King Ace” # Read into array variable. suite=($Suites) denomination=($Denominations) num_suites=${#suite[*]} num_denominations=${#denomination[*]} echo -n "${denomination[$((RANDOM%num_denominations))]} of " echo ${suite[$((RANDOM%num_suites))]} exit 0
Shell 编程

71. Script 2: Changes all filenames to lowercase
#!/bin/bash for filename in * # Traverse all files in directory. do fname=`basename $filename` # Change name to lowercase. n=`echo $fname | tr A-Z a-z` if [ “$fname” != “$n” ] # Rename only files not already lowercase. then mv $fname $n fi done exit 0
Shell 编程

72. Script 3: Compare two files with a script
#!/bin/bash ARGS=2 # Two args to script expected. if [ $# -ne “$ARGS” ]; then echo “Usage: `basename $0` file1 file2” ; exit 1 fi if [[ ! -r "$1" || ! -r "$2" ]] ; then echo “Both files must exist and be readable.” ; exit 2 cmp $1 $2 &> /dev/null # /dev/null buries the output of the “cmp” command. # Also works with 'diff', i.e., diff $1 $2 &> /dev/null if [ $? -eq 0 ] # Test exit status of “cmp” command. then echo “File \“$1\” is identical to file \“$2\”.” else echo “File \“$1\“ differs from file \“$2\”.” exit 0
Shell 编程

73. Exercise
#!/bin/bash MAX=10000 for((nr=1; nr<$MAX; nr++)) do let "t1 = nr % 5" if [ "$t1" -ne 3 ] then continue fi let "t2 = nr % 7" if [ "$t2" -ne 4 ] let "t3 = nr % 9" if [ "$t3" -ne 5 ] break # What happens when you comment out this line? Why? done echo "Number = $nr" exit 0
Shell 编程

74. Bourne Shell Programming

75. Bourne Shell Programming
Certainly the most popular shell is “bash”. Bash is the shell that will appear in the GNU operating system. Bash is an sh-compatible shell that incorporates useful features from the Korn shell (ksh) and C shell (csh).
bash is not only an excellent command line shell, but a scripting language in itself. Shell scripting allows us to use the shell's abilities and to automate a lot of tasks that would otherwise require a lot of commands.
Shell 编程

76. Borne Shell Background
Early Unix shell that was written by Steve Bourne of AT&T Bell Lab.
Basic shell provided with many commercial versions of UNIX
Many system shell scripts are written to run under Bourne Shell
A long and successful history
Shell 编程

77. Bourne Shell Programming
Control structures
if … then
for … in
while
until
case
break and continue
Shell 编程

78. if … then Structure if test-command then commands fi Example:
if test “$word1” = “$word2”
echo “Match”
Shell 编程

79. test Command test is a built-in command Syntax Argument
test expression
[ expression ]
The test command evaluate an expression
Returns a condition code indicating that the expression is either true (0) or false (not 0)
Argument
Expression contains one or more criteria
Logical AND operator to separate two criteria: -a
Logical OR operator to separate two criteria: -o
Negate any criterion: !
Group criteria with parentheses
Separate each element with a SPACE
Shell 编程

80. Test Criteria Test Operator for integers: int1 relop int2 Relop
Description
-gt
Greater than
-ge
Greater than or equal to
-eq
Equal to
-ne
Not euqal to
-le
Less than or equal to
-lt
Less than
Shell 编程

81. Exercise
Create a shell script to check there is at least one parameter
Something like this: …
if test $# -eq 0
then
echo “ you must supply at least one arguments”
exit 1 fi
Shell 编程

82. Test Criteria The test built-in options for files Option
Test Performed on file
-d filename
Exists and is a directory file
-f filename
Exists and is a regular file
-r filename
Exists and it readable
-s filename
Exists and has a length greater than 0
-u filename
Exists and has setuid bit set
-w filename
Exists and it writable
-x filename
Exists and it is executable
… …
Shell 编程

83. Exercise
Check weather or not the parameter is a non-zero readable file name
Continue with the previous script and add something like
if [ -r “$filename” –a –s “$filename” ]
then
… … fi
Shell 编程

84. Test Criteria String testing Criteria meaning String
True if string is not the null string
-n string
True if string has a length greater than zero
-z string
True if string has a length of zero
String1 = string2
True if string1 is equal to string2
String1 != string2
True if string1 is not equal to string2
Shell 编程

85. Exercise Check users confirmation Frist, read user input
echo -n “Please confirm: [Yes | No] “
read user_input
Then, compare it with standard answer ‘yes’
if [ “$user_input” = Yes ]
then
echo “Thanks for your confirmation!” Fi
Shell 编程

86. if…then…else Structure
if test-command
then
commands
else fi
You can use semicolon (;) ends a command the same way a NEWLINE does.
if [ … ]; then
… …
if [ 5 = 5 ]; then echo "equal"; fi
Shell 编程

87. if…then…elif Structure if test-command then commands elif test-command.
else fi
Shell 编程

88. Debugging Shell Scripts
Display each command before it runs the command
Set the –x option for the current shell
$set –x
Use the –x to invoke the script
$sh –x command arguments
Add the set command at the top of the script
set –x
Then each command that the script executes is preceded by a plus sign (+)
Distinguish the output of trace from any output that the script produces
Turn off the debug with set +x
Shell 编程

89. for… in Structure for loop-index in argument_list do commands done
Example:
for file in *
if [ -d “$file” ]; then
echo $file fi
Shell 编程

90. for
Structure
for loop-index do
commands
done
Automatically takes on the value of each of command line arguments, one at a time. Which implies
for arg in
Shell 编程

91. while Structure while test_command do commands done Example:
while [ “$number” –lt 10 ]
… …
number=`expr $number + 1`
Shell 编程

92. until Structure until test_command do commands done Example:
secretname=jenny
name=noname
until [ “$name” = “$secretname” ]
echo “ Your guess: \c”
read name
Shell 编程

93. break and continue Interrupt for, while or until loop
The break statement
transfer control to the statement AFTER the done statement
terminate execution of the loop
The continue statement
Transfer control to the statement TO the done statement
Skip the test statements for the current iteration
Continues execution of the loop
Shell 编程

94. Example:
for index in do if [ $index –le 3 ]; then echo continue continue fi echo $index if [ $index –ge 8 ]; then echo “break” break done
Shell 编程

95. case Structure default case: catch all pattern * ) case test_string in
commands_1 ;;
pattern-2 )
commands_2
… …
esac
default case: catch all pattern
* )
Shell 编程

96. case Special characters used in patterns Pattern Matches *
Matches any string of characters. ?
Matches any single character.
[…]
Defines a character class. A hyphen specifies a range of characters |
Separates alternative choices that satisfy a particular branch of the case structure
Shell 编程

97. Example
#!/bin/sh echo “\n Command MENU\n” echo “ a. Current data and time” echo “ b. Users currently logged in” echo “ c. Name of the working directory\n” echo “Enter a,b, or c: \c” read answer echo case “$answer” in a) date ;; b) who c) pwd *) echo “There is no selection: $answer” esac
Shell 编程

98. echo and read The backslash quoted characters in echo
\c suppress the new line
\n new line
\r return
\t tab
Read
read variable1 [variable2 …]
Read one line of standard input
Assign each word to the corresponding variable, with the leftover words assigned to last variables
If only one variable is specified, the entire line will be assigned to that variable.
Shell 编程

99. Example: bundle #!/bin/sh
#bundle: group files into distribution package
echo "# To Uble, sh this file"
for i do
echo "echo $i"
echo "cat >$i <<'END of $i' "
cat $i
echo "END of $i"
done
Will this program work for sure?
Shell 编程

100. Built-in: exec Execute a command: Syntax: exec command argument
Run a command without creating a new process
Quick start
Run a command in the environment of the original process
Exec does not return control to the original program
Exec can be the used only with the last command that you want to run in a script
Example, run the following command in your current shell, what will happen?
$exec who
Shell 编程

101. Built-in: exec
Redirect standard output, input or error of a shell script from within the script
exec < infile
exec > outfile 2> errfile
Example:
sh-2.05b$ more redirect.sh
exec > /dev/tty
echo "this is a test of redirection"
sh-2.05b$ ./redirect.sh 1 > /dev/null 2 >& 1
this is a test of redirection
Shell 编程

102. Catch a signal: builtin trap
Syntax: trap ‘commands’ signal-numbers
Shell executes the commands when it catches one of the signals
Then resumes executing the script where it left off.
Just capture the signal, not doing anything with it
trap ‘ ‘ signal_number
Often used to clean up temp files
Signals
SIGHUP 1 disconnect line
SIGINT 2 control-c
SIGKILL 9 kill with -9
SIGTERM 15 default kill
SIGSTP 24 control-z …
Shell 编程

103. Example [ruihong@dafinn ~/cs3451]$ more inter #!/bin/sh
trap 'echo PROGRAM INTERRUPTED' 2
while true do
echo "programming running."
sleep 1
done
Shell 编程

104. A partial list of built-in
bg, fg, jobs job control
break, continue change the loop
cd, pwd working directory
echo, read display/read
eval scan and evaluate the command
exec execute a program
exit exit from current shell
export, unset export/ remove a val or fun
test compare arguments
Shell 编程

105. A partial list of builtin
kill sends a signal to a process or job
set sets flag or argument
shift promotes each command line argument
times displays total times for the current shell and
trap traps a signal
type show whether unix command, build-in, function
umask file creation mask
wait waits for a process to terminate.
ulimit print the value of one or more resource limits
Shell 编程

106. functions A shell function is similar to a shell script
It stores a series of commands for execution at a later time.
The shell stores functions in the memory
Shell executes a shell function in the same shell that called it.
Where to define
In .profile
In your script
Or in command line
Remove a function
Use unset built-in
Shell 编程

107. functions Syntax function_name() { commands } Example:
sh-2.05b$ whoson()
> {
> date
> echo "users currently logged on"
> who
> }
sh-2.05b$ whoson
Tue Feb 1 23:28:44 EST 2005
users currently logged on
ruihong : Jan 31 08:46
ruihong pts/ Jan 31 08:54 (:0.0)
ruihong pts/ Jan 31 09:02 (:0.0)
Shell 编程

108. Example sh-2.05b$ more .profile setenv() { if [ $# -eq 2 ] then
eval $1=$2
export $1
else
echo "usage: setenv NAME VALUE" 1>&2 fi }
sh-2.05b$. .profile
sh-2.05b$ setenv T_LIBRARY /usr/local/t
sh-2.05b$ echo $T_LIBRARY
/usr/local/t
Shell 编程

109. Exercise Let’s look at some system scripts /etc/init.d/syslog
/etc/init.d/crond
Shell 编程

110. Summary Shell is a programming language
Programs written in this language are called shell scripts.
Variable
Built-in
Control structure
Function
Call utilities outside of shell
find, grep, awk
Shell 编程