Linux-shell_scripting for new user/shell script

Basic shell scripting
CS 2204
Class meeting 7
*Notes by Doug Bowman and other members of the
CS faculty at Virginia Tech. Copyright 2001-2003.

(C) Doug Bowman, Virginia Tech,
2001 2
Shell script/program
 A series of shell commands placed in
an ASCII text file
 Commands include
 Anything you can type on the command
line
 Shell variables
 Control statements (if, while, for)

2001 3
Resources
 Review UIAN chapter 4
 Online
 Advanced bash-scripting guide
http://www.tldp.org/LDP/abs/html/index.html
 Bash Reference Manual
http://www.gnu.org/manual/bash-2.05a/bashref.h
tml
 ksh Reference Manual
http://www.bolthole.com/solaris/ksh.html

2001 4
Script execution
 Provide script as an argument to the shell
program (e.g. bash my_script)
 Or specify which shell to use within the script
 First line of script is #!/bin/bash
 Make the script executable using chmod
 Make sure the PATH includes the current directory
 Run directly from the command line
 No compilation is necessary!

2001 5
Simple example script
#!/bin/bash
echo “Hello world!”
cd ~
pwd Output:
Hello world!
/home/grads/sgifford

2001 6
Quoting
 Quoting is necessary to use special characters
in a variable’s value or string
 ”” - shell only interprets $ and ‘‘ and
 $ - variable substitution
 ` - Command substitution
 ” - Literal double quote
 is used to escape characters
 echo ‘`date +%D`‘ will print: 10/06/03
 ’’ - shell doesn’t interpret special characters
 echo ‘`date +%D`‘ will print: `date +%D`

2001 7
Shell variables
 var refers to the name, $var to the value
 t = 100 #Sets var t to value 100
 echo ”$t = $t” #will print: $t = 100
 Remove a variable with unset var
 Names begin with alpha characters and include alpha, numeric,
or underscore
 Numeric
 Strings
 Arrays
 Command line arguments
 Functions
 Read only

2001 8
Numeric variables
 Integer variables are the only pure numeric
variables that can be used in bash
 Declaration and setting value:
declare -i var=100
 Expressions in the style of C:
 (( expression ))
 e.g. (( var+=1 ))
 +, -, *, /, %, &, |, ~, <, >, <=, >=, ==, !=, &&, ||

2001 9
String variables
 If you do not use the declare
keyword with option –i when using a
variable for the first time, it will be a
string
 var=100 makes var the string ‘100’.
 However, (( var=100 )) will treat var
as an integer even though it is a string

2001 10
Array variables
 Array is a list of values
 Don’t have to declare size
 Reference a value by ${name[index]}
 ${a[3]}
 $a (same as ${a[0]})
 Use the declare -a command to declare an
array
 declare –a sports
 sports=(ball frisbee puck)
 sports[3]=bat

2001 11
Arrays
 Array initialization
 sports=(football basketball)
 moresports=(${sports[*]} tennis)
 ${array[@]} or ${array[*]}
refers to the entire array contents
 echo ${moresports[*]} produces
football basketball tennis

2001 12
Command line arguments
 If arguments are passed to a script,
they can be referenced by $1, $2, $3,
…
 $0 refers to the name of the script
 $@ - array filled with arguments
excluding $0
 $# - number of arguments

2001 13
Exporting variables
 The export command, when used with
a variable name, allows child processes
of the shell to access the variable

2001 14
Output
 We have already seen echo
 More common in other shells including
ksh is print (does not exist in bash)
 echo –n does not print newline after
output

2001 15
Return values
 Scripts can return an integer value
 Use exit N
 The variable $? will contain the return
value of the last command run
 Can be used to test conditions

2001 16
Conditions
 If using integers: (( condition ))
 If using strings: [[ condition ]]
 Examples:
 (( a == 10 ))
 (( b >= 3 ))
 [[ $1 = -n ]]
 [[ ($v != fun) && ($v != games) ]]
 Special conditions for file existence, file
permissions, ownership, file type, etc.

2001 17
Conditions (continued…)
 [[ -e $file]] – File exists?
 [[ -f $file]] – Regular file?
 [[ -d $file]] – Directory?
 [[ -L $file]] – Symbolic link?
 [[ -r $file]] – File has read permission?
 [[ -w $file]] – File has write permission?
 [[ -x $file]] – File has execute perm?
 [[ -p $file]] – File is a pipe?

2001 18
If statements
 Syntax:
if condition
then
statements
elif condition
then
statements
else
statements
fi
optional

2001 19
If statement
 Example
if [[ -r $fname ]]
then
echo ’$fname is readable’
elif [[ -w $fname && -x $fname ]]
then
echo ’$fname is writeable and
executable’
fi

2001 20
For loops
 Syntax:
for var [in list]
do
statements
done
 If list is omitted, $@ is assumed
 Otherwise ${list[*]}
 where list is an array variable.

2001 21
For loop example
for colors in Red Blue Green
Yellow Orange Black Gray White
do
echo $colors
done
echo

2001 22
While loops
 Syntax:
while condition
do
statements
done
 The keywords break, continue, and
return have the same meaning as in
C/C++

2001 23
Case statements
 Syntax:
case expression in
pattern1)
statements ;;
pattern2)
statements ;;
…
*)
statements ;;
esac

2001 24
Case statement example
case $1 in
-a)
statements related to option a ;;
-b)
statements related to option b ;;
*)
all other options ;;
esac

2001 25
Command substitution
 Use the output of a command in a
variable, condition, etc. by:
 `command`
 $(command)

2001 26
Examples
 Arguments printed in a for loop
 Reformatting the wc command
 Performing a depth-first search

Linux-shell_scripting for new user/shell script

More Related Content

Similar to Linux-shell_scripting for new user/shell script

Recently uploaded

Linux-shell_scripting for new user/shell script