Basic Shell Scripting

Command Line Arguments:

Argument	Functionality
$#	Represents the parameter count. Useful for controlling loop constructs that need to process each parameter
$@	Expands to all the parameters separated by spaces. Useful for passing all the parameters to some other function or program
$*	Contains the string of all arguments (positional parameters) on the command line
The variable $@ is the same as $* except when enclosed in double quotes. Then, each argument contained in $@ is double quoted.
$-	Expands to the flags (options) the shell was invoked with. Useful for controlling program flow based on the flags set # echo $-
$$	Expands to the process id of the shell innovated to run the script/ current shell. Useful for creating unique temporary filenames relative to this instantiation of the script
$?	contains the exit status of the last command or shell program
$!	Contains the process ID number of the last command sent to the background. $ kill -9 $!
$0	Contains the name of the command (process) currently being executed
The variables $#,$* and $@ also change with the shiftcommand. Once a shift is performed, the first parameter is discarded. The $# variable is decremented, and the $* and $@ variables are updated.

Variables:

Assign a variable to a value	For string with space use "" $ var1="Hello World"
Read only variables	$ readonly variable_name
Assign variable from read line	$ read fname lname # Read 2 values separated by Space #"AA BB" "CC DD"/AA BB
Assign NULL to a variable	$ name=/ $ name=''/ $ name=""/ $ unset varname

Parameter substitution formats:

Substitution	Definition	Purpose
${parameter:-value}	If parameter exists and isn't null, return its value; otherwise return value	Returning a default value if the variable is undefined. $ echo ${var1:-$HOME}
${parameter:=value}	If parameter exists and isn't null, return its value; otherwise set it to value and then return its value	Setting a variable to a default value if it is undefined. $ echo ${var1:=$HOME}
${parameter:+value}	If parameter exists and isn't null, return value; otherwise return null.	Testing for the existence of a variable.
${parameter:?value}	If parameter exists and isn't null, return its value; otherwise print parameter: followed by message and abort the current command or script. Omitting message produces the default message parameter null or not set.	Catching errors that result from variables being undefined.

Command Substitution:

• $(command)
• `command`

Arithmetic Expansion:

• $((expression))
• echo $((1 + 3 + 4))

Relational Operators(works for numerical values 10, 20 or “10”, “20”):

• -eq EQUALS TO
• -ne NOT EQUALS TO
• -gt GREATER THAN
• -lt LESS THAN
• -ge GRATER THAN OR EQUALS TO
• -le LESS THAN OR EQUALS TO

Boolean Operators:

• ! NOT
• -o OR
• -a AND

String Operators:

• = EQUALS TO # [ $a = $b ]
• != NOT EQUALS TO # [ $a != $b ]
• -z RETURNS TRUE FOR ZERO LENGTH STRING # [ -z $a ]
• -n RETURNS TRUE FOR NON-ZERO LENGTH STRING # [ -n $a ]
• str RETURNS TRUE FOR NON-EMPTY STRING # [ $a ]

File Test Operators:

Operator	Description
-a file	True if file EXISTS
-b file	TRUE if file is a block special file
-c file	TRUE if file is a character special file
-d file	TRUE if file is a directory
-e file	TRUE if file exists. Is TRUE even if file is a directory but exists
-f file	TRUE if file is an ordinary file as opposed to a directory or special file
-g file	TRUE if file has its group ID (SGID) bit set
-k file	TRUE if file has its sticky bit set
-p file	TRUE if file is a named pipe
-t file	TRUE if file descriptor is open
-u file	TRUE if file has its user id (SUID) bit set
-r file	TRUE if file is readable/ read permission
-w file	TRUE if file is writable/ write permission
-x file	TRUE if file is execute/ execute permission
-s file	TRUE if file has size greater than 0
-O file	TRUE if you own file
-G	TRUE file your group id is same as file's
file1 -nt file2	TRUE if file1 is newer than file2
file1 -ot file2	TRUE if file1 is older than file2

Pattern Matching:

• *     Zero or More Characters, Except a Leading dot 
• ?     Any Single Character, Except a Leading dot
• []    Defines a Class of Characters ( - for Range, ! to Exclude)

If Statement:

if [ "$1" = "1" ]

then

echo "The first choice is nice"

elif [ "$1" = "2" ]

then

echo "The second choice is just as nice"

else

echo "I see you were wise enough not to choose"

echo "You win"

fi

Do...While:

count=$1 # Initialise count to first parameter

while [ $count -gt 0 ] # while count is greater than 10 do

do

echo $count seconds till supper time!

count=$(expr $count -1) # decrement count by 1

sleep 1 # sleep for a second using the Unix sleep command

done

For:

fruitlist="Apple Pear Tomato Peach Grape"

for fruit in $fruitlist

do

if [ "$fruit" = "Tomato" ] || [ "$fruit" = "Peach" ]

then

echo "I like ${fruit}es"

else

Unix Shell Scripting Tutorial

echo "I like ${fruit}s"

fi

done

Case:

case $1

in

1) echo 'First Choice';;

2) echo 'Second Choice';;

*) echo 'Other Choice';;

esac

Functions:

name() {

commands

}

·         For any local function variable we write: local value=4

#!/bin/sh

inc() { # The increment is defined first so we can use it

echo $(($1 + $2)) # We echo the result of the first parameter plus the second parameter

}

if [ "$1" "" ] || [ "$2" = "" ] || [ "$3" = "" ]

then

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echo USAGE:

echo " counter startvalue incrementvalue endvalue"

else

count=$1 # Rename are variables with clearer names

value=$2

end=$3

while [ $count -lt $end ] # Loop while count is less than end

do

echo $count

count=$(inc $count $value) # Call increment with count and value as parameters

count=`inc $count $value`

done # so that count is incremented by value

fi

·         Import functions from a script called common.sh in DIRECTORY

#!/bin/sh

. ./common.sh

if [ "$1" = "" ]; then

echo USAGE:

echo "sh test.sh type"

exit

fi

if `validtype $1`; then

echo Valid type

else

echo Invalid type

fi

Here is common.sh:

#!/bin/sh

validtype() {

if [ "$1" = "TYPEA" ] || [ "$1" = "TYPEB" ] || [ "$1" = "TYPEC" ] || [ "$1" = "TYPED" ] || [ "$1" = "TYPEE" ];

then

exit 0

else

exit 1

fi

}