Bash - 18.1 - Shell Scripts - Conditionals

Logical Operators for Conditional Execution

&&, || which is used to allow conditional execution of things.

cmd1 && cmd2 The cmd2 is executed only when the cmd1 succeeds.

cmd1 || cmd2 plays an inverse role, where cmd2 is executed if cmd1 fails. Here the exit status of grep is being used to redirect the command execution.

grep 'director' emp.lst && echo "pattern found in file"

grep 'manager' emp.lst || echo "pattern not found"

|| goes well with exit command, to terminate a script when a command fails.

grep 'manager' emp.lst || exit 2
echo "Pattern found - Job Over"

These two are enough for basic decision making but if is needed to handle complex ones.

if conditional

The if statement makes two-way decision depending on the fulfillment of a certain condition. if also requires then to execute. else and elif are not always required.
if is closed with corresponding fi.
The basic forms are:

if command is successful
then
	execute commands
else
	execute commands
fi

if command is successful
then
	execute commands
fi

if command is successful
then
	execute commands
elif command is successful
then ...
else ...
fi

All commands return a exit status and these solely determine the course of action pursued by if and while.

if grep "^$1" /etc/passwd 2>/dev/null
then 
	echo "pattern found"
else
	echo "pattern not found"
fi

Using test to evaluate expressions

The test statement is needed because the true or false values returned by the expressions can’t be handles by the if.
test doesn’t display any output but simply sets the parameter $?.

Tests work in three ways:

• Compares two numbers
• Compares two strings or a single one for a null value.
• Checks a file’s attributes

Numeric Comparison

They always begin with a hyphen -, followed by a two letter string enclosed on either side by whitespace.

$ x=5; y=7; z=7.2
$ test $x -eq $y ; echo $?
1

$test $x -lt $y ; echo $?
0

$test $z -gt $y ; echo $?
1

$ test $z -eq $y ; echo $?
0

Ensure that the messages meant to draw the attention of user (mainly from echo) are redirected to >/dev/tty. Even though it will work without it.

if test $# -eq 0; then
	echo "Usage: $0 pattern file" >/dev/tty
elif test $# -eq 2; then
	grep "$1" $2 || echo "$1 not found in $2" >/dev/tty
else
	echo "You did not enter two arguments" >/dev/tty
fi

The Short hand for test is placing the expression in [ ] .
[ $x -eq $y ] is same as test $x -eq $y
There must be white space around operators like -eq and operands like $x and inside [ ].

Shorthand for testing if a number is greater than zero can be if [ $x -gt 0 ] can be written as if [ $x ]

String Comparison

test can used to compare strings too using another set of operators like = where equality is performed and != for inequality and ! for negation also.
Also -n stg is String stg is a not a null string. -z stg is String stg is a null string.
stg when String stg is assigned and not null.

if [ $# -eq 0 ] ; then
	echo "Enter the string to be searched: \c"
	read pname
	if [ -z "$pname" ] ; then
		echo "you have not entered the string" ; exit 1
	fi
	echo "Enter the filename to be used: \c"
	read filename
	if [ ! -n "$filename" ] ; then     # ! -n is same as -z
		echo "You have not entered the filename" ; exit 2
	fi
	emp3a.sh "$pname" "$flname"
else
	emp3a.sh "$@" 
fi

Using and -a or -o operators for checking more than one condition in the same line.

if [ -n "$pname" -a -n "$flname" ] ; then
	emp3a.sh "$pname" "$flname"
else
	echo "At least one input was a null string" ; exit 1
fi

File Tests

test can be used to test the various file attributes like its type or its permissions.

. . . .

The case Conditional

The case statement is the second conditional offered by the shell. The statement matches an expression for more than one alternative, and uses a compact construction to permit multiway branching.
case also handles string tests better than if.

case expression in
	pattern1) commands1 ;;
	pattern2) commands2 ;;
	pattern3) commands3 ;;
		....
esac

echo "    Menue\n
1. List of files\n2. Process of user\n3. Today's Date
4. Users of system\n5. Quit to UNIX\nEnter Your Option: \c"
read choice
case "$choice" in
	1) ls -l ;;
	2) ps -f ;;
	3) date ;;
	4) who ;;
	5) exit ;;
	*) echo "Invalid option"  # ;; not requires for last option
esac

To handle multiple patterns using same case by using | to delimit multiple patterns like when input can be y or Y

echo "Do you want to continue? (y/n): \c"
read answer
case "$answer" in
	y|Y) ;;
	n|N) exit ;;
esac

expr : Computation and String Handling

Not needed for Korn or Bash shell