Shell Script
Shell script is a list of commands in a computer program that is run by the Unix shell which is a command line interpreter. A shell script usually has comments that describe the steps.
Important Things About Shell Scripting:
- Variables and Special Variables
$0 - The filename of the current script. $n - The Nth argument passed to script was invoked or function was called. $# - The number of argument passed to script or function. $@ - All arguments passed to script or function. $* - All arguments passed to script or function. $? - The exit status of the last command executed. $$ - The process ID of the current shell. For shell scripts, this is the process ID under which they are executing. $! - The process number of the last background command.
- Operators
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| # Shell operator
VAR=`expr 1 + 1`
echo $VAR
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- If … else
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| a="10"
b="20"
if [ $a == $b ]
then
echo "a is equal to b"
fi
if [ $a != $b ]
then
echo "a is not equal to b"
fi
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- Case … esac
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| FRUIT="apple"
case "$FRUIT" in
"apple") echo "Apple"
;;
"baana") echo "Banana"
;;
"mango") echo "????"
;;
esac
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- Loops
While loops
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| a=0
while [ "$a" -lt 10 ]
do
b="$a"
while [ "$b" -ge 0 ]
do
echo -n "$b "
b = `expr $b - 1`
done
echo
a=`expr $a + 1`
done
|
For loops
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| for var1 in 1 2 3
do
for var2 in 0 5
do
if [ $var1 -eq 2 -a $var2 -eq 0 ]
then
break 2
else
echo "$var1 $var2"
fi
done
done
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- Functions
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| # Define function
Hello () {
echo "Hello World"
}
# Invoke function
Hello
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