bikesharingdemand
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What can you compare in a shell script file?
- numbers, strings
- return value from the command
(if return value is 0, it means command was success)
- whether some file exists? what is the access permission of that file?
======================================================================
You can use if statement in one line
if TEST_COMMANDS; then COMMANDS; fi
You can use if statement in multiple lines
if condition
then
statements
elif condition
then
statements
else
statements
fi
======================================================================
Test on files
if [-f /etc/passwd ];
then
ACTION
fi
-e file: check whether the file exists
-d file: check whether the file is a directory
-f file: check whether the file is a regular file
(like not a symbolic link, directory, device node, etc)
-s file: check whether the file is non-zero size
-g file: check whether the file has sgid set
-u file: check whether the file has suid set
-r file: check whether the file is readable
-w file: check whether the file is writable
-x file: check whether the file is executable
======================================================================
if [ string1 == string2 ];
then
ACTION
fi
if [ string1 != string2 ];
then
ACTION
fi
#!/bin/bash
echo "Enter any color code for red or yellow or green, For example, type R or Y or G: "
# read input from the user
# and assign passed parameter into COLOR variable
read COLOR
echo $COLOR
# section that compares the entry and disply a message
# if you use $COLOR, it can lead to the situation where <[ == "R" ]>
# which is syntax error
# So, you use "" to make <if [ "" == "R" ]>
if [ "$COLOR" == "R" ]
then
echo "STOP. LEAVE WAY FOR OTHERS"
elif [ "$COLOR" == "Y" ]
then
echo "GET READY YOUR WAY WILL BE OPEN SHORTLY"
elif [ "$COLOR" == "Y" ]
then
echo "MOVE.. IT IS YOUR TURN TO GO"
else
echo "INCORRECT COLOR CODE"
fi
======================================================================
You can do numerical test
by using following operators inside of [ ]
-eq: equal to
-ne: not equal to
-gt: greather than
-lt: less than
-ge: greather than or equal to
-le: less than or equal to
======================================================================
You can perform arithmetic expression in a shell script file
by using 3 ways
1. Use expr utility
# expr 8 + 8: performs the opertion
# $(expr 8 + 8): Get value itself by referencing
echo $(expr 8 + 8)
2. Use $((...)) (mostly used)
echo $((x+1))
3. Use let
let x=(1+2); echo $x
======================================================================
vim agetest.sh
#!/bin/bash
echo "put your age: "
read AGE
# ||: or
if [ $AGE -lt 20 ] || [ $AGE -gt 50 ] ; then
echo "Sorry, your are out of the age range"
elif [ $AGE -gt 20 ] || [ $AGE -lt 30 ] ; then
echo "You are in 20s"
elif [ $AGE -gt 30 ] || [ $AGE -lt 40 ] ; then
echo "You are in 30s"
elif [ $AGE -gt 40 ] || [ $AGE -lt 50 ] ; then
echo "You are in 40s"
fi
sudo chmod +x agetest.sh
./agetest.sh
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