231 lines
3.6 KiB
Markdown
231 lines
3.6 KiB
Markdown
# Conditional Statements in Bash
|
|
|
|
Conditional statements in Bash allow you to control the flow of your script based on certain conditions. They enable your script to make decisions and execute different code blocks depending on whether specific conditions are true or false.
|
|
|
|
## 1. if statement
|
|
|
|
The most basic conditional statement is the 'if' statement. Its syntax is:
|
|
|
|
```
|
|
if [ condition ]; then
|
|
# commands to execute if condition is true
|
|
fi
|
|
```
|
|
|
|
Example:
|
|
|
|
```
|
|
#!/bin/bash
|
|
|
|
age=18
|
|
|
|
if [ $age -ge 18 ]; then
|
|
echo "You are an adult."
|
|
fi
|
|
```
|
|
|
|
## 2. if-else statement
|
|
|
|
The if-else statement allows you to specify actions for both when the condition is true and when it's false:
|
|
|
|
```
|
|
if [ condition ]; then
|
|
# commands to execute if condition is true
|
|
else
|
|
# commands to execute if condition is false
|
|
fi
|
|
```
|
|
|
|
Example:
|
|
|
|
```
|
|
#!/bin/bash
|
|
|
|
age=16
|
|
|
|
if [ $age -ge 18 ]; then
|
|
echo "You are an adult."
|
|
else
|
|
echo "You are a minor."
|
|
fi
|
|
```
|
|
|
|
## 3. if-elif-else statement
|
|
|
|
For multiple conditions, use the if-elif-else structure:
|
|
|
|
```
|
|
if [ condition1 ]; then
|
|
# commands for condition1
|
|
elif [ condition2 ]; then
|
|
# commands for condition2
|
|
elif [ condition3 ]; then
|
|
# commands for condition3
|
|
else
|
|
# commands if none of the conditions are true
|
|
fi
|
|
```
|
|
|
|
Example:
|
|
|
|
```
|
|
#!/bin/bash
|
|
|
|
grade=75
|
|
|
|
if [ $grade -ge 90 ]; then
|
|
echo "A"
|
|
elif [ $grade -ge 80 ]; then
|
|
echo "B"
|
|
elif [ $grade -ge 70 ]; then
|
|
echo "C"
|
|
elif [ $grade -ge 60 ]; then
|
|
echo "D"
|
|
else
|
|
echo "F"
|
|
fi
|
|
```
|
|
|
|
## 4. Comparison operators
|
|
|
|
Bash uses different operators for string and numeric comparisons:
|
|
|
|
Numeric comparisons:
|
|
- -eq: equal to
|
|
- -ne: not equal to
|
|
- -lt: less than
|
|
- -le: less than or equal to
|
|
- -gt: greater than
|
|
- -ge: greater than or equal to
|
|
|
|
String comparisons:
|
|
- =: equal to
|
|
- !=: not equal to
|
|
- <: less than (in ASCII alphabetical order)
|
|
- \>: greater than (in ASCII alphabetical order)
|
|
- -z: string is null (zero length)
|
|
- -n: string is not null
|
|
|
|
Example:
|
|
|
|
```
|
|
#!/bin/bash
|
|
|
|
num1=10
|
|
num2=20
|
|
str1="hello"
|
|
str2="world"
|
|
|
|
if [ $num1 -lt $num2 ]; then
|
|
echo "$num1 is less than $num2"
|
|
fi
|
|
|
|
if [ $str1 != $str2 ]; then
|
|
echo "$str1 is not equal to $str2"
|
|
fi
|
|
```
|
|
|
|
## 5. Logical operators
|
|
|
|
Bash supports logical AND and OR operations:
|
|
|
|
- &&: AND # Double ampersand Shift+7 (qwerty)
|
|
- ||: OR # Double Pipe | | with no spaces!
|
|
|
|
Example:
|
|
|
|
```
|
|
#!/bin/bash
|
|
|
|
age=25
|
|
has_license=true
|
|
|
|
if [ $age -ge 18 ] && [ "$has_license" = true ]; then
|
|
echo "You can drive a car."
|
|
fi
|
|
|
|
if [ $age -lt 18 ] || [ "$has_license" != true ]; then
|
|
echo "You cannot drive a car."
|
|
fi
|
|
```
|
|
|
|
## 6. Case statement
|
|
|
|
The case statement is useful when you have multiple conditions based on a single variable:
|
|
|
|
```
|
|
case $variable in
|
|
pattern1)
|
|
# commands for pattern1
|
|
;;
|
|
pattern2)
|
|
# commands for pattern2
|
|
;;
|
|
*) # This is the default or catch-all case
|
|
# default case
|
|
;;
|
|
esac
|
|
```
|
|
|
|
Example:
|
|
|
|
```
|
|
#!/bin/bash
|
|
|
|
fruit="apple"
|
|
|
|
case $fruit in
|
|
"apple")
|
|
echo "This is a red fruit."
|
|
;;
|
|
"banana")
|
|
echo "This is a yellow fruit."
|
|
;;
|
|
"grape")
|
|
echo "This is a purple fruit."
|
|
;;
|
|
*)
|
|
echo "Unknown fruit."
|
|
;;
|
|
esac
|
|
```
|
|
|
|
## 7. Test command
|
|
|
|
The test command is often used in conditional statements. It's equivalent to using square brackets []. You can use it like this:
|
|
|
|
```
|
|
if test $a -eq $b; then
|
|
echo "a is equal to b"
|
|
fi
|
|
```
|
|
|
|
This is the same as:
|
|
|
|
```
|
|
if [ $a -eq $b ]; then
|
|
echo "a is equal to b"
|
|
fi
|
|
```
|
|
|
|
## 8. Double square brackets
|
|
|
|
Bash also supports double square brackets [[ ]] for conditional tests. These provide more features than single brackets, such as pattern matching:
|
|
|
|
```
|
|
#!/bin/bash
|
|
|
|
string="Hello, World!"
|
|
|
|
if [[ $string == Hello* ]]; then
|
|
echo "String starts with 'Hello'"
|
|
fi
|
|
```
|
|
|
|
Double brackets also allow you to use && and || inside the condition:
|
|
|
|
```
|
|
if [[ $a -eq 5 && $b -gt 10 ]]; then
|
|
echo "Condition met"
|
|
fi
|
|
``` |