Introduction
In Bash scripting, loops allow you to automate repetitive tasks by iterating over a set of values or conditions.
Here are the common types of loops in Bash:
1. for loop
In Bash scripting, a for loop is used to iterate through a list of items (like numbers, filenames, or strings) and perform a specific action for each item. Below are some examples of how you can use for loops in Bash.
Basic Syntax
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| for variable in items_list
do
# commands to be executed
done
|
Example 1: Loop Through a List of Numbers
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| for i in 1 2 3 4 5 6 7 8 9
do
echo "Number is $i"
done
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Output:
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9
| Number is 1
Number is 2
Number is 3
Number is 4
Number is 5
Number is 6
Number is 7
Number is 8
Number is 9
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Example 2: Loop Through a Range of Numbers
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| for i in {1..9}
do
echo "Number is $i"
done
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Output:
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| Number is 1
Number is 2
Number is 3
Number is 4
Number is 5
Number is 6
Number is 7
Number is 8
Number is 9
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Example 3: Loop Through a List of Filenames
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| for file in *.txt
do
echo "Processing $file"
done
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Output:
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| Processing foo1.txt
Processing foo2.txt
# Assuming there are two text files named foo1.txt and foo2.txt in current directory
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Example 4: Loop Through a Command’s Output
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| for line in $(cat input.txt)
do
echo "Line is $line"
done
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Output:
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| Line is This is the first line.
Line is This is the second line.
# Assuming input.txt contains two lines as shown above
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Example 5: Loop Through a String
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| string="Hello"
for (( i=0; i<${#string}; i++ ))
do
echo "Character at position $i is ${string:$i:1}"
done
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Output:
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| Character at position 0 is H
Character at position 1 is e
Character at position 2 is l
Character at position 3 is l
Character at position 4 is o
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Example 6: Loop Through Multiple Commands
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| for cmd in ls pwd echo "Hello"
do
$cmd
done
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Output:
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| Desktop Documents Downloads Music Pictures
/home/user/foo
Hello
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Example 7: Nested Loops
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| for i in {1..3}
do
for j in {a..c}
do
echo "i is $i and j is $j"
done
done
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Output:
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| i is 1 and j is a
i is 1 and j is b
i is 1 and j is c
i is 2 and j is a
i is 2 and j is b
i is 2 and j is c
i is 3 and j is a
i is 3 and j is b
i is 3 and j is c
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| while read line; do
echo "Line is $line"
done <<< "Line1\nLine2\nLine3"
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Output:
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| Line is Line1
Line is Line2
Line is Line3
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2. while and until loop
A while loop in Bash is used to execute a set of commands as long as a specified condition remains true. Here’s how you can use a while loop:
Basic Syntax
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| while [ condition ]
do
command1
command2
...
done
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Example 1: Simple While Loop
This example reads lines from a file until the end of the file is reached.
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| #!/bin/bash
# Open the file and read line by line
while IFS= read -r line
do
echo "Line is $line"
done < input.txt
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Example 2: Using Condition in While Loop
This example prints numbers from 1 to 5.
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| #!/bin/bash
count=1
while [ $count -le 5 ]
do
echo "Number is $count"
((count++))
done
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Example 3: Infinite Loop with Break Statement
This example reads user input until the user enters ‘q’ to quit.
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| #!/bin/bash
while true
do
read -p "Enter a character (q to quit): " char
if [ "$char" = "q" ]; then
break
fi
done
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This example uses an until loop to prompt the user until they enter ‘y’.
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| #!/bin/bash
answer=""
until [ "$answer" = "y" ]
do
read -p "Do you want to continue? (y/n): " answer
done
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Example 5: Using Command Output in While Loop
This example checks if a file exists until it is created.
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| #!/bin/bash
file="newfile.txt"
while [ ! -f "$file" ]
do
echo "File $file does not exist. Waiting..."
sleep 1
done
echo "File $file has been created."
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Example 6: Using Arithmetic Condition in While Loop
This example counts down from 10 to 1.
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| #!/bin/bash
count=10
while [ $count -ge 1 ]
do
echo "Count is $count"
((count--))
done
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- While Loop: Executes as long as the condition is true.
- Until Loop: Opposite of while loop, it executes until the condition is false.
Summary
These examples should give you a good understanding of how to use for and while/until loops in Bash. These snippets can be easily modified to suit your specific needs.