For Loop in Bash

The for loop in Bash is used to iterate through a list of items, files, numbers, or any set of values. It executes a set of commands for each item in the list, making it an essential tool for automation and scripting.

Basic Syntax

for variable in list
do
  # Commands to execute
done

• variable: A placeholder for each item in the list.
• list: A space-separated list of items.
• do and done: Enclose the block of commands.

Examples of For Loops

1. Loop Through a List of Words

Script:

#!/bin/bash
for word in apple banana cherry
do
  echo "Fruit: $word"
done

Output:

Fruit: apple
Fruit: banana
Fruit: cherry

2. Loop Through Numbers

Script:

#!/bin/bash
for number in {1..5}
do
  echo "Number: $number"
done

Output:

Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

• {1..5}: Generates numbers from 1 to 5.

3. Loop Through a Range with Step

Script:

#!/bin/bash
for number in {1..10..2}
do
  echo "Odd Number: $number"
done

Output:

Odd Number: 1
Odd Number: 3
Odd Number: 5
Odd Number: 7
Odd Number: 9

• {1..10..2}: Generates numbers from 1 to 10, incrementing by 2.

4. Loop Through Files in a Directory

Script:

#!/bin/bash
for file in /path/to/directory/*
do
  echo "File: $file"
done

Output:

File: /path/to/directory/file1.txt
File: /path/to/directory/file2.txt
File: /path/to/directory/file3.txt

• /path/to/directory/*: Matches all files in the directory.

5. Loop Using Command Substitution

Script:

#!/bin/bash
for user in $(who | awk '{print $1}')
do
  echo "Logged-in user: $user"
done

Output:

Logged-in user: alice
Logged-in user: bob

• $(command): Executes a command and uses its output as the list.

6. C-Style For Loop

Bash also supports a C-like syntax for for loops.

Script:

#!/bin/bash
for ((i=1; i<=5; i++))
do
  echo "Count: $i"
done

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

• ((i=1; i<=5; i++)): Initializes, checks, and increments the loop variable.

7. Nested For Loop

You can nest for loops to iterate over multiple lists.

Script:

#!/bin/bash
for i in 1 2 3
do
  for j in A B C
  do
    echo "$i$j"
  done
done

Output:

1A
1B
1C
2A
2B
2C
3A
3B
3C

Best Practices

1. Quote Variables: Use double quotes around variables to handle spaces in values.

   for item in "apple pie" "banana split"
   do
     echo "$item"
   done
   

2. Avoid Hardcoding Paths: Use variables for paths or filenames to make scripts reusable.

   directory="/path/to/files"
   for file in "$directory"/*
   do
     echo "$file"
   done
   

3. Debugging Loops: Add set -x at the top of your script to trace execution.

Common Use Cases

1. Batch Renaming Files

   for file in *.txt
   do
     mv "$file" "${file%.txt}.bak"
   done
   

2. Performing Tasks on Multiple Servers

   for server in server1 server2 server3
   do
     ssh "$server" "uptime"
   done
   

3. Processing Log Files

   for log in /var/log/*.log
   do
     grep "ERROR" "$log"
   done
   

Conclusion

The for loop in Bash is a versatile tool for iterating over lists, processing files, and automating repetitive tasks. It’s a fundamental concept that every Bash user should master.

Let me know if you'd like examples of advanced use cases or troubleshooting tips!