Portable scripting (POSIX compliance) in Bash Scripting - Mini Project: Build & Apply

Portable scripting (POSIX compliance)

📖 Scenario: You work as a system administrator who needs to write scripts that run on many different Unix-like systems. Some systems have different shells or tools. To avoid errors, you want to write scripts that follow POSIX standards. This means your script will work on most systems without changes.

🎯 Goal: You will create a simple POSIX-compliant shell script that lists files in a directory, counts how many files there are, and prints the count. This script will use only POSIX features to ensure it runs everywhere.

📋 What You'll Learn

Create a shell script using only POSIX-compliant commands and syntax

Use ls to list files in a directory

Use a POSIX-compliant loop to count files

Print the total count using printf for portability

💡 Why This Matters

🌍 Real World

System administrators and developers often need scripts that run on many different Unix-like systems. Writing POSIX-compliant scripts ensures compatibility and reduces errors.

💼 Career

Knowing how to write portable shell scripts is valuable for roles in system administration, DevOps, and automation where scripts must work across diverse environments.

1

Create a POSIX-compliant shell script header and list files

Write the first line of the script as #!/bin/sh to specify the POSIX shell. Then create a variable called files that stores the output of ls command listing files in the current directory.

Bash Scripting

# Write the POSIX shell header and assign the output of ls to files
# Your code here

Hint

The first line #!/bin/sh tells the system to use the POSIX shell. Use $(ls) to capture the output of the ls command.

2

Add a counter variable to count files

Add a variable called count and set it to 0. This will keep track of how many files are in the directory.

Bash Scripting

#!/bin/sh
files=$(ls)
# Initialize count variable
# Your code here

Hint

Just write count=0 to start counting from zero.

3

Use a POSIX-compliant loop to count files

Use a for loop with the variable file to iterate over each item in $files. Inside the loop, increase count by 1 using POSIX syntax.

Bash Scripting

#!/bin/sh
files=$(ls)
count=0
# Loop over files and increment count
# Your code here

Hint

Use for file in $files; do ... done and increment count with count=$((count + 1)).

4

Print the total count using printf

Use printf to print the message Total files: X where X is the value of count. This ensures portable output formatting.

Bash Scripting

#!/bin/sh
files=$(ls)
count=0
for file in $files; do
    count=$((count + 1))
done
# Print the total count
# Your code here

Hint

Use printf "Total files: %d\n" "$count" to print the count.

Practice

1. Which shebang line is best for writing a portable POSIX-compliant shell script?

easy

A. #!/bin/sh

B. #!/bin/bash

C. #!/usr/bin/env bash

D. #!/bin/zsh

Solution

1. Step 1: Understand shebang purpose

   The shebang line tells the system which shell to use to run the script.

2. Step 2: Identify POSIX-compliant shell

   "/bin/sh" is the standard POSIX shell available on most Unix-like systems, ensuring portability.

3. Final Answer:

   #!/bin/sh -> Option A

4. Quick Check:

   POSIX shell = #!/bin/sh [OK]

Hint: Use #!/bin/sh for best portability across Unix systems [OK]

Common Mistakes:

• Using #!/bin/bash which may not exist on all systems
• Using #!/usr/bin/env bash which depends on env and bash presence
• Using #!/bin/zsh which is not POSIX standard

2. Which of the following is a POSIX-compliant way to write an if statement in a shell script?

easy

A. if [[ $var -eq 1 ]]; then echo yes; fi

B. if [ $var -eq 1 ]; then echo yes; fi

C. if (( $var == 1 )); then echo yes; fi

D. if test $var = 1; then echo yes; fi

Solution

1. Step 1: Identify POSIX test syntax

   POSIX shell supports the single bracket [ ] syntax for tests.

2. Step 2: Check each option

   if [ $var -eq 1 ]; then echo yes; fi uses [ $var -eq 1 ] which is POSIX compliant. if [[ $var -eq 1 ]]; then echo yes; fi uses [[ ]] which is bash-specific. if (( $var == 1 )); then echo yes; fi uses (( )) which is arithmetic syntax in bash. if test $var = 1; then echo yes; fi uses test but with = for numbers, which is string comparison, not numeric.

3. Final Answer:

   if [ $var -eq 1 ]; then echo yes; fi -> Option B

4. Quick Check:

   POSIX if test = [ ] syntax [OK]

Hint: Use single brackets [ ] for tests in POSIX scripts [OK]

Common Mistakes:

• Using [[ ]] which is not POSIX
• Using (( )) arithmetic syntax not POSIX
• Using = for numeric comparison instead of -eq

3. What will be the output of this POSIX-compliant script snippet?

count=3
while [ $count -gt 0 ]; do
  echo $count
  count=$((count - 1))
done

medium

A. 3
2
1

B. 3
2
1
0

C. 1
2
3

D. Error: syntax not supported

Solution

1. Step 1: Understand the loop condition

   The loop runs while count is greater than 0.

2. Step 2: Trace the loop iterations

   Initial count=3, prints 3, decrements to 2; prints 2, decrements to 1; prints 1, decrements to 0; loop ends.

3. Final Answer:

   3 2 1 -> Option A

4. Quick Check:

   Loop prints 3 to 1, stops before 0 [OK]

Hint: Loop runs while count > 0, prints then decrements [OK]

Common Mistakes:

• Expecting 0 to print
• Confusing -gt with -ge
• Thinking $(( )) is not POSIX (it is)

4. This script is intended to print all files in the current directory. What is the error?

for file in $(ls); do
  echo $file
 done

medium

A. The echo command is not POSIX compliant

B. Missing 'do' keyword in the for loop

C. Using $(ls) can break on filenames with spaces

D. Incorrect variable name 'file' used in loop

Solution

1. Step 1: Analyze command substitution usage

   Using $(ls) splits output on whitespace, breaking on filenames with spaces.

2. Step 2: Identify safer alternative

   Using 'for file in *' is POSIX compliant and handles spaces better.

3. Final Answer:

   Using $(ls) can break on filenames with spaces -> Option C

4. Quick Check:

   $(ls) splits on spaces, unsafe for filenames [OK]

Hint: Avoid $(ls); use 'for file in *' for portability [OK]

Common Mistakes:

• Thinking 'do' is missing (it is present)
• Assuming variable name matters
• Believing echo is not POSIX compliant

5. You want to write a POSIX-compliant shell script using only built-in shell commands (no external utilities like grep, cat, wc) that reads lines from a file and counts how many lines contain the word 'error'. Which approach is portable and correct?

hard

A. grep -c 'error' filename

B. for line in $(cat filename); do if echo $line | grep -q 'error'; then count=$((count+1)); fi; done; echo $count

C. cat filename | grep 'error' | wc -l

D. while read line; do case "$line" in *error*) count=$((count+1)) ;; esac; done < filename; echo $count

Solution

1. Step 1: Evaluate each option for POSIX compliance and correctness

   A: grep -c 'error' filename uses external grep utility. C: cat filename | grep 'error' | wc -l uses external cat, grep, wc utilities. B: while read line; do case "$line" in *error*) count=$((count+1)) ;; esac; done < filename; echo $count uses a while read loop with case pattern matching, fully POSIX compliant pure shell and handles lines safely. D: for line in $(cat filename); do if echo $line | grep -q 'error'; then count=$((count+1)); fi; done; echo $count uses for loop over $(cat filename) which breaks on spaces and uses external cat/grep.

2. Step 2: Choose the safest and most portable method

   while read line; do case "$line" in *error*) count=$((count+1)) ;; esac; done < filename; echo $count is the best because it reads line by line safely and uses POSIX shell features only.

3. Final Answer:

   while read line; do case "$line" in *error*) count=$((count+1)) ;; esac; done < filename; echo $count -> Option D

4. Quick Check:

   Line-by-line read with case is portable and safe [OK]

Hint: Use while read and case for safe POSIX line processing [OK]

Common Mistakes:

• Using external grep instead of pure shell constructs
• Using for loop over command substitution breaking on spaces
• Using unnecessary cat in pipelines