Portable scripting (POSIX compliance) in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a POSIX-compliant shell script snippet

What is the output of this POSIX-compliant shell script snippet?

Bash Scripting

count=0
for file in *; do
  case "$file" in
    *.sh) count=$((count + 1)) ;;
  esac
done
echo "$count"

A Number of files ending with .sh in the current directory

B Always outputs 0

C Total number of files in the current directory

D Syntax error due to missing semicolon

💡 Hint

Look at the pattern matching inside the case statement and the variable increment.

The script loops over all files in the current directory and increments count only for files ending with .sh. The echo prints that count.

📝 Syntax

intermediate

Identify the syntax error in this POSIX shell script

Which option correctly identifies the syntax error in this POSIX shell script snippet?

Bash Scripting

if [ "$var" = "value" ]
then
  echo "Match"
else
  echo "No match"
fi

A Missing spaces around the = operator inside [ ]

B Missing then keyword after the if condition

C No syntax error; script is valid POSIX shell

D Missing closing bracket ] in the if condition

💡 Hint

Check spacing and keywords carefully in POSIX shell if statements.

The script uses correct spacing around = inside [ ] and includes then and fi keywords properly.

🔧 Debug

advanced

Why does this POSIX shell script fail to assign variable correctly?

Consider this POSIX shell snippet: var=$(echo "Hello World" | awk '{print $2}') echo "$var" Why might this fail on some POSIX systems?

Bash Scripting

var=$(echo "Hello World" | awk '{print $2}')
echo "$var"

A The command substitution syntax $(...) is not POSIX compliant

B Some POSIX systems do not have awk installed by default

C The single quotes inside awk cause syntax errors in POSIX shell

D The echo command is not POSIX compliant

💡 Hint

Consider the availability of external commands on minimal POSIX systems.

While the syntax is POSIX compliant, some minimal POSIX systems might not have awk installed, causing the command to fail.

🧠 Conceptual

advanced

Which command is most portable for reading a file line by line in POSIX shell?

You want to read a file line by line in a POSIX-compliant shell script. Which command is the most portable and reliable?

A while IFS= read -r line; do echo "$line"; done < file.txt

B cat file.txt | while read line; do echo "$line"; done

C while read -r line; do echo "$line"; done < file.txt

D for line in $(cat file.txt); do echo "$line"; done

💡 Hint

Consider how to preserve leading/trailing spaces and backslashes in lines.

Option A uses IFS= and read -r to prevent trimming spaces and backslash interpretation, making it the most portable and reliable POSIX method.

🚀 Application

expert

Predict the output of this POSIX shell script with parameter expansion

What is the output of this POSIX shell script snippet? filename="archive.tar.gz" echo "${filename%.tar.gz}"

Bash Scripting

filename="archive.tar.gz"
echo "${filename%.tar.gz}"

A archive.tar

B archive.tar.gz

C Syntax error due to unsupported parameter expansion

D archive

💡 Hint

The % operator in parameter expansion removes the shortest matching suffix.

The expression ${filename%.tar.gz} removes the suffix .tar.gz from the variable, resulting in 'archive'.

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