Portable scripting (POSIX compliance) in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print the current working directory in a POSIX-compliant way.

Bash Scripting

echo [1]

Drag options to blanks, or click blank then click option'

Agetcwd

Bcwd

Cpwd

Ddir

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-POSIX commands like 'cwd' or 'getcwd' which may not exist.

Using 'dir' which is not guaranteed to be POSIX-compliant.

The pwd command prints the current working directory and is POSIX-compliant.

2fill in blank

medium

Complete the code to check if a file named 'data.txt' exists in a POSIX-compliant way.

Bash Scripting

if [ [1] "data.txt" ]; then echo "File exists"; fi

Drag options to blanks, or click blank then click option'

A-d

B-e

C-f

D-s

Attempts:

3 left

💡 Hint

Common Mistakes

Using -d which checks only for directories.

Using -f which checks only for regular files.

Using -s which checks if file size is greater than zero.

The -e test checks if a file or directory exists and is POSIX-compliant.

3fill in blank

hard

Fix the error in the script to assign the output of the 'date' command to a variable in a POSIX-compliant way.

Bash Scripting

current_date=[1]

Drag options to blanks, or click blank then click option'

A`date`

B$(date +%Y-%m-%d)

C${date}

D$(date)

Attempts:

3 left

💡 Hint

Common Mistakes

Using ${date} which tries to expand a variable named date.

Using backticks which can cause issues in nested commands.

Using $(date +%Y-%m-%d) when the question wants just the output of date command.

Using $(date) is POSIX-compliant for command substitution.

4fill in blank

hard

Fill both blanks to create a POSIX-compliant loop that prints numbers 1 to 5.

Bash Scripting

i=1
while [ [1] ]; do
  echo $i
  i=[2]
 done

Drag options to blanks, or click blank then click option'

A$i -le 5

B$i -lt 5

C$((i + 1))

D$i + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using '$i -lt 5' which stops at 4, not 5.

Using 'i=$i + 1' which is not arithmetic expansion.

Using 'i=$i++' which is not POSIX-compliant.

The condition [ $i -le 5 ] checks if i is less or equal to 5, and i=$((i + 1)) increments i in a POSIX-compliant way.

5fill in blank

hard

Fill all three blanks to create a POSIX-compliant script that creates a directory if it does not exist and then changes into it.

Bash Scripting

if [ ! -d [1] ]; then
  [2] [3]
fi

Drag options to blanks, or click blank then click option'

Amyfolder

Bmkdir

C-p

Dcd

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'cd' in the blank where directory creation is expected.

Omitting the '-p' option which is safer for nested directories.

Using a different directory name than 'myfolder'.

The script checks if 'myfolder' directory does not exist, then creates it with mkdir -p. The cd command is not used here because the instruction only asks to create and then change into it, but the blank is only for creation.

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