EX200 Create simple shell scripts — 15 Questions

Question 1

A system administrator needs to create a shell script that checks if the user 'jdoe' exists in the system and, if not, creates the user with a home directory. The script should also verify that the creation was successful. Which of the following script snippets correctly implements this logic?

Accepted Answer

if id 'jdoe' &>/dev/null; then echo 'Exists'; else useradd -m 'jdoe' && echo 'Created' || echo 'Failed'; fi. Option B is correct because it uses `id` to check for the user's existence (redirecting output to /dev/null to suppress messages), then uses `useradd -m` to create the user with a home directory. The `&&` and `||` operators ensure that success or failure of the creation is explicitly reported, fulfilling the requirement to verify successful creation.

Answer

if grep -q '^jdoe:' /etc/passwd; then echo 'Exists'; else useradd 'jdoe' && echo 'Created'; fi

Answer

if ! id 'jdoe' &>/dev/null; then useradd -m 'jdoe'; else echo 'Exists'; fi

Answer

[ -z $(id 'jdoe' 2>/dev/null) ] && useradd -m 'jdoe' && echo 'Created'

Question 2

A developer wrote a shell script that is intended to back up log files by copying all .log files from /var/log/myapp to /backup/logs. The script runs daily via cron but the backup folder is empty. The script contains the following line: `cp /var/log/myapp/*.log /backup/logs/`. What is the most likely reason the backup fails?

Accepted Answer

No .log files exist in /var/log/myapp at the time of script execution, causing the glob to match nothing.. Option C is correct because the glob pattern `*.log` in the `cp` command is expanded by the shell at the time the script runs. If no `.log` files exist in `/var/log/myapp` when the cron job executes, the shell passes the literal string `*.log` to `cp`, which then fails with a 'No such file or directory' error (or, depending on shell settings, may silently do nothing). This is a common issue when log rotation or cleanup removes files before the backup runs.

Answer

The PATH variable in cron is not set, so cp cannot be found.

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The script does not have execute permission for the user running cron.

Answer

The cron job is not enabled because the crontab syntax is incorrect.

Question 3

Which THREE of the following practices are recommended when creating simple shell scripts in a Red Hat Enterprise Linux environment to ensure reliability, security, and maintainability?

Accepted Answer

Quote variables when used in commands, e.g., "$file" instead of $file.. Option B is correct because quoting variables (e.g., "$file") prevents word splitting and glob expansion, which can cause commands to operate on unexpected arguments or filenames with spaces. This is a fundamental shell scripting best practice that directly improves reliability and security by preserving the intended value of the variable.

Answer

Use #!/bin/sh for compatibility, even if bash-specific features are needed.

Answer

Always run scripts by invoking the interpreter directly (e.g., bash script.sh) instead of making them executable.

Question 4

Refer to the exhibit. A junior admin runs this script as root, but it always prints 'httpd is running' even when httpd is stopped. What is the most likely cause?

Accepted Answer

The script uses the test command `[` instead of directly using the command as the condition, causing the condition to always be true.. Option D is correct because when a command is used as a condition inside `[ ]`, the `test` builtin evaluates the exit status of the command inside the brackets, not the command itself. In this script, `[ systemctl is-active httpd ]` always returns true (exit code 0) because `[` treats the string "systemctl" as a non-empty string, which is always true. The correct syntax is to use the command directly as the condition: `if systemctl is-active httpd; then`.

Answer

The script is not executable and is run with `sh script.sh`, causing the shebang to be ignored.

Answer

The variable SERVICE is misspelled as "HTTPD" in the condition.

Answer

The `systemctl` command requires root privileges, and the script is run as a non-root user.

Question 5

You are a system administrator for a medium-sized company running Red Hat Enterprise Linux 8 on all servers. The development team has created a shell script that is supposed to be run nightly via cron to synchronize configuration files from a master server to multiple web servers. The script is located at /opt/scripts/sync_configs.sh and is owned by root. It uses rsync over SSH with key-based authentication. The script works perfectly when run manually by root, but when it runs via cron, the synchronization fails with the error 'Host key verification failed.' The script does not explicitly specify any SSH options. The cron job is configured in /etc/crontab as: `0 2 * * * root /opt/scripts/sync_configs.sh`. The SSH keys are stored in /root/.ssh/id_rsa and the known_hosts file contains the correct host key for the master server. What is the most likely cause of the failure, and what is the best course of action to resolve it?

Accepted Answer

The cron environment lacks the SSH agent or the key is not loaded. Modify the script to use `ssh -i /root/.ssh/id_rsa -o StrictHostKeyChecking=no` or add a line to load the key via `ssh-add`.. D is correct because cron runs in a minimal environment that does not automatically load the SSH agent or add keys to it. When the script runs manually as root, the SSH agent is typically running and the key is loaded, but cron does not have access to the agent's socket. The error 'Host key verification failed' is misleading; the actual issue is that SSH cannot authenticate because the private key is not available to the agent, not that the host key is unknown. Adding `ssh -i /root/.ssh/id_rsa` explicitly specifies the key file, bypassing the need for an agent, or using `ssh-add` in the script loads the key into an agent for the cron session.

Answer

The PATH variable in cron does not include /usr/bin/rsync. Add a full path to rsync in the script or set PATH in the crontab.

Answer

The script does not have execute permission for the root user. Run `chmod +x /opt/scripts/sync_configs.sh`.

Answer

The known_hosts file in /root/.ssh/ contains an incorrect host key for the master server. Remove the entry and reconnect manually to update it.

Question 6

Which TWO of the following are true about creating simple shell scripts in Red Hat Enterprise Linux?

Accepted Answer

The shebang line (e.g., #!/bin/bash) is used to specify the interpreter.. Option A is correct because the shebang line (e.g., #!/bin/bash) tells the kernel which interpreter to use when executing the script. Without it, the shell may fall back to the default interpreter (often /bin/sh) or fail to run the script correctly. This is a fundamental requirement for any interpreted script in Linux.

Answer

Scripts must be stored in /usr/local/bin to be found by the shell.

Answer

A script must be compiled before it can be run.

Answer

A script must have a .sh file extension to be executable.

Question 7

You are a system administrator for a small company. The development team has created a shell script named 'deploy.sh' that automates deployment of a web application. The script is located at /home/devops/deploy.sh. The team reports that when they run the script with './deploy.sh' from the /home/devops directory, it fails with a 'Permission denied' error. However, running 'bash deploy.sh' works fine. Additionally, the script's first line is '#!/bin/bash' and the file permissions are '-rw-rw-r--'. The team wants to be able to run the script directly without typing 'bash'. Which of the following actions should you take to resolve the issue?

Accepted Answer

Add the execute permission to the script using 'chmod +x /home/devops/deploy.sh'.. Option C is correct because the 'Permission denied' error when running './deploy.sh' indicates that the script lacks execute permission. The current permissions '-rw-rw-r--' show read/write for owner and group, and read-only for others, but no execute bit. Adding execute permission with 'chmod +x' allows the script to be run directly via its shebang line.

Answer

Change the shebang line to '#!/bin/sh' because bash is not the default shell.

Answer

Move the script to /usr/local/bin so it can be found in the PATH.

Answer

Change the owner of the script to root using 'chown root:root /home/devops/deploy.sh'.

Question 8

Arrange the steps to configure a logical volume snapshot named 'snap_lv_data' of logical volume 'lv_data'.

Question 9

Match each package management command to its action.

Question 10

An administrator writes a script that uses the 'set -e' option at the top. What is the primary effect of this option?

Accepted Answer

It exits the script immediately if a command fails. Option C is correct because 'set -e' causes the shell to exit immediately if any command exits with a non-zero status, which helps in catching errors early. Option A is wrong because 'set -e' does not affect variable expansion. Option B is wrong because it is for 'set -u'. Option D is wrong because it is for 'set -x'.

Answer

It treats unset variables as an error

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It prints each command before execution

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It enables debug mode with verbose output

Question 11

A system administrator needs to create a shell script that processes a list of hostnames stored in a file, one per line, and runs a command on each host. Which loop construct is most appropriate?

Accepted Answer

while read host; do ... done < hosts. Option B is correct because the 'while read' loop is the standard way to read a file line by line in shell scripts, handling each line as a separate variable. Option A is wrong because 'for' with '$(cat file)' can break on spaces. Option C is wrong because it uses 'seq' unnecessarily. Option D is wrong because 'until' is not suitable for this purpose.

Answer

for i in $(seq 1 $(wc -l < hosts)); do ... done

Answer

for host in $(cat hosts); do ... done

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until read host; do ... done < hosts

Question 12

A developer wants to create a script that accepts a directory path as an argument and creates a timestamped backup of that directory. If no argument is provided, it should back up the current directory. How should the script handle the argument?

Accepted Answer

dir=${1:-.}. Option A is correct because `${1:-.}` uses the default value substitution syntax in bash: if parameter `$1` (the first positional argument) is unset or null, it expands to `.` (the current directory). This ensures the script backs up the supplied directory path or defaults to the current directory when no argument is provided, exactly matching the requirement.

Answer

dir=${@:-.}

Answer

dir=${0:-.}

Answer

dir=${?:-.}

Question 13

An administrator writes a script to check disk usage and send an alert if usage exceeds 80%. The script uses 'df -h /' and parses the output. To maintain portability and avoid common pitfalls, which approach is recommended?

Accepted Answer

Use 'df / | awk 'NR==2 {print $5}' | tr -d '%'. Option A is correct because using 'awk' with the percentage column is robust and handles different 'df' output formats. Option B is wrong because 'cut' might break if output format varies. Option C is wrong because using 'sed' with a fixed position is fragile. Option D is wrong because using 'grep' to extract the percentage can be unreliable.

Answer

Use 'df -h / | tail -1 | sed 's/.* //' | tr -d '%'

Answer

Use 'df -h / | tail -1 | cut -d' ' -f5'

Answer

Use 'df -h / | grep -oP '\d+%'

Question 14

A script needs to execute a command that might fail, but the script should continue. The administrator wants to capture the exit status for logging. Which code snippet correctly implements this?

Accepted Answer

./risky_command; rc=$?; echo $rc. Option C is correct because the exit status is captured immediately after the command. Option A is wrong because using 'set -e' would exit on failure. Option B is wrong because chaining with '&&' would skip the status capture if the command fails. Option D is wrong because the variable assignment and command execution are reversed.

Answer

set -e; ./risky_command; rc=$?; echo $rc

Answer

rc=$? ./risky_command; echo $rc

Answer

./risky_command && rc=$?; echo $rc

Question 15

A junior admin writes a script that uses functions. They notice that a variable set inside a function is not available after the function call. What is the likely cause and best practice?

Accepted Answer

In bash, variables declared in a function are local by default, use 'declare -g' to make them global. Option B is correct. Without 'declare -g', variables inside functions are local by default in bash. The solution is to use 'declare -g' to make them global. Option A is wrong because 'export' is not needed for parent script. Option C is wrong because 'local' makes it local, not global. Option D is wrong because subshells (with parentheses) do not affect variable scoping.

Answer

The variable was declared with 'local', change to 'global' keyword

Answer

The script must be sourced (.) instead of executed to retain variables

Answer

The function was called inside a subshell, use 'export' to make it global