3. Create Simple Shell Scripts

📘Red Hat Certified System Administrator (RHCSA – EX200)

This section is very important for the RHCSA exam. You must know how to make decisions inside a shell script.

Conditional execution allows a script to run certain commands only if a condition is true.

In simple words:

“If something is true, do this. Otherwise, do something else.”

In IT environments, this is used to:

Check if a user exists before creating one
Verify if a file exists before modifying it
Check if a service is running
Validate input before continuing
Confirm success or failure of commands

1. Understanding Exit Status (Very Important for RHCSA)

Before learning if, you must understand exit codes.

In Linux:

Every command returns an exit status
0 → Success (TRUE)
Non-zero (1–255) → Failure (FALSE)

Example:

ls /etc
echo $?

If the directory exists, $? will show 0.

If it does not exist:

ls /wrongdir
echo $?

You will see a non-zero value.

💡 For shell scripting:

0 = TRUE
Non-zero = FALSE

This is opposite of many programming languages.

2. Basic if Statement

Syntax

if CONDITION
then
    commands
fi

Example:

if ls /etc > /dev/null 2>&1
then
    echo "Directory exists"
fi

Explanation:

If the command succeeds → execute the echo
If it fails → do nothing

3. Using if with test Command

The test command checks conditions.

Syntax:

test CONDITION

Or more commonly:

[ CONDITION ]

Important:

There MUST be a space after [ and before ]
[ is actually a command

4. File Condition Tests (Very Important for Exam)

These are commonly tested in RHCSA.

Check if file exists

[ -e filename ]

Check if regular file

[ -f filename ]

Check if directory

[ -d directory ]

Check if file is readable

[ -r filename ]

Check if file is writable

[ -w filename ]

Check if file is executable

[ -x filename ]

Example: Check Before Creating a Backup

#!/bin/bashif [ -f /etc/passwd ]
then
    cp /etc/passwd /backup/passwd.bak
    echo "Backup created"
fi

In an IT environment:

Always verify a file exists before copying or modifying it.

5. String Comparisons

Used when checking usernames, input values, service names, etc.

Equal

[ "$var" = "value" ]

Not equal

[ "$var" != "value" ]

Example:

read usernameif [ "$username" = "admin" ]
then
    echo "Administrative user"
fi

⚠ Always use double quotes around variables:

"$var"

This prevents errors if variable is empty.

6. Numeric Comparisons

Used when checking numbers like disk space, process count, user input, etc.

Operators:

Meaning	Operator
Equal	-eq
Not equal	-ne
Greater than	-gt
Less than	-lt
Greater or equal	-ge
Less or equal	-le

Example:

read numberif [ "$number" -gt 10 ]
then
    echo "Number is greater than 10"
fi

7. if-else Statement

Used when you want two possible outcomes.

Syntax

if CONDITION
then
    commands
else
    commands
fi

Example:

if [ -d /backup ]
then
    echo "Backup directory exists"
else
    mkdir /backup
    echo "Backup directory created"
fi

This is very common in server setup scripts.

8. if-elif-else Statement

Used when multiple conditions exist.

Syntax

if CONDITION1
then
    commands
elif CONDITION2
then
    commands
else
    commands
fi

Example:

read usertypeif [ "$usertype" = "admin" ]
then
    echo "Full privileges"
elif [ "$usertype" = "support" ]
then
    echo "Limited privileges"
else
    echo "Regular user"
fi

9. Logical Operators

You can combine conditions.

AND (both must be true)

[ condition1 ] && [ condition2 ]

OR inside single test:

[ condition1 -a condition2 ]

Better practice for RHCSA:

if [ -f file ] && [ -r file ]
then
    echo "File exists and is readable"
fi

OR (one must be true)

[ condition1 ] || [ condition2 ]

Example:

if [ "$user" = "admin" ] || [ "$user" = "root" ]
then
    echo "Privileged user"
fi

10. Using Double Brackets [[ ]]

Modern bash supports:

[[ condition ]]

Advantages:

Safer string comparison
No need to escape some characters
Better pattern matching

Example:

if [[ "$file" == *.conf ]]
then
    echo "Configuration file"
fi

For RHCSA, basic [ ] is usually enough, but knowing [[ ]] is helpful.

11. Checking Command Success Directly

You can test a command directly in if.

Example:

if systemctl is-active sshd > /dev/null 2>&1
then
    echo "Service is running"
else
    echo "Service is not running"
fi

This is extremely common in IT automation scripts.

12. Negation (NOT)

Use ! to reverse a condition.

Example:

if [ ! -f /etc/myconfig.conf ]
then
    echo "Configuration file missing"
fi

Meaning:

If file does NOT exist

13. Important Exam Tips

For RHCSA, you must:

✔ Know correct syntax of if, elif, else, fi
✔ Remember spaces inside [ ]
✔ Use correct numeric operators (-eq, -gt, etc.)
✔ Know file test options (-f, -d, -r, -w, -x)
✔ Understand exit status (0 = success)
✔ Quote variables properly: "$var"
✔ Be able to write small working scripts

14. Common Mistakes (Avoid in Exam)

❌ Missing space:

if["$a"="b"]

Correct:

if [ "$a" = "b" ]

❌ Using = for numeric comparison:

[ "$num" = 5 ]

Correct:

[ "$num" -eq 5 ]

❌ Forgetting fi at the end

❌ Not making script executable:

chmod +x script.sh

15. Complete RHCSA-Level Example Script

#!/bin/bashread -p "Enter username: " userif id "$user" > /dev/null 2>&1
then
    echo "User exists"
else
    useradd "$user"
    echo "User created"
fi

What this script does:

Checks if user exists
If yes → prints message
If not → creates user

This type of logic is commonly expected in the exam.

Final Summary

For RHCSA EX200, conditional execution means:

Using if, elif, else, fi
Using [ ] or test
Understanding exit codes
Performing file, string, and numeric comparisons
Combining conditions using AND/OR
Checking command success
Using negation

If you can:

Write a script that checks file existence
Compare numbers correctly
Verify user/service status
Handle multiple conditions

Then you are fully prepared for this section of the RHCSA exam.