Day 10 of DevOps: Python, Linux and

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Pic from Hackerrank by the author

Let’s learn some syntax…

Linux/Bash for DevOps

1. while loop

A while loop keeps executing as long as a condition is true.

Syntax

while [ condition ]
do
    commands
done

Example: print numbers 1–5

count=1

while [ $count -le 5 ]
do
    echo $count
    count=$((count + 1))
done

Output:

1
2
3
4
5

How it works

count=1
   ↓
Is count <= 5?
   ↓ Yes
echo count
   ↓
count = count + 1
   ↓
Check again

Common numerical operators:

OperatorMeaning-eqequal-nenot equal-ltless than-leless than or equal-gtgreater than-gegreater than or equal

Example:

while [ $count -lt 10 ]
do
    echo $count
    count=$((count + 1))
done

2. for loop

A for loop is generally used when you want to iterate over a list of values.

Syntax

for variable in list
do
    commands
done

Example

for name in Alice Bob Charlie
do
    echo $name
done

Output:

Alice
Bob
Charlie

Very common DevOps example

Suppose you want to check several servers:

for server in server1 server2 server3
do
    echo "Checking $server"
done

Output:

Checking server1
Checking server2
Checking server3

Using a range

for i in {1..5}
do
    echo $i
done

Output:

1
2
3
4
5

You can also specify an increment:

for i in {1..10..2}
do
    echo $i
done

Output:

1
3
5
7
9

3. if statement

if is used when you want Bash to make a decision.

Syntax

if [ condition ]
then
    commands
fi

Example:

age=25

if [ $age -ge 18 ]
then
    echo "Adult"
fi

Output:

Adult

if-else

if [ condition ]
then
    commands
else
    commands
fi

Example:

age=16

if [ $age -ge 18 ]
then
    echo "Adult"
else
    echo "Minor"
fi

Output:

Minor

if-elif-else

if [ condition1 ]
then
    commands
elif [ condition2 ]
then
    commands
else
    commands
fi

Example:

marks=75

if [ $marks -ge 90 ]
then
    echo "A"
elif [ $marks -ge 60 ]
then
    echo "B"
else
    echo "C"
fi

Output:

B

4. Comparing numbers

This is very important in Bash.

Do not normally use <, > or = for numerical comparisons inside [ ].

Use:

-eq
-ne
-lt
-le
-gt
-ge

Examples

a=10
b=20

Equal

if [ $a -eq $b ]
then
    echo "Equal"
fi

Not equal

if [ $a -ne $b ]
then
    echo "Not equal"
fi

Less than

if [ $a -lt $b ]
then
    echo "$a is smaller"
fi

Less than or equal

if [ $a -le $b ]
then
    echo "a <= b"
fi

Greater than

if [ $a -gt $b ]
then
    echo "a is greater"
fi

Greater than or equal

if [ $a -ge $b ]
then
    echo "a >= b"
fi

5. Comparing characters/strings

For strings, you use different operators.

=
!=

Example:

name="Shreya"

if [ "$name" = "Shreya" ]
then
    echo "Name matches"
fi

Output:

Name matches

Not equal

name="Shreya"

if [ "$name" != "Rahul" ]
then
    echo "Names are different"
fi

Why do we put quotes around variables?

Prefer:

if [ "$name" = "Shreya" ]

rather than:

if [ $name = "Shreya" ]

Because if the variable is empty or contains spaces, quoting prevents many Bash parsing problems.

For example:

name=""

This is safer:

if [ "$name" = "Shreya" ]

6. String comparison with [[ ]]

You’ll often see modern Bash scripts use:

if [[ "$name" == "Shreya" ]]
then
    echo "Matched"
fi

Notice:

[ "$name" = "Shreya" ]

versus:

[[ "$name" == "Shreya" ]]

For Bash scripting, I recommend becoming comfortable with [[ ]] because it is more robust and gives you more capabilities.

7. Character/string operators

OperatorMeaning= or ==equal!=not equal-zstring is empty-nstring is not empty

Example:

name="Shreya"

if [ -n "$name" ]
then
    echo "Name exists"
fi

Check if empty

name=""

if [ -z "$name" ]
then
    echo "Name is empty"
fi

8. Combining conditions

You can combine conditions with:

&&
||

Example:

age=25
country="India"

if [[ $age -ge 18 && $country == "India" ]]
then
    echo "Condition satisfied"
fi

&& means AND.

|| means OR.

Example:

if [[ $age -lt 18 || $age -gt 60 ]]
then
    echo "Special category"
fi

9. A very useful DevOps example

Suppose you have a script that checks whether a service is running:

service="nginx"

if systemctl is-active --quiet "$service"
then
    echo "$service is running"
else
    echo "$service is not running"
fi

This is extremely common in Bash automation.

Another example using a loop:

for service in nginx docker ssh
do
    if systemctl is-active --quiet "$service"
    then
        echo "$service is running"
    else
        echo "$service is down"
    fi
done

This combines **for + if** — exactly the kind of pattern you'll use in DevOps scripts.

Quick cheat sheet

# WHILE
while [ condition ]
do
    commands
done

# FOR
for variable in list
do
    commands
done

# IF
if [ condition ]
then
    commands
fi

# IF / ELSE
if [ condition ]
then
    commands
else
    commands
fi

# NUMBER COMPARISON
-eq   # ==
-ne   # !=
-lt   # <
-le   # <=
-gt   # >
-ge   # >=

# STRING COMPARISON
= / == # equal
!=     # not equal
-z     # empty
-n     # not empty

# LOGICAL
&&     # AND
||     # OR

$(()) -> for doing arithmetic operations

One important distinction to remember

Think of it this way:

Numbers       → -eq -ne -lt -le -gt -ge

Strings       → = == != -z -n

Files         → -f -d -e -r -w -x

For DevOps, the file operators are the next important piece because you’ll frequently write scripts like “if this file exists, do X; if this directory doesn’t exist, create it.”

The Python equivalent, keeping the same structure as the Bash notes.

1. while loop

A while loop keeps running as long as a condition is True.

Syntax

while condition:
    commands

Example: print 1–5

count = 1

while count <= 5:
    print(count)
    count = count + 1

Output:

1
2
3
4
5

You can also write:

count += 1

instead of:

count = count + 1

2. for loop

Python’s for loop is used to iterate over a sequence/iterable.

Syntax

for variable in sequence:
    commands

Example

for name in ["Alice", "Bob", "Charlie"]:
    print(name)

Output:

Alice
Bob
Charlie

Very common DevOps example

servers = ["server1", "server2", "server3"]

for server in servers:
    print(f"Checking {server}")

Output:

Checking server1
Checking server2
Checking server3

Using range()

for i in range(1, 6):
    print(i)

Output:

1
2
3
4
5

Remember: the end value is excluded.

range(1, 6)

means:

1, 2, 3, 4, 5

Increment by 2

for i in range(1, 10, 2):
    print(i)

Output:

1
3
5
7
9

3. if statement

Python uses if to make decisions.

Syntax

if condition:
    commands

Example:

age = 25

if age >= 18:
    print("Adult")

Output:

Adult

Important difference from Bash

Python uses indentation instead of then and fi.

Bash:

if [ $age -ge 18 ]
then
    echo "Adult"
fi

Python:

if age >= 18:
    print("Adult")

4. if-else

Syntax

if condition:
    commands
else:
    commands

Example:

age = 16

if age >= 18:
    print("Adult")
else:
    print("Minor")

Output:

Minor

5. if-elif-else

Python uses elif instead of Bash's elif syntax.

marks = 75

if marks >= 90:
    print("A")
elif marks >= 60:
    print("B")
else:
    print("C")

Output:

B

6. Comparing numbers

Python is much simpler than Bash here.

You can directly use:

==    equal
!=    not equal
<     less than
<=    less than or equal
>     greater than
>=    greater than or equal

Examples

a = 10
b = 20

Equal

if a == b:
    print("Equal")

Not equal

if a != b:
    print("Not equal")

Less than

if a < b:
    print("a is smaller")

Less than or equal

if a <= b:
    print("a <= b")

Greater than

if a > b:
    print("a is greater")

Greater than or equal

if a >= b:
    print("a >= b")

7. Comparing characters / strings

Python uses:

==
!=

Example

name = "Shreya"

if name == "Shreya":
    print("Name matches")

Output:

Name matches

Not equal

name = "Shreya"

if name != "Rahul":
    print("Names are different")

8. Empty strings

Python doesn’t have Bash’s -z and -n.

Instead, Python commonly uses:

if not name:

to check whether a string is empty.

Example:

name = ""

if not name:
    print("Name is empty")

You can also explicitly check:

if name == "":
    print("Name is empty")

For non-empty:

name = "Shreya"

if name:
    print("Name exists")

9. Comparing characters

A character is simply a string of length 1 in Python.

character = "A"

if character == "A":
    print("Character is A")

You can also compare characters alphabetically because strings have an ordering:

character = "B"

if character > "A":
    print("B comes after A")

For example:

if "b" > "a":
    print("b is greater")

This is based on the characters’ Unicode values.

Be careful with uppercase/lowercase:

print("A" < "a")

This is True because uppercase and lowercase letters have different Unicode values.

10. Combining conditions

Python uses:

and
or
not

AND

age = 25
country = "India"

if age >= 18 and country == "India":
    print("Condition satisfied")

Both conditions must be True.

OR

if age < 18 or age > 60:
    print("Special category")

At least one condition must be True.

NOT

is_running = False

if not is_running:
    print("Service is not running")

11. Very useful DevOps example

Suppose you want to check whether a service is running.

With Python, you can execute a Linux command using subprocess:

import subprocess

service = "nginx"

result = subprocess.run(
    ["systemctl", "is-active", "--quiet", service]
)

if result.returncode == 0:
    print(f"{service} is running")
else:
    print(f"{service} is not running")

And you can combine this with a for loop:

import subprocess

services = ["nginx", "docker", "ssh"]

for service in services:
    result = subprocess.run(
        ["systemctl", "is-active", "--quiet", service]
    )

    if result.returncode == 0:
        print(f"{service} is running")
    else:
        print(f"{service} is down")

That’s a very realistic DevOps Python scripting pattern.

12. Bash vs Python cheat sheet

ConceptBashPythonEqual number-eq==Not equal-ne!=Less than-lt<Less/equal-le<=Greater than-gt>Greater/equal-ge>=AND&&andORNOT!notEqual string===Empty string-znot stringwhilewhile [ condition ]while condition:forfor x in listfor x in list:Block endingfi,done`indentation

The biggest thing to remember

Bash:

if [ $age -ge 18 ]
then
    echo "Adult"
fi

Python:

if age >= 18:
    print("Adult")

So Python is generally much cleaner for conditions because you don’t have Bash’s -eq, -gt, [ ], then, fi, etc.