Master Linux Shell Flow Control: If, For, While, Until, and Case Explained

Shell Conditional Statements (if)

Linux shell provides conditional statements using the if, then, elif, else, and fi keywords.

if [ condition ]; then
    action
elif [ condition ]; then
    action
else
    action
fi

Condition tests can use logical operators such as [[ and ]]. Example:

#!/bin/sh
scores=40
if [[ $scores -gt 90 ]]; then
    echo "very good!"
elif [[ $scores -gt 80 ]]; then
    echo "good!"
elif [[ $scores -gt 60 ]]; then
    echo "pass!"
else
    echo "no pass!"
fi

Shell Loop Statements (for, while, until)

for loop iterates over a list of values.

for var in list; do
    action
done

Example using seq to generate numbers 1‑10:

#!/bin/sh
for i in $(seq 10); do
    echo $i
done

Another form uses C‑style syntax:

#!/bin/sh
for ((i=1;i<=10;i++)); do
    echo $i
done

while loop repeats while a condition is true.

#!/bin/sh
i=10
while [[ $i -gt 5 ]]; do
    echo $i
    ((i--))
 done

Reading a file line‑by‑line:

#!/bin/sh
while read line; do
    echo $line
done < /etc/hosts

until loop runs until its condition becomes true.

#!/bin/sh
a=10
until [[ $a -lt 0 ]]; do
    echo $a
    ((a--))
done

Shell Selection Statements (case, select)

The case statement matches a variable against patterns.

case $arg in
    pattern|sample) # action ;;
    pattern1) # action ;;
    *) # default action ;;
esac

Example:

#!/bin/sh
case $1 in
    start|begin) echo "start something" ;;
    stop|end)   echo "stop something"  ;;
    *)         echo "Ignorant"       ;;
esac

The select statement creates a simple menu for user choice, often used together with case.

#!/bin/sh
select ch in "begin" "end" "exit"; do
    case $ch in
        "begin") echo "start something" ;;
        "end")   echo "stop something"   ;;
        "exit")  echo "exit"; break      ;;
        *)        echo "Ignorant"        ;;
    esac
done

These constructs form the core of shell flow‑control, enabling conditional execution, iteration, and menu‑driven selection in scripts.