Master Linux Shell Flow Control: If, For, While, Case & Select with Examples

Shell Conditional Statements (if)

The if statement in a Linux shell follows the structure

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

. Conditions are evaluated using test operators such as -gt, -lt, etc. 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

Running the script prints the appropriate message based on the value of scores. The double‑bracket [[ ]] syntax and spaces around variables are required.

Shell Loop Statements

for Loop

The classic for loop iterates over a list of words or the output of seq:

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

This prints numbers 1 through 10, each on a new line. An alternative C‑style syntax is for (( init; condition; increment )):

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

while Loop

The while loop repeats as long as its condition is true:

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

Output: 10 9 8 7 6 (each on a separate line).

until Loop

The until loop runs until its condition becomes true:

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

Result: counts down from 10 to 0.

Shell Selection Statements

case Statement

The case construct matches a variable against patterns:

case $1 in
  start|begin)
    echo "start something";;
  stop|end)
    echo "stop something";;
  *)
    echo "Ignorant";;
esac

Running testcase.sh start prints start something.

select Statement

select

creates a simple menu from a list of items, usually combined with case to handle the choice:

#!/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

The user selects a number; the script executes the corresponding branch.

These examples cover the three categories of shell flow‑control statements—conditional, looping, and selection—providing the essential syntax and practical scripts for everyday automation tasks.