This article explores the use of while loops in scripts to handle rocket status checks during launch sequences.
In this section, we’ll explore the use of while loops in our scripts. Our previous implementation reported errors by using the rocket debug command if the rocket’s status was “failed.” Recall that the rocket status command can return one of three states: launching, success, or failed. After liftoff, the rocket typically remains in the launching state for a period, eventually transitioning to either a success or failed state.For example, executing the following command shortly after liftoff might display the status as “launching”:
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$ rocket-status lunar-missionlaunching
Initially, our script checked the status only once immediately after liftoff:
If the rocket stays in the launching state for a few seconds and then fails, a single status check might miss the failure. Consider this case:
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$ rocket-status lunar-missionsuccess
In this example, our script could potentially overlook a failure if it happens after the lone check.
To handle scenarios where the rocket’s status remains “launching” beyond the initial check, it’s important to add a mechanism that waits and keeps checking until the state transitions to either “success” or “failed.”
One way to address this is to add nested conditionals with pauses:
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mkdir $mission_namerocket-add $mission_namerocket-start-power $mission_namerocket-internal-power $mission_namerocket-start-sequence $mission_namerocket-start-engine $mission_namerocket-lift-off $mission_namerocket_status=$(rocket-status $mission_name)if [ "$rocket_status" = "launching" ]; then sleep 2 rocket_status=$(rocket-status $mission_name) if [ "$rocket_status" = "launching" ]; then sleep 2 rocket_status=$(rocket-status $mission_name) if [ "$rocket_status" = "launching" ]; then # Continue with additional nested checks… : fi fifiif [ "$rocket_status" = "failed" ]; then rocket-debug $mission_namefi
However, this method quickly becomes unwieldy when dealing with longer launch durations, as it requires an impractical number of nested conditionals to handle every possible state check.A more robust solution is to replace the nested conditionals with a while loop. The while loop repeatedly executes commands as long as the specified condition remains true. Below is an improved version of our script using a while loop:
With this approach, the script pauses for two seconds and then rechecks the rocket’s status repeatedly until it is no longer “launching.” Once the status changes, the loop exits, and if the final state is “failed,” the script invokes the debug command.
Looping structures like the while loop are essential when you need to repeat operations without knowing the exact number of iterations in advance. Unlike for loops that work with a predetermined count, while loops continuously execute until a specific condition is met.
Another common scenario where while loops shine is in creating menu-driven programs or infinite loops. Consider the example below that displays a menu with three options: shutdown, restart, or exit the menu. Based on the user’s input, the corresponding action is executed:
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while true; do echo "1. Shutdown" echo "2. Restart" echo "3. Exit Menu" read -p "Enter your choice: " choice if [ "$choice" -eq 1 ]; then shutdown now elif [ "$choice" -eq 2 ]; then shutdown -r now elif [ "$choice" -eq 3 ]; then break else continue fidone
In this script:
Choosing option 1 triggers an immediate shutdown.
Option 2 restarts the system.
Option 3 breaks the loop, thus exiting the menu.
The continue statement ensures the loop restarts when an invalid choice is entered.
While loops offer a clean and efficient method to repeatedly execute commands until a desired condition is met, making them indispensable for scripting scenarios with unpredictable durations or outcomes.
