Why does Bash not continue to the next iteration as expected?

Introduction Have you ever faced an unexpected behavior in your Bash scripts, particularly regarding loops? In this article, we will delve into a common issue with the continue statement in Bash, as illustrated by your provided script. You're correct in observing that the continue statement should skip the remaining commands in the loop and jump to the next iteration. So, why isn’t it working as expected in your case? Understanding the Issue The primary reason the continue statement may not be functioning as you expected lies in the structure of your loop and the way Bash interprets commands. A common mistake is how command substitutions and pipes are handled in Bash scripts. When you use a pipe (|), the commands are run in separate subshells. This means the continue statement is scoped to the subshell and does not affect the parent shell's loop. The Role of Subshells in Piping When you pipe the output of a command to another (as you're doing with | while read ...), Bash creates a subshell for the commands within the pipe. continue operates at the level of the shell where the loop is defined, not in the subshell. Therefore, the continue statement behaves as if it is executed in a different context, and doesn’t affect the main loop you expected. Steps to Fix the Issue To resolve this issue, you should avoid using a pipe to read the output directly from the inotifywait command. Instead, you can implement the following changes to keep the loop in the same shell: 1. Use a Function Instead of a Pipe Here's a refined version of your script that maintains the loop in the same shell while processing the input from inotifywait: #!/bin/bash TARGET=/sd/REMOTE_BACKUP LOGDIR=/sd DEBUG=: # Init daily information (reset on each run and at midnight) TODAY=$(date +%Y-%m-%d) TODAYCOUNT=0 TODAYDELTA=0 RECOMPUTESTORAGE() { echo 123 456 } inotifywait -m -e close_write -e moved_to -e delete --recursive --format "%w %f" $TARGET/ | while read DIRNAME FILENAME; do tput el $DEBUG "DIRNAME=$DIRNAME FILENAME=$FILENAME" # Exit request [[ -f $TARGET/.exit ]] && { echo "Exit request received..." rm -v $TARGET/.exit exit } # Recompute storage request [[ -f $TARGET/.recompute ]] && { read FULL USED

May 10, 2025 - 22:18

Why does Bash not continue to the next iteration as expected?

Introduction

Have you ever faced an unexpected behavior in your Bash scripts, particularly regarding loops? In this article, we will delve into a common issue with the continue statement in Bash, as illustrated by your provided script. You're correct in observing that the continue statement should skip the remaining commands in the loop and jump to the next iteration. So, why isn’t it working as expected in your case?

Understanding the Issue

The primary reason the continue statement may not be functioning as you expected lies in the structure of your loop and the way Bash interprets commands. A common mistake is how command substitutions and pipes are handled in Bash scripts. When you use a pipe (|), the commands are run in separate subshells. This means the continue statement is scoped to the subshell and does not affect the parent shell's loop.

The Role of Subshells in Piping

When you pipe the output of a command to another (as you're doing with | while read ...), Bash creates a subshell for the commands within the pipe. continue operates at the level of the shell where the loop is defined, not in the subshell. Therefore, the continue statement behaves as if it is executed in a different context, and doesn’t affect the main loop you expected.

Steps to Fix the Issue

To resolve this issue, you should avoid using a pipe to read the output directly from the inotifywait command. Instead, you can implement the following changes to keep the loop in the same shell:

1. Use a Function Instead of a Pipe

Here's a refined version of your script that maintains the loop in the same shell while processing the input from inotifywait:

#!/bin/bash

TARGET=/sd/REMOTE_BACKUP
LOGDIR=/sd
DEBUG=:

# Init daily information (reset on each run and at midnight)
TODAY=$(date +%Y-%m-%d)
TODAYCOUNT=0
TODAYDELTA=0

RECOMPUTESTORAGE() {
    echo 123 456
}

inotifywait -m -e close_write -e moved_to -e delete --recursive --format "%w %f" $TARGET/ |
while read DIRNAME FILENAME; do
    tput el
    $DEBUG "DIRNAME=$DIRNAME        FILENAME=$FILENAME"

    # Exit request
    [[ -f $TARGET/.exit ]] && {
        echo "Exit request received..."
        rm -v $TARGET/.exit
        exit
    }

    # Recompute storage request
    [[ -f $TARGET/.recompute ]] && {
        read FULL USED <<<$(RECOMPUTESTORAGE)
        DELTA=$USED
        rm -v $TARGET/.recompute
        continue
    }

    if [ "$FILENAME" == "" ]; then
        echo "${FILENAME} REMoVED"
    else
        echo -e "\nUnencrypted file $FILENAME *NOT* externalized"
    fi

    echo -n -e "\rWaiting event..."
done | tee ${LOGDIR}$(basename $0).log

2. Simplifying Condition Checks

In addition to maintaining context for the loop, improving the conditions can enhance readability:

Use {} for grouping commands instead of using && to ensure clarity.

Frequently Asked Questions

What does `inotifywait` do?

inotifywait is a command used to watch for changes to files in a directory and can trigger actions based on those changes. It’s powerful for scripts that manage backups or synchronize changes.

Why is using `continue` important in a loop?

The continue statement is essential when you need to skip the current iteration of the loop based on conditions. It allows you to control the flow of your script effectively.

Can I avoid using a subshell in other scenarios?

Yes, wherever possible, try to avoid piping if you need command control flow to affect the entire script or need access to its variables.

Conclusion

In summary, the unexpected behavior you encountered stems from how pipes create subshells, affecting the scope of statements like continue. By restructuring your script to avoid piping, you can ensure the loop behaves as intended, facilitating better control and execution. Always remember that proper context for execution is key to avoiding bugs in Bash scripts. Happy scripting!