Loops

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This session focuses on automating repetitive tasks within shell scripts using various looping constructs available in Bash.

Learning Objectives

  • Implement for loops to iterate over lists, sequences, and file sets.
  • Utilize while loops for processing streams of data, such as reading files line-by-line.
  • Understand the application of until loops.
  • Learn flow control statements within loops (break and continue).

Topics Covered

1. The for Loop

  • Iterating Over a List: for item in item1 item2 item3; do ... done.
  • Iterating Over Sequences: Using brace expansion, e.g., for i in {1..10} or seq.
  • C-Style For Loop (Advanced):

bash for ((i=0; i<10; i++)); do echo "Iteration $i" done

  • File Globbing: Iterating directly over files matched by a pattern (e.g., for file in *.log; do ... done).

2. The while Loop

  • Reading Input: The standard pattern for processing files: while IFS= read -r line; do ... done < input_file.
  • IFS=: Prevents word splitting.
  • -r: Prevents backslash interpretation.
  • Conditional Loops: Using while with a command whose exit status determines continuation (e.g., while ping -c 1 google.com; do ... done).
  • Infinite Loops:

bash while true; do echo "System pulse check..." sleep 5 done

3. The until Loop

  • Inverted Condition: Executes as long as the command's exit status is non-zero (i.e., executes until the command succeeds).

4. Loop Control & Advanced Techniques

  • break: Exiting the loop immediately.
  • continue: Skipping the remainder of the current iteration.
  • The select Loop (User Menus):

bash PS3="Choose an option: " select opt in "Update" "Status" "Quit"; do case $opt in "Update") sudo apt update ;; "Status") uptime ;; "Quit") break ;; esac done

Lab/Assessment Focus

Goal: Create an advanced bulk_rename.sh.

  1. Safety: Before renaming, use [[ -w "$1" ]] to verify write permissions.
  2. Iterate: Use a for loop to find all .log files.
  3. String Manipulation: Use ${file%.log}_backup.log to generate the new name.
  4. Batching: Add a while loop that asks the user "Are you sure?" before each rename, unless the -y flag is passed as the second argument.
  5. Menu: At the end, use a select loop to ask the user if they want to view the renamed files or exit.

Advanced Topic References

Control Structures Functions