Scriptflow

Understanding and controlling your shell script’s execution path is crucial for writing reliable Bash scripts. By default, a script runs sequentially, but you can alter this sequence using control constructs such as conditionals, loops, sourcing files, and functions. These tools allow you to:

• Run commands only if specific conditions are met
• Repeat commands multiple times with varying inputs

A Real-World Analogy: Buying a Movie Ticket

Imagine you walk up to a theater ticket booth. If you hand over a valid ticket, you enter; if not, you’re turned away. This decision-making process mirrors how an if statement in Bash evaluates conditions and branches accordingly.

A Factory Analogy for Complex Workflows

Consider a widget factory where each item travels along a conveyor. At an inspection station, defective widgets are removed while good ones proceed to the next stage. This inspection step functions like a control construct in your script, deciding whether data moves forward or is handled differently.

Key Constructs That Alter Scriptflow

Shell scripts use these four core constructs to modify the default linear execution:

Conditional Statements

if Statement

Bash’s [[ … ]] test command provides richer conditional checks than [ … ]:

#!/usr/bin/env bash
# This block never runs because 3 is not greater than 4
if [[ 3 -gt 4 ]]; then
    echo "This will never be printed"
fi

case Statement

Use case for clear branching when matching a variable against multiple patterns:

#!/usr/bin/env bash
action="$1"

case "$action" in
  start)
    echo "Starting service";;
  stop)
    echo "Stopping service";;
  restart)
    echo "Restarting service";;
  *)
    echo "Usage: $0 {start|stop|restart}";;
esac

Loops

Loops are ideal for executing commands multiple times, either a fixed count or until a condition changes.

Here are five versatile loop patterns:

1. while loop with a counter

   #!/usr/bin/env bash
   i=1
   while [[ $i -le 3 ]]; do
       echo "Iteration $i"
       i=$(( i + 1 ))
   done
   

2. for loop with brace expansion

   #!/usr/bin/env bash
   for i in {1..3}; do
       echo "Iteration $i"
   done
   

3. until loop counting down

   #!/usr/bin/env bash
   i=3
   until [[ $i -eq 0 ]]; do
       echo "Iteration $i"
       i=$(( i - 1 ))
   done
   

4. Piping seq into while

   #!/usr/bin/env bash
   seq 1 3 | while read -r i; do
       echo "Iteration $i"
   done
   

5. Reading lines from a file

   #!/usr/bin/env bash
   while read -r line; do
       echo "Line: $line"
   done < fds.txt
   

Sourcing External Files

You can import another script or configuration file mid-execution using source or the shorthand .. This merges the external content into your current shell environment.

Example v1: Basic Sourcing

# .conf content:
#!/usr/bin/env bash
source .conf
echo "${name}"

$ ./conf_read-v1.sh
Bob Doe

Example v2: Safe Sourcing with Fallback

#!/usr/bin/env bash
readonly CONF_FILE=".conf"

if [[ -f "${CONF_FILE}" ]]; then
    source "${CONF_FILE}"
else
    name="Bob"
fi

echo "${name}"
exit 0

$ ./conf_read-v2.sh
Bob

When .conf exists:

$ echo 'name="Juan Carlos"' > .conf
$ ./conf_read-v2.sh
Juan Carlos

Next Steps

In the next section, we explore how functions can modularize your scriptflow, making your Bash scripts more maintainable and reusable.

Links and References

• Bash Reference Manual
• Advanced Bash-Scripting Guide
• ShellCheck: Bash Linter