How do I translate this `for` loop for the fish shell?

Translating Bash/Zsh 'for' Loops to Fish Shell Syntax

Learn how to convert traditional shell 'for' loops into idiomatic Fish Shell syntax, covering common patterns like iterating over ranges, arrays, and command output.

The Fish Shell (Friendly Interactive SHell) offers a more user-friendly and powerful command-line experience compared to traditional shells like Bash or Zsh. While its syntax is often simpler and more intuitive, it can differ significantly, especially for scripting constructs like for loops. This article will guide you through translating common for loop patterns from Bash/Zsh to Fish Shell, ensuring your scripts run smoothly in your preferred shell.

Understanding Fish Shell's Loop Philosophy

Fish Shell's design prioritizes clarity and consistency. Unlike Bash/Zsh, which often requires specific syntax for array expansion or command substitution within loops, Fish aims for a more natural expression. Variables in Fish are always arrays, even if they contain a single element, simplifying iteration. The for loop syntax is generally more straightforward, often eliminating the need for do and done keywords or explicit seq commands.

flowchart TD
    A[Start with Bash/Zsh Loop] --> B{Identify Loop Type}
    B -->|Numeric Range| C[Use 'for i in (seq ...)' or 'for i in (math ...)']
    B -->|Array/List| D[Use 'for item in $array_var']
    B -->|Command Output| E[Use 'for line in (command)']
    C --> F[Implement Fish Syntax]
    D --> F
    E --> F
    F --> G[Test and Refine]
    G --> H[End]

Decision flow for translating Bash/Zsh loops to Fish Shell

Translating Common Loop Patterns

Let's look at some common for loop scenarios and how they translate from Bash/Zsh to Fish Shell.

Iterating a Numeric Range

Bash/Zsh

for i in $(seq 1 5);
do
  echo "Number: $i"
done

# Or with brace expansion
for i in {1..5};
do
  echo "Number: $i"
done

Fish Shell

for i in (seq 1 5)
  echo "Number: $i"
end

# Or using math for more complex ranges
for i in (math '1..5')
  echo "Number: $i"
end

Iterating Over an Array/List

Bash/Zsh

my_array=("apple" "banana" "cherry")
for item in "${my_array[@]}";
do
  echo "Fruit: $item"
done

Fish Shell

set my_array "apple" "banana" "cherry"
for item in $my_array
  echo "Fruit: $item"
end

Iterating Over Command Output

Bash/Zsh

for file in $(ls *.txt);
do
  echo "Processing file: $file"
done

# Safer way with find
find . -name "*.txt" -print0 | while IFS= read -r -d $'' file;
do
  echo "Processing file: $file"
done

Fish Shell

for file in (ls *.txt)
  echo "Processing file: $file"
end

# For more robust handling of filenames with spaces, use string split
# This example assumes `find` output is newline-separated
for file in (find . -name "*.txt" | string split \n)
  # Ensure 'file' is not empty if find returns nothing or has trailing newline
  if test -n "$file"
    echo "Processing file: $file"
  end
end

Looping with a C-style Counter

Bash/Zsh

for (( i=0; i<5; i++ ));
do
  echo "Count: $i"
done

Fish Shell

for i in (seq 0 4)
  echo "Count: $i"
end

# Or using a while loop for more complex conditions
set i 0
while test $i -lt 5
  echo "Count: $i"
  set i (math $i + 1)
end

Best Practices for Fish Shell Loops

When writing loops in Fish Shell, consider these best practices to ensure your code is robust and idiomatic:

1. Embrace seq and math: For numeric ranges, seq is your friend. For more complex arithmetic within loops, math provides a powerful and intuitive way to perform calculations.
2. Leverage Fish's array handling: Fish handles arrays naturally. You rarely need special syntax like ${array[@]} to expand an array for iteration.
3. Use string split for command output: When iterating over the output of a command, especially if it might contain spaces or special characters, string split is a powerful tool to parse the output reliably.
4. Prefer for over while for simple iteration: If you're just iterating over a known list or range, for is generally more concise and readable than while with a manual counter.
5. Always end your loops: Fish requires an explicit end keyword to close loop blocks, similar to fi for if statements.

# Example: Iterating over files and performing an action
for file in (ls *.log)
  if test -f "$file"
    echo "Analyzing log file: $file"
    # Add your processing logic here, e.g., grep, awk, etc.
    grep "ERROR" "$file"
  end
end

# Example: Looping with a step increment
for i in (seq 0 2 10) # Start 0, step 2, end 10
  echo "Current value: $i"
end

Practical Fish Shell loop examples