Conditionals

Conditional execution based on command exit status or test expressions.

Source: src/scripting/control_flow.f90:129-272, src/ast/evaluator_simple_real.f90:2349-2495

If Statements

Basic Syntax

if condition; then
    commands
fi

With Elif and Else

if condition1; then
    commands
elif condition2; then
    commands
else
    commands
fi

Condition Types

Conditions can be:

1. Test expressions - [ ... ] or [[ ... ]]
2. Commands - exit status 0 = true, non-zero = false

# Test expression
if [[ -f "$file" ]]; then
    echo "File exists"
fi

# Command as condition
if grep -q "pattern" file.txt; then
    echo "Found"
fi

# Negated condition
if ! command; then
    echo "Command failed"
fi

Evaluation

The condition is evaluated by evaluate_condition() (control_flow.f90:25). When the condition evaluates to true (exit status 0), the then-block executes. Otherwise, elif conditions are checked in order until one matches, or the else-block executes.

# Multiple conditions
if [[ $x -gt 10 ]]; then
    echo "x > 10"
elif [[ $x -gt 5 ]]; then
    echo "5 < x <= 10"
elif [[ $x -gt 0 ]]; then
    echo "0 < x <= 5"
else
    echo "x <= 0"
fi

One-Liners

# Compact form
if [[ $debug ]]; then echo "Debug on"; fi

# Using && and ||
[[ -f file ]] && echo "exists" || echo "missing"

Case Statements

Pattern-based branching with glob matching.

Source: src/scripting/control_flow.f90:1221-1419

Basic Syntax

case word in
    pattern1) commands ;;
    pattern2) commands ;;
esac

Multiple Patterns

Use | to match multiple patterns:

case $answer in
    y|Y|yes|Yes|YES)
        echo "Affirmative"
        ;;
    n|N|no|No|NO)
        echo "Negative"
        ;;
    *)
        echo "Unknown: $answer"
        ;;
esac

Glob Patterns

Case patterns support full glob syntax:

case $file in
    *.txt)
        echo "Text file"
        ;;
    *.sh)
        echo "Shell script"
        ;;
    [0-9]*)
        echo "Starts with digit"
        ;;
    ???)
        echo "Three characters"
        ;;
    *)
        echo "Other"
        ;;
esac

Character Classes

case $char in
    [[:alpha:]])
        echo "Letter"
        ;;
    [[:digit:]])
        echo "Digit"
        ;;
    [[:space:]])
        echo "Whitespace"
        ;;
esac

First Match Wins

Once a pattern matches, subsequent patterns are skipped:

case "abc" in
    a*)   echo "Matches a*" ;;    # This matches
    abc)  echo "Matches abc" ;;   # Never reached
esac

Exit Status

• Returns exit status of last executed command
• Returns 0 if no pattern matched

Pattern Matching Details

Pattern matching uses pattern_matches_no_dotfile_check() from the glob module. Multi-pattern matching is handled by check_multi_pattern() (control_flow.f90:1376), which splits on | and checks each pattern.

Combining Conditions

Logical AND

if [[ -f "$file" ]] && [[ -r "$file" ]]; then
    echo "File exists and is readable"
fi

# Or within [[ ]]
if [[ -f "$file" && -r "$file" ]]; then
    echo "File exists and is readable"
fi

Logical OR

if [[ -z "$var" ]] || [[ "$var" == "default" ]]; then
    var="fallback"
fi

Negation

if ! [[ -d "$dir" ]]; then
    mkdir "$dir"
fi

# Within [[ ]]
if [[ ! -d "$dir" ]]; then
    mkdir "$dir"
fi

Nested Conditionals

if [[ -d "$path" ]]; then
    if [[ -w "$path" ]]; then
        echo "Writable directory"
    else
        echo "Read-only directory"
    fi
elif [[ -f "$path" ]]; then
    echo "It's a file, not a directory"
else
    echo "Path doesn't exist"
fi

Common Patterns

Default Values

if [[ -z "$CONFIG" ]]; then
    CONFIG="/etc/myapp/config"
fi

# Or more concisely
: "${CONFIG:=/etc/myapp/config}"

File Type Dispatch

case $(file -b --mime-type "$1") in
    text/*)
        cat "$1"
        ;;
    image/*)
        display "$1"
        ;;
    application/pdf)
        evince "$1"
        ;;
    *)
        echo "Unknown type"
        ;;
esac

Option Parsing

while [[ $# -gt 0 ]]; do
    case $1 in
        -v|--verbose)
            verbose=1
            shift
            ;;
        -o|--output)
            output="$2"
            shift 2
            ;;
        --)
            shift
            break
            ;;
        -*)
            echo "Unknown option: $1" >&2
            exit 1
            ;;
        *)
            break
            ;;
    esac
done