Syntax Highlighting

fortsh provides real-time syntax highlighting as you type, similar to fish shell.

Source: src/io/syntax_highlight.f90

Overview

Commands are colored in real-time to indicate:

• Valid commands - Green
• Invalid commands - Red
• Options/flags - Blue
• Strings - Yellow
• Variables - Magenta
• Comments - Gray
• Operators - Cyan
• Paths - Light blue

Color Scheme

Source: syntax_highlight.f90:54-62

Command Validation

As you type, fortsh checks whether commands exist:

ls        # Green - valid command
ks        # Red - no such command
./script  # Green if executable, red otherwise

Validation Process

1. Check if command is a shell builtin
2. Search PATH for executable
3. Check if path exists and is executable

Source: syntax_highlight.f90:782-899

Validation Cache

To avoid repeated filesystem lookups, fortsh caches command validation results:

type :: cache_entry_t
  character(len=256) :: command = ''
  logical :: is_valid = .false.
  integer :: timestamp = 0
end type

Source: syntax_highlight.f90:72-81

Cache size: 256 entries

Token Types

Source: syntax_highlight.f90:43-51

TOKEN_COMMAND = 1    ! Valid/invalid command
TOKEN_OPTION = 2     ! Flags starting with -
TOKEN_STRING = 3     ! Quoted strings
TOKEN_VARIABLE = 4   ! $ variables
TOKEN_COMMENT = 5    ! # comments
TOKEN_OPERATOR = 6   ! |, &, >, <, ;
TOKEN_NUMBER = 7     ! Numeric values
TOKEN_PATH = 8       ! Paths with /

Highlighting Rules

Commands

The first word of a command is checked for validity:

echo hello      # "echo" highlighted as valid command
git status      # "git" highlighted as valid command

After pipes and semicolons, the next word is also treated as a command:

ls | grep foo   # Both "ls" and "grep" highlighted

Options

Words starting with - are highlighted as options:

ls -la --color=auto
   ^^^ ^^^^^^^^^^^^ Blue

Strings

Both quote styles are highlighted:

echo "double quoted"  # Yellow
echo 'single quoted'  # Yellow

Strings span until the closing quote, including across escape sequences.

Variables

Variables are highlighted in magenta:

echo $HOME              # $HOME in magenta
echo ${PATH}            # ${PATH} in magenta
echo "$USER is $(whoami)"
     ^^^^^     ^^^^^^^  # Variables highlighted

Comments

From # to end of line:

ls  # list files
    ^^^^^^^^^^^^ Gray

Operators

Shell operators are highlighted in cyan:

ls && echo done
   ^^          ^^ Cyan

cat file | grep pattern > output
         ^             ^        Cyan

Implementation

Main Function

Source: syntax_highlight.f90:170-254

subroutine highlight_command_line(input, output, shell)
  ! 1. Tokenize input
  ! 2. Determine token types
  ! 3. Apply colors
  ! 4. Build output with ANSI codes
end subroutine

Tokenization

Source: syntax_highlight.f90:294-431

The tokenizer handles:

• Quoted strings (preserving quotes)
• Comments (# to end of line)
• Operators (as single tokens)
• Whitespace separation

Color Code Generation

Source: syntax_highlight.f90:919-929

function color_code(color) result(code)
  ! Returns ANSI escape sequence: ESC[{color}m
end function

Configuration

Disable Highlighting

For scripts or testing:

export FORTSH_NO_HIGHLIGHT=1

Terminal Compatibility

Highlighting is automatically disabled on:

• Dumb terminals (TERM=dumb)
• Test mode (FORTSH_TEST_MODE=1)

Source: syntax_highlight.f90:93-128

Performance

Incremental Updates

When editing, fortsh uses incremental highlighting for single character changes:

subroutine highlight_single_char(ch, context, output)
  ! Context-aware single character highlighting
end subroutine

Source: syntax_highlight.f90:258-291

Caching

• Command validity is cached (256 entries)
• Cache entries have timestamps for freshness
• Invalid cache entries are revalidated

Builtin Recognition

Source: syntax_highlight.f90:820-842

These commands are recognized as valid without PATH lookup:

cd, echo, pwd, exit, export, unset, source, alias, unalias,
type, test, read, declare, local, readonly, return, break,
continue, shift, set, true, false, history, jobs, fg, bg,
wait, kill, trap, eval, exec, hash, command, builtin,
complete, compgen, printf, getopts, pushd, popd, dirs

Troubleshooting

Colors Not Showing

1. Check terminal supports colors: echo $TERM
2. Verify not in test mode: unset FORTSH_TEST_MODE
3. Check highlighting not disabled: unset FORTSH_NO_HIGHLIGHT

Wrong Colors

• Some terminals map colors differently
• Try a modern terminal emulator (kitty, alacritty, iTerm2)

Performance Issues

If highlighting causes lag:

• Check disk speed (PATH searches hit filesystem)
• Reduce PATH length
• Commands in cache are fast; first use may be slow