subst

Perform substitutions on a string.

Syntax

subst ?options? string

Parameters

• string: The string to perform substitutions on

Options

Option	Description
-nocommands	Do not substitute command results (text in [])
-novariables	Do not substitute variable values (text with $)
-nobackslashes	Do not substitute backslash sequences

Description

Performs command substitution, variable substitution, and backslash substitution on the given string, similar to how the Tcl interpreter processes double-quoted strings. The options allow selective disabling of specific substitution types.

Note that subst does not give any special treatment to double quotes or curly braces (except within command substitutions) - they are treated as literal characters.

Variable Substitution

The following variable syntax is supported:

• Simple variables: $name
• Braced variables: ${name}
• Namespace-qualified variables: $namespace::name
• Array variables with index: $name(index) (the index is also substituted)

Backslash Escape Sequences

The following backslash escape sequences are supported:

• Standard escapes: \a, \b, \f, \n, \r, \t, \v, \\
• Backslash-newline: \<newline> collapses to a single space (including trailing whitespace)
• Hexadecimal: \xNN (up to 2 hex digits)
• Octal: \NNN (up to 3 octal digits)
• Unicode 16-bit: \uNNNN (exactly 4 hex digits, e.g., \u00A9 produces the copyright symbol)
• Unicode 32-bit: \UNNNNNNNN (exactly 8 hex digits, e.g., \U0001F44B produces the waving hand emoji)

Exception Handling in Command Substitution

When a command within [...] raises an exception:

• break - stops substitution and returns the result up to that point
• continue - substitutes an empty string for that command and continues processing
• return - substitutes the returned value
• Custom return codes (>= 5) - substitutes the returned value

Examples

All substitutions (default)

Output

Disable command substitution

Output

Disable variable substitution

Output

Disable backslash substitution

Output

Multiple options

Output

Building dynamic strings

Output

See Also

• format - Printf-style formatting
• string - String operations
• eval - Evaluate a script