Tcl append Command

last modified October 20, 2026

The Tcl append command concatenates strings to a variable. It's more efficient than using set for string concatenation. The command modifies the variable in place.

Basic Definition

The append command appends one or more values to a variable. If the variable doesn't exist, it creates it. The command returns the new value.

Syntax: append varName ?value value ...?. The first argument is the variable name. Subsequent arguments are values to append.

Basic String Concatenation

This shows the simplest usage of append to concatenate strings.

basic_append.tcl

set str "Hello"
append str " " "World"
puts $str

This creates a variable str with value "Hello", then appends a space and "World". The result is "Hello World" printed to standard output.

Appending Multiple Values

The append command can append multiple values in a single call.

append_multiple.tcl

set sentence "Tcl"
append sentence " is" " a" " powerful" " language"
puts $sentence

Here we build a sentence by appending multiple words in one command. This is shorter than several separate append calls and avoids repeated command overhead.

Appending to Non-existent Variables

append creates the variable if it doesn't exist, unlike some other commands.

append_new_var.tcl

# No prior definition of 'text'
append text "Creating" " new" " variable"
puts $text

This demonstrates that append will create the text variable automatically. The variable is initialized with the concatenated values.

Appending Numbers

append works with numbers by converting them to strings.

append_numbers.tcl

set result "The answer is: "
append result 4 2
puts $result

Numbers 4 and 2 are converted to strings and appended. The result is "The answer is: 42". Note this is string concatenation, not arithmetic addition.

Appending in Loops

append is often used in loops to build strings incrementally.

append_loop.tcl

set output ""
foreach word {Tcl is great for scripting} {
    append output $word " "
}
puts [string trim $output]

This loop builds a sentence by appending each word with a space. The final string trim removes the trailing space. This pattern is common for building output strings.

Appending to List Elements

The append command works on variables, not on list elements, so changing one element of a list takes a few steps with lindex and lset.

append_list.tcl

set colors {red green blue}
set second [lindex $colors 1]
append second "ish"
lset colors 1 $second
puts $colors

The second element (index 1) is copied into the second variable, "ish" is appended to it, and lset stores the result back in the list. The list becomes "red greenish blue".

Best Practices

Performance: Use append instead of set for concatenation.
Clarity: Split very long argument lists into several appends.
Whitespace: Remember to include spaces between words.
Lists: For complex data, consider using lists instead.
Initialization: Initialize variables for clarity when needed.

This tutorial covered the Tcl append command with practical examples showing its usage in different scenarios.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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