Tcl puts Command

last modified October 20, 2026

The Tcl puts command is used to output text to the console or files. It's the primary command for displaying information in Tcl scripts. The command is simple yet versatile.

Basic Definition

The puts command writes strings to the standard output by default. It can also write to files when a file channel is specified. The command automatically adds a newline unless instructed otherwise.

Syntax: puts ?-nonewline? ?channelId? string. The optional -nonewline suppresses the newline. channelId specifies an output channel.

Basic Output

This example demonstrates the simplest usage of puts to display text.

basic_puts.tcl

puts "Hello, Tcl World!"

This command outputs the string "Hello, Tcl World!" followed by a newline. The string must be quoted with double quotes or braces when it contains spaces; otherwise puts receives too many arguments and raises an error.

Output Without Newline

The -nonewline option prevents puts from adding a newline.

puts_nonewline.tcl

puts -nonewline "Enter your name: "
gets stdin name
puts "Hello, $name"

This creates a prompt that waits for user input on the same line. The gets command reads input, and the final puts displays a greeting. The prompt has no newline, so on a buffered stdout it may only appear after the input unless you call flush stdout. Here standard input is empty, so name is empty and the greeting ends after the comma.

Output to a File

puts can write to files by specifying a file channel.

puts_file.tcl

set file [open "output.txt" w]
puts $file "This goes to a file"
close $file

This opens a file for writing, writes a string to it, then closes the file. The w mode truncates existing files or creates new ones.

Output Variables

puts can display variable values using substitution.

puts_variable.tcl

set language "Tcl"
set version 8.6
puts "Using $language version $version"

This shows how to output variable values within a string. The dollar sign triggers variable substitution in a double-quoted string; inside braces, puts {Using $language} would print the text unchanged.

Formatted Output

puts can display formatted output using the format command.

puts_format.tcl

set pi 3.14159265359
puts [format "Pi rounded to 2 decimals: %.2f" $pi]

This demonstrates formatting a floating-point number to 2 decimal places. The format command provides C-style formatting capabilities.

Multi-line Output

puts can output multiple lines using newline characters.

puts_multiline.tcl

puts "Line 1\nLine 2\nLine 3"

This shows how to create multi-line output with a single puts command. The \n escape sequence represents a newline character. It is replaced inside double quotes; a string in braces keeps the backslash and the n as they are.

Best Practices

Newlines: Use -nonewline only when needed.
Files: Always close files after writing.
Buffering: Use flush for immediate output.
Errors: Check file operations for success, and write diagnostics to stderr with puts stderr.
Formatting: Use format for complex output.

This tutorial covered the Tcl puts 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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