Tcl Script Cheatsheet

Basic Language Syntax

Print to the screen

puts "Hello, world!"


Variables are strings

# Set x to string hello
set x hello
# Set my_variable to string 22
set my_variable 22
# Expand a variable 
puts "The value of my_variable is $my_variable" 
> The value of my_variable is 22

White space is used to delimit items in a list

set y "One two three"
# Print a list with one item per line
join $y \n
> One
> Two
> Three

If you want a variable to be a string including whitespace and not a list, use { }

set z {One two three}
join $z \n
> One two three

Get the length of a list

set a "One two three"
llength $a
> 3

Arrays are zero indexed

set a "One two three"
lindex $a 1
> two


The expr function performs mathematical operations on string variables that are numbers.

set a 10
set b 20
expr $a * $b
expr ($a * $b) + 10
> 200
> 210

Integer maths is used unless one variable is a float

set a 10
set b 3
set c 3.0
expr $a / $b
expr $a / $c
> 3
> 3.3333333333333335

Command expansion

Square brackets allow inline command expansion

set x 10
set y 2
set z 3.0
puts "10 / 2  is [expr $x / $y]"
puts "(10 / 2) / 3.0  is [expr [expr $x / $y] / $z]"
> 10 / 2  is 5
> (10 / 2) / 3.0  is 1.6666666666666667


Step through each item in a list

set values "1 2 3"
foreach value $values {puts "Value: $value"}
> Value: 1
> Value: 2
> Value: 3

Curly braces can be split over multiple lines

foreach value $values {
    puts "Value: $value"

While loops can also be used

set x 0
while {x < 3} {
    puts "[incr x]"
> 1
> 2
> 3

If statements

If, elseif, and else are all allowed

set x 3
if {$x > 5} {
  puts "X is > 5"
} else {
  puts "X is <= 5"
> X is <= 5
set y 2
if {$y == 1} {
  puts "Y is 1"
} elseif {$y == 2} {
  puts "Y is 2"
} else {
  puts "Y is not 1 or 2"
> Y is 2

File handling

Open a file for writing using open file_name.ext w. This returns a file handle.

Write to a file using puts followed by the file handle and the string to write.

set fh [open test.txt w]
puts $fh "Hello, World"
puts $fh "Hi!"
close $fh

Open a file for reading using open file_name.ext r.

Use gets to read a line from the file. If gets is called with a single argument (the file handle), it returns a line from the file as a string.

set fh [open test.txt r]
puts [gets $fh]
close $fh

If gets is called with two arguments, it returns the line length, or -1 when at end of the file. The line's contents is stored in the second variable.

set fh [open test.txt r]
puts "Line length: [gets $fh line]"
puts "Line contents $line"
close $fh

You can loop through a whole file like this:

set fh [open test.txt r]
set line_number 0
while {[gets $fh line] >= 0} {
    puts "[incr line_number]: $line"
close $fh


You can make procedures like this

proc print_file filename {
  set fh [open $filename r]
  puts [read $fh]
  close $fh
print_file test.txt

Or with multiple arguments

proc say_hi {name1 name2} {
  puts "Hi $name1"
  puts "Hi $name2"
say_hi Bob Steve
> Hi Bob
> Hi Steve


Get a unix timestamp using clock seconds. You can format it as a nice date / time using clock format.

clock seconds
clock format [clock seconds] -format "%Y-%m-%d %H:%M:%S"
2022-10-15 20:10:34

Get the last modified time of a file:

file mtime file_name.ext
clock format [clock seconds] -format "%Y-%m-%d %H:%M:%S"
2022-10-15 20:10:34