Basic commands in Tcl

In this part of the Tcl tutorial, we cover some basic Tcl commands.

The covered Tcl commands include the puts, open, gets, flush, incr, info, set, and unset commands.

The puts command

In the first example, we will mention the puts command. The puts command is used to print messages to the console or to other channels like a file. The command has the following syntax:

puts ?-nonewline? ?channelId? string

The puts is the command name. Optional parameters are specified between question marks.
The -nonewline switch suppresses the newline character. By default, the command puts a newline to each message. The channelId must be an identifier for an open channel such as the Tcl standard input channel stdin, the return value from an invocation of open or socket. It defaults to standard output channel stdout if not specified. Finally, the string is the message to be printed.

#!/usr/bin/tclsh

puts "This is Tcl tutorial"
puts stdout "This is Tcl tutorial"

The puts command prints a message to the console. Both command invocations do the same thing.

The open command

The open command opens a file, a serial port, or a command pipeline and returns a channel identifier. In the following example, we use the command to open a file.

#!/usr/bin/tclsh

puts [open messages w] "This is Tcl tutorial"

The puts command is used to write to a file, opened for writing with the open command.

$ cat messages 
This is Tcl tutorial

We show the contents of the messages file created by the above Tcl script.

The gets and flush commands

The gets command reads a line from a channel, and the flush command flushes buffered output of a channel. In the following example we create a script that greets the user.

#!/usr/bin/tclsh

puts -nonewline "What is your name? "
flush stdout
gets stdin name
puts "Hello $name"

In this example, we request an input from the user and print the input in a custom greeting.

puts -nonewline "What is your name? "

The -nonewline option suppresses the newline. The prompt remains on the same line.

flush stdout

The output is buffered. To see the output immediately after the command runs, we can use the flush command. The stdout is the standard output. In our case it is a terminal; it is called a channel id in Tcl.

gets stdin name

The gets command reads a line from the standard input. The result is stored in the name variable.

puts "Hello $name"

Finally, we greet the user.

$ ./name.tcl 
What is your name? Jan
Hello Jan

Running the example.

The incr command

The incr increments the value of a variable. It has the following syntax:

incr varName ?increment?

If a parameter is passed to the command, then its value is added to the value of the variable; otherwise the value is incremented by 1.

#!/usr/bin/tclsh

# incr_cmd.tcl

set x 5

incr x  
puts $x

incr x 4
puts $x

The code example set a variable and increments it twice.

set x 5

Value 5 is set to the x variable.

incr x  
puts $x

The x variable is incremented by 1. Number 6 is printed to the console.

incr x 4
puts $x

Number 4 is added to the x variable. The puts command prints 10 to the console.

$ ./incr_cmd.tcl 
6
10

This is the output of the incr_cmd.tcl script.

The info command

The info command returns information about the state of the Tcl interpreter.

#!/usr/bin/tclsh

puts [info tclversion]
puts [info host]
puts [info exists var]

The info command has several options. We show three of them.

puts [info tclversion]

Here we print the version of the Tcl interpreter.

puts [info host]

This line prints the host name.

puts [info exists var]

Finally we check if the variable var is set.

The set and unset commands

The set command is used to create and read variables. The unset command destroys a variable.

#!/usr/bin/tclsh

set x 23
puts $x
puts [set x]

unset x
puts [info exists x]

An example showing the set and unset commands.

set x 23

We create an x variable and assign a value 23 to it.

puts $x

We print the value of the x variable.

puts [set x]

This line also prints the value of the x variable. The set command with one parameter reads the value of the variable. The value is passed to the puts command and is printed to the terminal.

unset x

The variable x is destroyed.

puts [info exists x]

We verify the existence of the variable using the info exists command.

Command line arguments

Tcl scripts like any other scripts can take command line arguments. Tcl has three predefined variables.

#!/usr/bin/tclsh

puts "The script has $argc arguments"
puts "The list of arguments: $argv"
puts "The name of the script is $argv0"

We use all the predefined variables in this script.

$ ./args.tcl 1 2 3 
The script has 3 arguments
The list of arguments: 1 2 3
The name of the script is ./args.tcl

Running the example.

This chapter covered some basics of the Tcl language.