Basic commands in Tcl programming

Basic commands in Tcl

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

#!/usr/bin/tclsh

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

Here we use the puts command to write to a file. We open a file for writing using the open command.

$ cat messages 
This is Tcl tutorial

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

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Greeting a 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 stdoutis the standard output. In our case 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.

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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.

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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 putscommand and 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 existscommand.

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

• $argc - the number of arguments passed to the script
• $argv - the list of arguments
• $argv0 - the name of the script

#!/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.