Tcl - File I/O

Tcl supports file handling with the help of the built in commands open, read, puts, gets, and close.

A file represents a sequence of bytes, does not matter if it is a text file or binary file.

Opening Files

Tcl uses the open command to open files in Tcl. The syntax for opening a file is as follows −

open fileName accessMode

Here, filename is string literal, which you will use to name your file and accessMode can have one of the following values −

Sr.No. Mode & Description
1

r

Opens an existing text file for reading purpose and the file must exist. This is the default mode used when no accessMode is specified.
2

w

Opens a text file for writing, if it does not exist, then a new file is created else existing file is truncated.
3

a

Opens a text file for writing in appending mode and file must exist. Here, your program will start appending content in the existing file content.
4

r+

Opens a text file for reading and writing both. File must exist already.
5

w+

Opens a text file for reading and writing both. It first truncate the file to zero length if it exists otherwise create the file if it does not exist.
6

a+

Opens a text file for reading and writing both. It creates the file if it does not exist. The reading will start from the beginning, but writing can only be appended.

Closing a File

To close a file, use the close command. The syntax for close is as follows −

close fileName

Any file that has been opened by a program must be closed when the program finishes using that file. In most cases, the files need not be closed explicitly; they are closed automatically when File objects are terminated automatically.

Writing a File

Puts command is used to write to an open file.

puts $filename "text to write"

A simple example for writing to a file is shown below.

#!/usr/bin/tclsh

set fp [open "input.txt" w+]
puts $fp "test"
close $fp

When the above code is compiled and executed, it creates a new file input.txt in the directory that it has been started under (in the program's working directory).

Reading a File

Following is the simple command to read from a file −

set file_data [read $fp]

A complete example of read and write is shown below −

Live Demo 
#!/usr/bin/tclsh

set fp [open "input.txt" w+]
puts $fp "test"
close $fp
set fp [open "input.txt" r]
set file_data [read $fp]
puts $file_data
close $fp

When the above code is compiled and executed, it reads the file created in previous section and produces the following result −

test

Here is another example for reading file till end of file line by line −

Live Demo 
#!/usr/bin/tclsh

set fp [open "input.txt" w+]
puts $fp "test\ntest"
close $fp
set fp [open "input.txt" r]

while { [gets $fp data] >= 0 } {
   puts $data
}
close $fp

When the above code is compiled and executed, it reads the file created in previous section and produces the following result −

test
test
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