Tcl supports file I/O handling with the help of the built in commands open, read, puts, gets, and close. A file I/O represents a sequence of bytes, does not matter if it is a text file or binary file in Tcl.
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 | rOpens an existing text file for reading purpose and the file must exist. This is the default mode used when no accessMode is specified. |
| 2 | wOpens a text file for writing, if it does not exist, then a new file is created else existing file is truncated. |
| 3 | aOpens 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 have 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 reading and write is shown below −
#!/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 the previous section and produces the following result −
test
Here is another example for reading file till the end of the file line by line −
#!/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 the previous section and produces the following result −
test test
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