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Introduction | A Simplest Example | Basic | Contorl | Procedure | Object


Tcl (Tool Command Language), a a scripting language with simple and elegant syntax rules, is created by Dr. John Ousterhout in 1989. The original Tcl has been designed as a small language that can be embedded and extended easily. After long time evolution, Tcl has been available on many platforms such as Windows, Unix, and Macintosh. Tcl is a great productive tool for programmers.

The Simplest Example

pack [button .b -text "Hello" -command {puts "Hello World!"}]

This simplest Tk example shows a Hello button on the Main window. When you click the button the console window will display the string "Hello World!" on the console. button command creates an instance .b button. -text parameter shows the button title. -command parameter is the responsed to the button's action. pack command layouts the button on the window. This example demonstrates geometry management, font management, and callbacks.


Tcl is a typeless language. Its basic syntax and structure looks like.

    command argument1 argument2 ...    

where command is a basic function or a defined procedure in the Tcl interpreter. The arguments are separeted by the white space. A newline or semicolon is the termination of a command. Each command will return a string or a null value. The arguments are string value. Grouping, substitution, and dispatch are basic operating mechanisms of Tcl.

  • Grouping

    square brackets [] represents the nested command. The command is executed and return a value.

    [clock seconds]

    curly braces {} groups words into a single argument without interpreting the words in the braces.

    set words {the time is [clock seconds]}

    double quotation "" marks groups words as a single argument and interprets command and substitution inside.

    set words "the time is [clock seconds]"

  • Substitution

    $variable dollar sign $ evals variable and get its value return.

    set variable "variable value"

    \ backslash is used to quote special character such as \, ", and $. \n returens the newline. \t returns a tab. \{, \}, \$ are quote for these special character.

    puts "The \{first\} line.\nthis is the second line."

    #comment is a comment in the beginning of a line or after the semicolon.

    # comment here please

  • Dispatch

    After grouping and substitution, Tcl command calls a build-in function or procedure to execute. This is the procedure of dispatch. In the Following paragraphs we are going to describe some building commands in the Tcl.


  • if boolean body_true else body_false

    it is a conditional command. If the value of boolean_expression is true then execute body_true, otherwise execute body_false.

        if {$variable < 0} { 
            set retval -1
        } elseif ($variable > 0) { 
            set retval +1 
        } else { 
            set retval 0 
  • while boolean body

    it evaluates the boolean if the result is true execute the body and repeates the procedure, otherwise the while command terminates.

        set count 7 
        while {$count > 0} { 
            puts 2*$count; incr count -1 
  • for initial boolean increment body

    its initial expression initializes the loop. If the second argument boolean returns non-zero value the loop continues. The third argument increment is executed {after} the execution of the body

        for {set count 0 } {$count < 7} {incr count 1} {
            puts 2*$count
  • braek, continue, and return

    break command exits the loop. continue command executes next iteration. return command returns from a procedure.


  • proc name arguments body

    it provides a method to define a sequence of commands. The variables in the procedure have a local scope. The first argument is the procedure name. The arguments is the list of argument name. The body is the command sequence of the procedure. The return command can be used inside the body to returen a value.

        proc distance {a, b, x, y} {
            return sqrt(($x-$a)*($x-$a) + ($y-$b)*($y-$b)
  • global variable ...

    global scope is the top level scope. After a variable is defined by global it is visible in the global scope. It is not necessary to define a global variable in the global scope. You need to declare the global variable in every procs you use it. Its replacment is ::variable that puts prefix :: on a global variable.

        global variable1 variable2
  • namespace

    namespaces speccificate new scopes for global variables and procedures. By using namespaces programmer can organize the large Tcl programs. Neatware strongly suggests you use the namespaces for your Tcl project although it may be little bit complexity to use with the callback commands. The format of namespace is as follow:

        namespace eval name {
            variable var initvalue
            namespace export proc1 proc2 ...
        proc name::proc1 {args} {
            variable var
            # commands ...
        proc name::procn {args} {
            # commands ...

    name is the namespace name that is declared by namespace eval. variable declares the var variable and its init value. namespace export declares the procedure names that will be available to call. The proc statements define the procedures. You may call a procedure by name::procn args. A variable var must be declared when you want to use it in a procedure.

    The namespaces may be nested. You may save to write namespace header in each calls with namespace import command. The callback commands such as after are evaluated later. Their default scopes are global. By using the namespace code prefix in front of the callback commands you can execute the callback command later in current scope.

    Namespace has the same stack level like procedure. All the variables can be accessed by uplevel or upvar command.


  • String

    String is a primitive object of Tcl. It consists of any characters even unicode characters. Some characters may have special meaning for a string in the context.

    String Construction

    (i) 'append v s1 s2 ...' command concatenates s1 s2 ... onto the variable v; (ii) the 'join list s' command joins the elements of list together and distinguishes them with a string s. The default string s is a space.

      set v "one "; append v "two " "threr "     

    after join l is "a::{b c}::d"

      set l {a {b c} d}; join l "::"

    String Access

    (i) 'string length s' command returns the number of characters of the s; (ii) 'string range s i j' command returns the substring of the s from i to j; (iii) to get an element, 'string index s i' returns the character in the position i. The starting position of a string is 0; (iv) to find the occurrence of a string, 'string first s1 s2' command returns the first occurrence of s1 in the s2; (v) the 'string last s1 s2' command finds the last occurrence.

        set s "abc defe"
        puts [string length $s] ;# display 8
        puts [string range $s 1 5] ;# display "bc de"
        puts [string index $s 0] ;# display "a"
        puts [string first "ef" $s] ;# display 5
        puts [string last "e" $s] ;# display 7

    String Operation

    (i) 'string compare s1 s2' compares two strings. It returns 0 if they are equal, or -1 if s1 is less than the s2, otherwise +1; (ii) to find a string in a pattern, 'string match pat s' command completes pattern matching. pat may be the combination of characters and the special matching characters where * for any characters, ? for any single character, and [xyz] for one of a character in the [ ]. If s matches the pat it returns 1, otherwise returns 0. (iii) 'string tolower s' and 'string toupper s' will convert the s to lower and upper case respectively. Examples of string compare are:

        set s "abc "    
        set r [string compare $s "abc"]
        if {$r == 0} {
          puts "s == 'abc'"
        } elseif {$r == -1} {
          puts "s < 'abc'"
        } else {
          puts "s > 'abc'"

    string match example:

        if {[string match {a?[xyz]} $s] == 0} {
          puts "matched"

    string conversion example:

        puts [string tolower $s]
        puts [string toupper $s] 

    String Format

    'format s v1 v2 ...' command is similar to printf() function of C. It returns a formatting string. s is the format specification. v1, v2 ... are corresponding values.)

        set s "32"
        set d [format "%2d" $s]
  • List

    List is a useful type to represent embedded structure. It is a general expression of tree structure.

    List Construction

    (i) 'list a1 a2 ...' command constructs a list from its arguments a1, a2, ... . The curly brace { } also represents a list; (ii) 'lappend ls a1 a2 ...' command appends arguments a1, a2, ... to the the end of the ls as elements; (iii) to merger lists, 'concat ls1 ls2 ...' joins the elements in ls1, ls2, ... together to form a new list; finally, (iv) 'split s c' command returns a list that splits the s with the character c.

        set lx [list a b c {d e}]
        set ly [lappend $lx {g h}]
        set lxy [concat $lx $ly] 
        set s "a, b, c"
        set l [split $s ","]

    List Access

    (i) 'llength ls' returns the number of elements in the ls; (ii) to get a sub-list, 'lrange ls i j' command returns elements of ls from i to j; finally, (iii) 'lindex ls i' returns the ith element of the ls.

        set l {a b {c d}}
        puts [llength $l]    ;# output 3
        puts [lrange $l 0 1] ;# output {a b}
        puts [lindex $l 2]   ;# output {c d}

    List Operation

    'lsearch option ls v' returns the index of ls that matches the value v in one of an option or return -1 if no value is found. -glob, -exact, and -regexp are possible option values. The default option value is -glob.

    (i) to add a new element into a list, 'linsert ls i a1 a2 ...' command inserts elements a1, a2, ... before the index i of the list ls. It returns the new list; (ii)to update elements, 'lreplace ls i j a1 a2 ...' command replaces elements from i to j in ls by elements a1, a2, ... and returns the new list; (iii) 'lsort option ls' sorts elements in ls according to the options value (-ascii, -integer, -real, -dictionary, -increasing, -decreasing, -command, -index i). The default options are '-ascii -increasing'. It returns the new list.

        set l {a b {c d}}
        puts [lsearch -exact "b"] ;# output 1
        puts [linsert $l 1 "f"] ;# output {a f b {c d}}
        puts [lreplace $l 0 0 "g"] ;# output {g f b {c d}}
        puts [lsort -increasing $l] ;# output {b {c d} f g} 
  • Array

    Tcl array is more like a map rather than traditional array in C. It is a collection of key/value pairs. The key is the index and the value is the element of an array. An element of array 'a' with index 'key' is represented as a(key). Its value is $a(key). A Tcl array is implemented as a hash table.

    Array Construction

    'array names ary [pat]' command returns the list of ary keys that matches the pat. If no pat item it returns the list of all the keys of the ary.

        set a(x) "abc"
        set a(y) "def"
        set l [array names a] ;# l is {x y}

    Array Access

    (i) 'array exists ary' returns 1 if ary is an array variable, otherwise it returns 0; (ii) to get the number of elements of ary, 'array size ary' returns this value.

        if {[array exists a] == 1} {
          puts "a is an array"
          set n [array size a]
        } else {
          puts "a is not an array"
          set n 0

    Array Operation

    (i) 'array get ary [pat]' returns a key/value pair list. pat is used for matching keys. Without pat 'array get' command will return all the pairs; (ii) 'array set ary ls' command sets ary with the ls in the key/value form.

        set a(x) "abc"
        set a(y) "def"
        set l [array get a] ;# l is a list {x abc y def}
        array set b $l ;# b is an array same as a 
  • Regexp

    Regular Expression (Regexp) is a powerful tool for text processing. It provides pattern matching for string data. Much of following description of regular expressions is copied verbatim from Henry Spencer's manual entry. Henry Spencer makes a very efficient implementation of the regular expression.

    A regular expression is zero or more branches, separated by ``|''. It matches anything that matches one of the branches. A branch is zero or more pieces, concatenated. It matches a match for the first, followed by a match for the second, etc. A piece is an atom possibly followed by ``*'', ``+'', or ``?''. An atom followed by ``*'' matches a sequence of 0 or more matches of the atom. An atom followed by ``+'' matches a sequence of 1 or more matches of the atom. An atom followed by ``?'' matches a match of the atom, or the null string.

    An atom is a regular expression in parentheses (matching a match for the regular expression), a range (see below), ``.'' (matching any single character), ``^'' (matching the null string at the beginning of the input string), ``$'' (matching the null string at the end of the input string), a ``\'' followed by a single character (matching that character), or a single character with no other significance (matching that character).

    A range is a sequence of characters enclosed in ``[]''. It normally matches any single character from the sequence. If the sequence begins with ``^'', it matches any single character not from the rest of the sequence. If two characters in the sequence are separated by ``-'', this is shorthand for the full list of ASCII characters between them (e.g. ``[0-9]'' matches any decimal digit). To include a literal ``]'' in the sequence, make it the first character (following a possible ``^''). To include a literal ``-'', make it the first or last character.


    The regexp command has the syntax, 'regexp [switches] regexp str [matchvar] [submatchvar ...]'.

    It returns 1 if part of or all str matches regular expression regexp, otherwise it returns 0. The 'matchvar' stores the string that matched entire regexp. 'submatchvar' stores the string that matched the leftmost parenthesized subexpression within regexp.

    The optional switches may have value -nocase, -indices, and --. Here, '-nocase' treates upper-case characters in str as lower- case. '-indices' specifies that the matchvar contains a pair of numbers that is the position of matched substring in the str, otherwise matchvar stores the matched substring. Finally, '--' marks the end of the switches.

      regexp (ab|a)(b*)c "abc" match submatch1 submatch2

    where match has the value "abc"; submatch1 has the value "ab"; and submatch2 has the empty value.


    The regsub command substitutes string according to the matching of regular expression. Its syntax is:

      regsub [switches] regexp str substitute variable

    If there is no match in the str for regexp, regsub command returns 0, otherwise it returns the number of matches. In addition, regsub replaces the matched regexp in the str with the substitute. The new replaced string is stored in the variable. The '&' and '\1' to '\9' characters are special characters during the substitution. The '&' is replaced by the matched string. \1 to \9 are the first and nine matched substrings. The 'switches' may be the value -all, -nocase, or --. '-all', option will replace all the occurrences of the regexp pattern. Only first occurrence is replaced in default. '-nocase' means case insensitive when makes a match. '--' is the end of the switches.

      regsub -all {%([0-9a-hA-H][0-9a-hA-H]])} \\
             $url {[format %c 0x\1]} newurl

    This is an example in CGI program where URL with special characters is encoded with a hexadecimal code %xx. Then the format command will decode the corresponding character with hexadecimal code.

  • File

    File commands are divided into directory and file operations.

    Directory Status

    (i) 'file dirname name' returns a directory name in a path. If name is a relative file name and only contains one path element, then returns ``.'' (or ``:'' on the Macintosh). If name refers to a root directory, then the root directory is returned. (ii) 'file tail name' returns the name after the last directory separator. If name contains no separators then returns name itself (iii) 'file isdirectory name' returns 1 if file name is a directory, otherwise returns 0. (iv)'file mkdir dir ...' creates one or more directories. For each pathname dir specified, this command will create all non-existing parent directories as well as dir itself. If a directory exists, then no action is taken and no error is returned. Trying to overwrite an existing file with a directory will result in an error. dir arguments are processed in the order specified, halting at the first error, if any.

        file dirname ~/src/foo.c  # returns ~/src 
        file tail ~/src/foo.c # returns foo.c
        file mkdir src  # create src directory

    File Status

    (i) 'file size name' returns the file size;

    (ii) 'file atime name' returns a decimal string giving the time at which file name was last accessed. The time is measured in the standard POSIX fashion as seconds from a fixed starting time (often January 1, 1970). If the file doesn't exist or its access time cannot be queried then an error is generated;

    (iii) 'file stat name varname' invokes the stat kernel call on name, and uses the variable given by varname to hold information returned from the kernel call. varname is treated as an array variable, and the following elements of that variable are set: atime, ctime, dev, gid, ino, mode, mtime, nlink, size, type, uid.

    Each element except type is a decimal string with the value of the corresponding field from the stat return structure; see the manual entry for stat for details on the meanings of the values. The type element gives the type of the file in the same form returned by the command file type. This command returns an empty string;

    (iv) 'file attributes name [option]' this subcommand returns a list of the platform specific flags and their values. The 'file attributes name [option value ...] sets one or more of the values. The values are as follows:

    On Unix, -group gets or sets the group name for the file. A group id can be given to the command, but it returns a group name. -owner gets or sets the user name of the owner of the file. The command returns the owner name, but the numerical id can be passed when setting the owner. -permissions sets or retrieves the octal code that chmod(1) uses. This command does not support the symbolic attributes for chmod(1) at this time.

    On Windows, -archive gives the value or sets or clears the archive attribute of the file. -hidden gives the value or sets or clears the hidden attribute of the file. -longname will expand each path element to its long version. This attribute cannot be set. -readonly gives the value or sets or clears the readonly attribute of the file. -shortname gives a string where every path element is replaced with its short (8.3) version of the name. This attribute cannot be set. -system gives or sets or clears the value of the system attribute of the file.

    On Macintosh, -creator gives or sets the Finder creator type of the file. -hidden gives or sets or clears the hidden attribute of the file. -readonly gives or sets or clears the readonly attribute of the file. Note that directories can only be locked if File Sharing is turned on. -type gives or sets the Finder file type for the file.

        set time [file atime filename]
        set len  [file size filename] 

    File Operation

    (i) 'file copy source target' copies source file to the target file or directory; (ii) 'file delete pathname ...' remove files and directories; (iii) 'file rename source target' rename source file name to the target; (iv) 'file join name [name ...]' takes one or more file names and combines them, using the correct path separator for the current platform; (v) 'file split name' returns a list whose elements are the path components in name. The first element of the list will have the same path type as name. All other elements will be relative.

      file copy src.sal dest.sal
  • Channel

    A channel maybe either a file or a pipeline processes. You may create, open, read/write, and close a channel.

    Channel Creation and Close

    (i) 'open name [access] [permission]' returns a channel id. The 'name' maybe a file name or a pipeline specification. The 'access' maybe fopen like format or POSIX format. The 'permission' is the permission access on the new file. Its default value is 0666 that permits read/write for anyone. (ii) 'close cid' will close the channel cid. (iii) 'flush cid' writes the content of buffer to the channel.

    FOPEN Access Format

    r for read; r+ for read & write; w for write if file exists truncate, otherwis create; w+ for write & read if file exists truncate, otherwise create; a for write and append; a+ for read & write and append.

    POSIX Access Format

    RDONLY for read; WRONLY for write; RDWR for read & write; APPEND for append; CREAT for create if file not exists; TRUNC for truncate.

    use with catch example:

      if [catch {open filename "w+"} fid] {
        error "open file error!"
      } else {
        # processing
        close $fid

    Channel Status

    (i) 'seek cid offset [org]' sets the current position to offset from the org. org may have the value start, current, or end. (ii) 'tell cid' returns current position of the channel (iii) 'eof cid' if it is the end of file.

      set pos [seek cid 256 start]

    Channel Access

    (i) 'read cid [nbytes]' reads nbytes or entire data from cid. It returns the reading data. (ii) 'write cid string' writes a string to the cid.

      set data [read cid 256]