Go to main content

man pages section 1: User Commands

Exit Print View

Updated: Wednesday, February 9, 2022

format (1t)


format - Format a string in the style of sprintf


format formatString ?arg arg ...?


format(1t)                   Tcl Built-In Commands                  format(1t)


       format - Format a string in the style of sprintf

       format formatString ?arg arg ...?

       This  command  generates a formatted string in a fashion similar to the
       ANSI C sprintf procedure.  FormatString indicates  how  to  format  the
       result, using % conversion specifiers as in sprintf, and the additional
       arguments, if any, provide values to be substituted  into  the  result.
       The return value from format is the formatted string.

       The command operates by scanning formatString from left to right.  Each
       character from the format string  is  appended  to  the  result  string
       unless  it  is  a percent sign.  If the character is a % then it is not
       copied to the result string.  Instead, the characters following  the  %
       character are treated as a conversion specifier.  The conversion speci-
       fier controls the conversion of the next successive arg to a particular
       format  and the result is appended to the result string in place of the
       conversion specifier.  If there are multiple conversion  specifiers  in
       the  format  string, then each one controls the conversion of one addi-
       tional arg.  The format command must be given enough args to  meet  the
       needs of all of the conversion specifiers in formatString.

       Each  conversion  specifier  may  contain up to six different parts: an
       XPG3 position specifier, a set of flags, a minimum field width, a  pre-
       cision,  a  size  modifier,  and  a conversion character.  Any of these
       fields may be omitted except for the conversion character.  The  fields
       that  are present must appear in the order given above.  The paragraphs
       below discuss each of these fields in turn.

       If the % is followed by a decimal number and a $, as  in  "%2$d",  then
       the  value  to  convert is not taken from the next sequential argument.
       Instead, it is taken from the argument indicated by the number, where 1
       corresponds  to  the  first  arg.  If the conversion specifier requires
       multiple arguments because of * characters in the specifier  then  suc-
       cessive  arguments  are  used,  starting with the argument given by the
       number.  This follows the XPG3 conventions for  positional  specifiers.
       If  there are any positional specifiers in formatString then all of the
       specifiers must be positional.

       The second portion of a conversion specifier may  contain  any  of  the
       following flag characters, in any order:

       -         Specifies  that  the converted argument should be left-justi-
                 fied in its field (numbers are normally right-justified  with
                 leading spaces if needed).

       +         Specifies that a number should always be printed with a sign,
                 even if positive.

       space     Specifies that a space should be added to  the  beginning  of
                 the number if the first character is not a sign.

       0         Specifies  that  the number should be padded on the left with
                 zeroes instead of spaces.

       #         Requests an alternate output form. For o and O conversions it
                 guarantees that the first digit is always 0.  For x or X con-
                 versions, 0x or 0X (respectively) will be added to the begin-
                 ning  of the result unless it is zero.  For b conversions, 0b
                 will be added to the beginning of the  result  unless  it  is
                 zero.  For all floating-point conversions (e, E, f, g, and G)
                 it guarantees that the result always  has  a  decimal  point.
                 For  g  and  G  conversions it specifies that trailing zeroes
                 should not be removed.

       The third portion of a conversion specifier is a decimal number  giving
       a  minimum  field  width  for this conversion.  It is typically used to
       make columns line up in tabular printouts.  If the  converted  argument
       contains  fewer characters than the minimum field width then it will be
       padded so that it is as wide as the minimum field width.  Padding  nor-
       mally  occurs by adding extra spaces on the left of the converted argu-
       ment, but the 0 and - flags may be used to specify padding with  zeroes
       on  the left or with spaces on the right, respectively.  If the minimum
       field width is specified as * rather than a number, then the next argu-
       ment  to the format command determines the minimum field width; it must
       be an integer value.

       The fourth portion of a conversion specifier is a precision, which con-
       sists  of a period followed by a number.  The number is used in differ-
       ent ways for different conversions.  For e, E,  and  f  conversions  it
       specifies  the  number  of digits to appear to the right of the decimal
       point.  For g and G conversions it specifies the total number of digits
       to appear, including those on both sides of the decimal point (however,
       trailing zeroes after the decimal point will still  be  omitted  unless
       the  # flag has been specified).  For integer conversions, it specifies
       a minimum number of digits to print (leading zeroes will  be  added  if
       necessary).  For s conversions it specifies the maximum number of char-
       acters to be printed; if the string is longer than this then the trail-
       ing  characters  will be dropped.  If the precision is specified with *
       rather than a number then the  next  argument  to  the  format  command
       determines the precision; it must be a numeric string.

       The fifth part of a conversion specifier is a size modifier, which must
       be ll, h, or l.  If it is ll it specifies  that  an  integer  value  is
       taken  without  truncation for conversion to a formatted substring.  If
       it is h it specifies that an integer value is  truncated  to  a  16-bit
       range  before converting.  This option is rarely useful.  If it is l it
       specifies that the integer value is truncated to the same range as that
       produced  by the wide() function of the expr command (at least a 64-bit
       range).  If neither h nor l are present, the integer value is truncated
       to  the  same  range as that produced by the int() function of the expr
       command (at least a 32-bit range, but determined by the  value  of  the
       wordSize element of the tcl_platform array).

       The  last  thing  in  a conversion specifier is an alphabetic character
       that determines what kind of conversion to perform.  The following con-
       version characters are currently supported:

       d         Convert integer to signed decimal string.

       u         Convert integer to unsigned decimal string.

       i         Convert integer to signed decimal string (equivalent to d).

       o         Convert integer to unsigned octal string.

       x or X    Convert  integer to unsigned hexadecimal string, using digits
                 "0123456789abcdef" for x and "0123456789ABCDEF" for X).

       b         Convert integer to binary string, using digits 0 and 1.

       c         Convert integer to the Unicode character it represents.

       s         No conversion; just insert string.

       f         Convert number to signed decimal string of the  form  xx.yyy,
                 where  the  number  of  y's  is  determined  by the precision
                 (default: 6).  If the precision is 0 then no decimal point is

       e or E    Convert number to scientific notation in the form x.yyye+-zz,
                 where the number  of  y's  is  determined  by  the  precision
                 (default: 6).  If the precision is 0 then no decimal point is
                 output.  If the E form is used then E is printed  instead  of

       g or G    If  the  exponent is less than -4 or greater than or equal to
                 the precision, then convert number as for %e or  %E.   Other-
                 wise convert as for %f.  Trailing zeroes and a trailing deci-
                 mal point are omitted.

       %         No conversion: just insert %.

       The behavior of the format command is the same as the  ANSI  C  sprintf
       procedure except for the following differences:

       [1]    Tcl guarantees that it will be working with UNICODE characters.

       [2]    %p and %n specifiers are not supported.

       [3]    For  %c conversions the argument must be an integer value, which
              will then be converted to the corresponding character value.

       [4]    The size modifiers are ignored  when  formatting  floating-point
              values.   The  ll  modifier  has  no sprintf counterpart.  The b
              specifier has no sprintf counterpart.

       Convert the numeric value of  a  UNICODE  character  to  the  character

              set value 120
              set char [format %c $value]

       Convert the output of time into seconds to an accuracy of hundredths of
       a second:

              set us [lindex [time $someTclCode] 0]
              puts [format "%.2f seconds to execute" [expr {$us / 1e6}]]

       Create a packed X11 literal color specification:

              # Each color-component should be in range (0..255)
              set color [format "#%02x%02x%02x" $r $g $b]

       Use XPG3 format codes to allow reordering of fields (a  technique  that
       is  often  used  in  localized  message  catalogs;  see msgcat) without
       reordering the data values passed to format:

              set fmt1 "Today, %d shares in %s were bought at $%.2f each"
              puts [format $fmt1 123 "Global BigCorp" 19.37]

              set fmt2 "Bought %2\$s equity ($%3$.2f x %1\$d) today"
              puts [format $fmt2 123 "Global BigCorp" 19.37]

       Print a small table of powers of three:

              # Set up the column widths
              set w1 5
              set w2 10

              # Make a nice header (with separator) for the table first
              set sep +-[string repeat - $w1]-+-[string repeat - $w2]-+
              puts $sep
              puts [format "| %-*s | %-*s |" $w1 "Index" $w2 "Power"]
              puts $sep

              # Print the contents of the table
              set p 1
              for {set i 0} {$i<=20} {incr i} {
                  puts [format "| %*d | %*ld |" $w1 $i $w2 $p]
                  set p [expr {wide($p) * 3}]

              # Finish off by printing the separator again
              puts $sep

       See attributes(7) for descriptions of the following attributes:

       |Availability   | runtime/tcl-8    |
       |Stability      | Uncommitted      |

       scan(n), sprintf(3), string(n)

       conversion specifier, format, sprintf, string, substitution

       Source code for open source software components in Oracle  Solaris  can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-

       This    software    was    built    from    source     available     at
       https://github.com/oracle/solaris-userland.    The  original  community
       source was downloaded from  http://prdownloads.sourceforge.net/tcl/tcl-

       Further information about this software can be found on the open source
       community website at https://www.tcl.tk/.

Tcl                                   8.1                           format(1t)