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GNU Info File | 1994-11-17 | 48KB | 837 lines

This is Info file gawk.info, produced by Makeinfo-1.55 from the input file /gnu/src/amiga/gawk-2.15.5/gawk.texi. This file documents `awk', a program that you can use to select particular records in a file and perform operations upon them. This is Edition 0.15 of `The GAWK Manual', for the 2.15 version of the GNU implementation of AWK. Copyright (C) 1989, 1991, 1992, 1993 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation. File: gawk.info, Node: Top, Next: Preface, Prev: (dir), Up: (dir) General Introduction ******************** This file documents `awk', a program that you can use to select particular records in a file and perform operations upon them. This is Edition 0.15 of `The GAWK Manual', for the 2.15 version of the GNU implementation of AWK. * Menu: * Preface:: What you can do with `awk'; brief history and acknowledgements. * Copying:: Your right to copy and distribute `gawk'. * This Manual:: Using this manual. Includes sample input files that you can use. * Getting Started:: A basic introduction to using `awk'. How to run an `awk' program. Command line syntax. * Reading Files:: How to read files and manipulate fields. * Printing:: How to print using `awk'. Describes the `print' and `printf' statements. Also describes redirection of output. * One-liners:: Short, sample `awk' programs. * Patterns:: The various types of patterns explained in detail. * Actions:: The various types of actions are introduced here. Describes expressions and the various operators in detail. Also describes comparison expressions. * Expressions:: Expressions are the basic building blocks of statements. * Statements:: The various control statements are described in detail. * Arrays:: The description and use of arrays. Also includes array-oriented control statements. * Built-in:: The built-in functions are summarized here. * User-defined:: User-defined functions are described in detail. * Built-in Variables:: Built-in Variables * Command Line:: How to run `gawk'. * Language History:: The evolution of the `awk' language. * Installation:: Installing `gawk' under various operating systems. * Gawk Summary:: `gawk' Options and Language Summary. * Sample Program:: A sample `awk' program with a complete explanation. * Bugs:: Reporting Problems and Bugs. * Notes:: Something about the implementation of `gawk'. * Glossary:: An explanation of some unfamiliar terms. * Index:: File: gawk.info, Node: Preface, Next: Copying, Prev: Top, Up: Top Preface ******* If you are like many computer users, you would frequently like to make changes in various text files wherever certain patterns appear, or extract data from parts of certain lines while discarding the rest. To write a program to do this in a language such as C or Pascal is a time-consuming inconvenience that may take many lines of code. The job may be easier with `awk'. The `awk' utility interprets a special-purpose programming language that makes it possible to handle simple data-reformatting jobs easily with just a few lines of code. The GNU implementation of `awk' is called `gawk'; it is fully upward compatible with the System V Release 4 version of `awk'. `gawk' is also upward compatible with the POSIX (draft) specification of the `awk' language. This means that all properly written `awk' programs should work with `gawk'. Thus, we usually don't distinguish between `gawk' and other `awk' implementations in this manual. This manual teaches you what `awk' does and how you can use `awk' effectively. You should already be familiar with basic system commands such as `ls'. Using `awk' you can: * manage small, personal databases * generate reports * validate data * produce indexes, and perform other document preparation tasks * even experiment with algorithms that can be adapted later to other computer languages * Menu: * History:: The history of `gawk' and `awk'. Acknowledgements. File: gawk.info, Node: History, Prev: Preface, Up: Preface History of `awk' and `gawk' =========================== The name `awk' comes from the initials of its designers: Alfred V. Aho, Peter J. Weinberger, and Brian W. Kernighan. The original version of `awk' was written in 1977. In 1985 a new version made the programming language more powerful, introducing user-defined functions, multiple input streams, and computed regular expressions. This new version became generally available with System V Release 3.1. The version in System V Release 4 added some new features and also cleaned up the behavior in some of the "dark corners" of the language. The specification for `awk' in the POSIX Command Language and Utilities standard further clarified the language based on feedback from both the `gawk' designers, and the original `awk' designers. The GNU implementation, `gawk', was written in 1986 by Paul Rubin and Jay Fenlason, with advice from Richard Stallman. John Woods contributed parts of the code as well. In 1988 and 1989, David Trueman, with help from Arnold Robbins, thoroughly reworked `gawk' for compatibility with the newer `awk'. Current development (1992) focuses on bug fixes, performance improvements, and standards compliance. We need to thank many people for their assistance in producing this manual. Jay Fenlason contributed many ideas and sample programs. Richard Mlynarik and Robert J. Chassell gave helpful comments on early drafts of this manual. The paper `A Supplemental Document for `awk'' by John W. Pierce of the Chemistry Department at UC San Diego, pinpointed several issues relevant both to `awk' implementation and to this manual, that would otherwise have escaped us. David Trueman, Pat Rankin, and Michal Jaegermann also contributed sections of the manual. The following people provided many helpful comments on this edition of the manual: Rick Adams, Michael Brennan, Rich Burridge, Diane Close, Christopher ("Topher") Eliot, Michael Lijewski, Pat Rankin, Miriam Robbins, and Michal Jaegermann. Robert J. Chassell provided much valuable advice on the use of Texinfo. Finally, we would like to thank Brian Kernighan of Bell Labs for invaluable assistance during the testing and debugging of `gawk', and for help in clarifying numerous points about the language. File: gawk.info, Node: Copying, Next: This Manual, Prev: Preface, Up: Top GNU GENERAL PUBLIC LICENSE ************************** Version 2, June 1991 Copyright (C) 1989, 1991 Free Software Foundation, Inc. 675 Mass Ave, Cambridge, MA 02139, USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. Preamble ======== The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too. 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It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found. ONE LINE TO GIVE THE PROGRAM'S NAME AND A BRIEF IDEA OF WHAT IT DOES. Copyright (C) 19YY NAME OF AUTHOR This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. Also add information on how to contact you by electronic and paper mail. If the program is interactive, make it output a short notice like this when it starts in an interactive mode: Gnomovision version 69, Copyright (C) 19YY NAME OF AUTHOR Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'. This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details. The hypothetical commands `show w' and `show c' should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than `show w' and `show c'; they could even be mouse-clicks or menu items--whatever suits your program. You should also get your employer (if you work as a programmer) or your school, if any, to sign a "copyright disclaimer" for the program, if necessary. Here is a sample; alter the names: Yoyodyne, Inc., hereby disclaims all copyright interest in the program `Gnomovision' (which makes passes at compilers) written by James Hacker. SIGNATURE OF TY COON, 1 April 1989 Ty Coon, President of Vice This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License. File: gawk.info, Node: This Manual, Next: Getting Started, Prev: Copying, Up: Top Using this Manual ***************** The term `awk' refers to a particular program, and to the language you use to tell this program what to do. When we need to be careful, we call the program "the `awk' utility" and the language "the `awk' language." The term `gawk' refers to a version of `awk' developed as part the GNU project. The purpose of this manual is to explain both the `awk' language and how to run the `awk' utility. While concentrating on the features of `gawk', the manual will also attempt to describe important differences between `gawk' and other `awk' implementations. In particular, any features that are not in the POSIX standard for `awk' will be noted. The term "`awk' program" refers to a program written by you in the `awk' programming language. *Note Getting Started with `awk': Getting Started, for the bare essentials you need to know to start using `awk'. Some useful "one-liners" are included to give you a feel for the `awk' language (*note Useful "One-liners": One-liners.). A sample `awk' program has been provided for you (*note Sample Program::.). If you find terms that you aren't familiar with, try looking them up in the glossary (*note Glossary::.). The entire `awk' language is summarized for quick reference in *Note `gawk' Summary: Gawk Summary. Look there if you just need to refresh your memory about a particular feature. Most of the time complete `awk' programs are used as examples, but in some of the more advanced sections, only the part of the `awk' program that illustrates the concept being described is shown. * Menu: * Sample Data Files:: Sample data files for use in the `awk' programs illustrated in this manual. File: gawk.info, Node: Sample Data Files, Prev: This Manual, Up: This Manual Data Files for the Examples =========================== Many of the examples in this manual take their input from two sample data files. The first, called `BBS-list', represents a list of computer bulletin board systems together with information about those systems. The second data file, called `inventory-shipped', contains information about shipments on a monthly basis. Each line of these files is one "record". In the file `BBS-list', each record contains the name of a computer bulletin board, its phone number, the board's baud rate, and a code for the number of hours it is operational. An `A' in the last column means the board operates 24 hours a day. A `B' in the last column means the board operates evening and weekend hours, only. A `C' means the board operates only on weekends. aardvark 555-5553 1200/300 B alpo-net 555-3412 2400/1200/300 A barfly 555-7685 1200/300 A bites 555-1675 2400/1200/300 A camelot 555-0542 300 C core 555-2912 1200/300 C fooey 555-1234 2400/1200/300 B foot 555-6699 1200/300 B macfoo 555-6480 1200/300 A sdace 555-3430 2400/1200/300 A sabafoo 555-2127 1200/300 C The second data file, called `inventory-shipped', represents information about shipments during the year. Each record contains the month of the year, the number of green crates shipped, the number of red boxes shipped, the number of orange bags shipped, and the number of blue packages shipped, respectively. There are 16 entries, covering the 12 months of one year and 4 months of the next year. Jan 13 25 15 115 Feb 15 32 24 226 Mar 15 24 34 228 Apr 31 52 63 420 May 16 34 29 208 Jun 31 42 75 492 Jul 24 34 67 436 Aug 15 34 47 316 Sep 13 55 37 277 Oct 29 54 68 525 Nov 20 87 82 577 Dec 17 35 61 401 Jan 21 36 64 620 Feb 26 58 80 652 Mar 24 75 70 495 Apr 21 70 74 514 If you are reading this in GNU Emacs using Info, you can copy the regions of text showing these sample files into your own test files. This way you can try out the examples shown in the remainder of this document. You do this by using the command `M-x write-region' to copy text from the Info file into a file for use with `awk' (*Note Misc File Ops: (emacs)Misc File Ops, for more information). Using this information, create your own `BBS-list' and `inventory-shipped' files, and practice what you learn in this manual. File: gawk.info, Node: Getting Started, Next: Reading Files, Prev: This Manual, Up: Top Getting Started with `awk' ************************** The basic function of `awk' is to search files for lines (or other units of text) that contain certain patterns. When a line matches one of the patterns, `awk' performs specified actions on that line. `awk' keeps processing input lines in this way until the end of the input file is reached. When you run `awk', you specify an `awk' "program" which tells `awk' what to do. The program consists of a series of "rules". (It may also contain "function definitions", but that is an advanced feature, so we will ignore it for now. *Note User-defined Functions: User-defined.) Each rule specifies one pattern to search for, and one action to perform when that pattern is found. Syntactically, a rule consists of a pattern followed by an action. The action is enclosed in curly braces to separate it from the pattern. Rules are usually separated by newlines. Therefore, an `awk' program looks like this: PATTERN { ACTION } PATTERN { ACTION } ... * Menu: * Very Simple:: A very simple example. * Two Rules:: A less simple one-line example with two rules. * More Complex:: A more complex example. * Running gawk:: How to run `gawk' programs; includes command line syntax. * Comments:: Adding documentation to `gawk' programs. * Statements/Lines:: Subdividing or combining statements into lines. * When:: When to use `gawk' and when to use other things. File: gawk.info, Node: Very Simple, Next: Two Rules, Prev: Getting Started, Up: Getting Started A Very Simple Example ===================== The following command runs a simple `awk' program that searches the input file `BBS-list' for the string of characters: `foo'. (A string of characters is usually called, a "string". The term "string" is perhaps based on similar usage in English, such as "a string of pearls," or, "a string of cars in a train.") awk '/foo/ { print $0 }' BBS-list When lines containing `foo' are found, they are printed, because `print $0' means print the current line. (Just `print' by itself means the same thing, so we could have written that instead.) You will notice that slashes, `/', surround the string `foo' in the actual `awk' program. The slashes indicate that `foo' is a pattern to search for. This type of pattern is called a "regular expression", and is covered in more detail later (*note Regular Expressions as Patterns: Regexp.). There are single-quotes around the `awk' program so that the shell won't interpret any of it as special shell characters. Here is what this program prints: fooey 555-1234 2400/1200/300 B foot 555-6699 1200/300 B macfoo 555-6480 1200/300 A sabafoo 555-2127 1200/300 C In an `awk' rule, either the pattern or the action can be omitted, but not both. If the pattern is omitted, then the action is performed for *every* input line. If the action is omitted, the default action is to print all lines that match the pattern. Thus, we could leave out the action (the `print' statement and the curly braces) in the above example, and the result would be the same: all lines matching the pattern `foo' would be printed. By comparison, omitting the `print' statement but retaining the curly braces makes an empty action that does nothing; then no lines would be printed. File: gawk.info, Node: Two Rules, Next: More Complex, Prev: Very Simple, Up: Getting Started An Example with Two Rules ========================= The `awk' utility reads the input files one line at a time. For each line, `awk' tries the patterns of each of the rules. If several patterns match then several actions are run, in the order in which they appear in the `awk' program. If no patterns match, then no actions are After processing all the rules (perhaps none) that match the line, `awk' reads the next line (however, *note The `next' Statement: Next Statement.). This continues until the end of the file is reached. For example, the `awk' program: /12/ { print $0 } /21/ { print $0 } contains two rules. The first rule has the string `12' as the pattern and `print $0' as the action. The second rule has the string `21' as the pattern and also has `print $0' as the action. Each rule's action is enclosed in its own pair of braces. This `awk' program prints every line that contains the string `12' *or* the string `21'. If a line contains both strings, it is printed twice, once by each rule. If we run this program on our two sample data files, `BBS-list' and `inventory-shipped', as shown here: awk '/12/ { print $0 } /21/ { print $0 }' BBS-list inventory-shipped we get the following output: aardvark 555-5553 1200/300 B alpo-net 555-3412 2400/1200/300 A barfly 555-7685 1200/300 A bites 555-1675 2400/1200/300 A core 555-2912 1200/300 C fooey 555-1234 2400/1200/300 B foot 555-6699 1200/300 B macfoo 555-6480 1200/300 A sdace 555-3430 2400/1200/300 A sabafoo 555-2127 1200/300 C sabafoo 555-2127 1200/300 C Jan 21 36 64 620 Apr 21 70 74 514 Note how the line in `BBS-list' beginning with `sabafoo' was printed twice, once for each rule. File: gawk.info, Node: More Complex, Next: Running gawk, Prev: Two Rules, Up: Getting Started A More Complex Example ====================== Here is an example to give you an idea of what typical `awk' programs do. This example shows how `awk' can be used to summarize, select, and rearrange the output of another utility. It uses features that haven't been covered yet, so don't worry if you don't understand all the details. ls -l | awk '$5 == "Nov" { sum += $4 } END { print sum }' This command prints the total number of bytes in all the files in the current directory that were last modified in November (of any year). (In the C shell you would need to type a semicolon and then a backslash at the end of the first line; in a POSIX-compliant shell, such as the Bourne shell or the Bourne-Again shell, you can type the example as shown.) The `ls -l' part of this example is a command that gives you a listing of the files in a directory, including file size and date. Its output looks like this: -rw-r--r-- 1 close 1933 Nov 7 13:05 Makefile -rw-r--r-- 1 close 10809 Nov 7 13:03 gawk.h -rw-r--r-- 1 close 983 Apr 13 12:14 gawk.tab.h -rw-r--r-- 1 close 31869 Jun 15 12:20 gawk.y -rw-r--r-- 1 close 22414 Nov 7 13:03 gawk1.c -rw-r--r-- 1 close 37455 Nov 7 13:03 gawk2.c -rw-r--r-- 1 close 27511 Dec 9 13:07 gawk3.c -rw-r--r-- 1 close 7989 Nov 7 13:03 gawk4.c The first field contains read-write permissions, the second field contains the number of links to the file, and the third field identifies the owner of the file. The fourth field contains the size of the file in bytes. The fifth, sixth, and seventh fields contain the month, day, and time, respectively, that the file was last modified. Finally, the eighth field contains the name of the file. The `$5 == "Nov"' in our `awk' program is an expression that tests whether the fifth field of the output from `ls -l' matches the string `Nov'. Each time a line has the string `Nov' in its fifth field, the action `{ sum += $4 }' is performed. This adds the fourth field (the file size) to the variable `sum'. As a result, when `awk' has finished reading all the input lines, `sum' is the sum of the sizes of files whose lines matched the pattern. (This works because `awk' variables are automatically initialized to zero.) After the last line of output from `ls' has been processed, the `END' rule is executed, and the value of `sum' is printed. In this example, the value of `sum' would be 80600. These more advanced `awk' techniques are covered in later sections (*note Overview of Actions: Actions.). Before you can move on to more advanced `awk' programming, you have to know how `awk' interprets your input and displays your output. By manipulating fields and using `print' statements, you can produce some very useful and spectacular looking reports. File: gawk.info, Node: Running gawk, Next: Comments, Prev: More Complex, Up: Getting Started How to Run `awk' Programs ========================= There are several ways to run an `awk' program. If the program is short, it is easiest to include it in the command that runs `awk', like this: awk 'PROGRAM' INPUT-FILE1 INPUT-FILE2 ... where PROGRAM consists of a series of patterns and actions, as described earlier. When the program is long, it is usually more convenient to put it in a file and run it with a command like this: awk -f PROGRAM-FILE INPUT-FILE1 INPUT-FILE2 ... * Menu: * One-shot:: Running a short throw-away `awk' program. * Read Terminal:: Using no input files (input from terminal instead). * Long:: Putting permanent `awk' programs in files. * Executable Scripts:: Making self-contained `awk' programs. File: gawk.info, Node: One-shot, Next: Read Terminal, Prev: Running gawk, Up: Running gawk One-shot Throw-away `awk' Programs ---------------------------------- Once you are familiar with `awk', you will often type simple programs at the moment you want to use them. Then you can write the program as the first argument of the `awk' command, like this: awk 'PROGRAM' INPUT-FILE1 INPUT-FILE2 ... where PROGRAM consists of a series of PATTERNS and ACTIONS, as described earlier. This command format instructs the shell to start `awk' and use the PROGRAM to process records in the input file(s). There are single quotes around PROGRAM so that the shell doesn't interpret any `awk' characters as special shell characters. They also cause the shell to treat all of PROGRAM as a single argument for `awk' and allow PROGRAM to be more than one line long. This format is also useful for running short or medium-sized `awk' programs from shell scripts, because it avoids the need for a separate file for the `awk' program. A self-contained shell script is more reliable since there are no other files to misplace. File: gawk.info, Node: Read Terminal, Next: Long, Prev: One-shot, Up: Running gawk Running `awk' without Input Files --------------------------------- You can also run `awk' without any input files. If you type the command line: awk 'PROGRAM' then `awk' applies the PROGRAM to the "standard input", which usually means whatever you type on the terminal. This continues until you indicate end-of-file by typing `Control-d'. For example, if you execute this command: awk '/th/' whatever you type next is taken as data for that `awk' program. If you go on to type the following data: Kathy Ben Tom Beth Seth Karen Thomas `Control-d' then `awk' prints this output: Kathy Beth Seth as matching the pattern `th'. Notice that it did not recognize `Thomas' as matching the pattern. The `awk' language is "case sensitive", and matches patterns exactly. (However, you can override this with the variable `IGNORECASE'. *Note Case-sensitivity in Matching: Case-sensitivity.) File: gawk.info, Node: Long, Next: Executable Scripts, Prev: Read Terminal, Up: Running gawk Running Long Programs --------------------- Sometimes your `awk' programs can be very long. In this case it is more convenient to put the program into a separate file. To tell `awk' to use that file for its program, you type: awk -f SOURCE-FILE INPUT-FILE1 INPUT-FILE2 ... The `-f' instructs the `awk' utility to get the `awk' program from the file SOURCE-FILE. Any file name can be used for SOURCE-FILE. For example, you could put the program: /th/ into the file `th-prog'. Then this command: awk -f th-prog does the same thing as this one: awk '/th/' which was explained earlier (*note Running `awk' without Input Files: Read Terminal.). Note that you don't usually need single quotes around the file name that you specify with `-f', because most file names don't contain any of the shell's special characters. Notice that in `th-prog', the `awk' program did not have single quotes around it. The quotes are only needed for programs that are provided on the `awk' command line. If you want to identify your `awk' program files clearly as such, you can add the extension `.awk' to the file name. This doesn't affect the execution of the `awk' program, but it does make "housekeeping" easier. File: gawk.info, Node: Executable Scripts, Prev: Long, Up: Running gawk Executable `awk' Programs ------------------------- Once you have learned `awk', you may want to write self-contained `awk' scripts, using the `#!' script mechanism. You can do this on many Unix systems (1) (and someday on GNU). For example, you could create a text file named `hello', containing the following (where `BEGIN' is a feature we have not yet discussed): #! /bin/awk -f # a sample awk program BEGIN { print "hello, world" } After making this file executable (with the `chmod' command), you can simply type: hello at the shell, and the system will arrange to run `awk' (2) as if you had typed: awk -f hello Self-contained `awk' scripts are useful when you want to write a program which users can invoke without knowing that the program is written in `awk'. If your system does not support the `#!' mechanism, you can get a similar effect using a regular shell script. It would look something like this: : The colon makes sure this script is executed by the Bourne shell. awk 'PROGRAM' "$@" Using this technique, it is *vital* to enclose the PROGRAM in single quotes to protect it from interpretation by the shell. If you omit the quotes, only a shell wizard can predict the results. The `"$@"' causes the shell to forward all the command line arguments to the `awk' program, without interpretation. The first line, which starts with a colon, is used so that this shell script will work even if invoked by a user who uses the C shell. ---------- Footnotes ---------- (1) The `#!' mechanism works on Unix systems derived from Berkeley Unix, System V Release 4, and some System V Release 3 systems. (2) The line beginning with `#!' lists the full pathname of an interpreter to be run, and an optional initial command line argument to pass to that interpreter. The operating system then runs the interpreter with the given argument and the full argument list of the executed program. The first argument in the list is the full pathname of the `awk' program. The rest of the argument list will either be options to `awk', or data files, or both. File: gawk.info, Node: Comments, Next: Statements/Lines, Prev: Running gawk, Up: Getting Started Comments in `awk' Programs ========================== A "comment" is some text that is included in a program for the sake of human readers, and that is not really part of the program. Comments can explain what the program does, and how it works. Nearly all programming languages have provisions for comments, because programs are typically hard to understand without their extra help. In the `awk' language, a comment starts with the sharp sign character, `#', and continues to the end of the line. The `awk' language ignores the rest of a line following a sharp sign. For example, we could have put the following into `th-prog': # This program finds records containing the pattern `th'. This is how # you continue comments on additional lines. /th/ You can put comment lines into keyboard-composed throw-away `awk' programs also, but this usually isn't very useful; the purpose of a comment is to help you or another person understand the program at a later time.