This is Info file gawk.info, produced by Makeinfo-1.55 from the input file /gnu-src/gawk-2.15.6/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: Statements/Lines, Next: When, Prev: Comments, Up: Getting Started `awk' Statements versus Lines ============================= Most often, each line in an `awk' program is a separate statement or separate rule, like this: awk '/12/ { print $0 } /21/ { print $0 }' BBS-list inventory-shipped But sometimes statements can be more than one line, and lines can contain several statements. You can split a statement into multiple lines by inserting a newline after any of the following: , { ? : || && do else A newline at any other point is considered the end of the statement. (Splitting lines after `?' and `:' is a minor `gawk' extension. The `?' and `:' referred to here is the three operand conditional expression described in *Note Conditional Expressions: Conditional Exp.) If you would like to split a single statement into two lines at a point where a newline would terminate it, you can "continue" it by ending the first line with a backslash character, `\'. This is allowed absolutely anywhere in the statement, even in the middle of a string or regular expression. For example: awk '/This program is too long, so continue it\ on the next line/ { print $1 }' We have generally not used backslash continuation in the sample programs in this manual. Since in `gawk' there is no limit on the length of a line, it is never strictly necessary; it just makes programs prettier. We have preferred to make them even more pretty by keeping the statements short. Backslash continuation is most useful when your `awk' program is in a separate source file, instead of typed in on the command line. You should also note that many `awk' implementations are more picky about where you may use backslash continuation. For maximal portability of your `awk' programs, it is best not to split your lines in the middle of a regular expression or a string. *Warning: backslash continuation does not work as described above with the C shell.* Continuation with backslash works for `awk' programs in files, and also for one-shot programs *provided* you are using a POSIX-compliant shell, such as the Bourne shell or the Bourne-again shell. But the C shell used on Berkeley Unix behaves differently! There, you must use two backslashes in a row, followed by a newline. When `awk' statements within one rule are short, you might want to put more than one of them on a line. You do this by separating the statements with a semicolon, `;'. This also applies to the rules themselves. Thus, the previous program could have been written: /12/ { print $0 } ; /21/ { print $0 } *Note:* the requirement that rules on the same line must be separated with a semicolon is a recent change in the `awk' language; it was done for consistency with the treatment of statements within an action. File: gawk.info, Node: When, Prev: Statements/Lines, Up: Getting Started When to Use `awk' ================= You might wonder how `awk' might be useful for you. Using additional utility programs, more advanced patterns, field separators, arithmetic statements, and other selection criteria, you can produce much more complex output. The `awk' language is very useful for producing reports from large amounts of raw data, such as summarizing information from the output of other utility programs like `ls'. (*Note A More Complex Example: More Complex.) Programs written with `awk' are usually much smaller than they would be in other languages. This makes `awk' programs easy to compose and use. Often `awk' programs can be quickly composed at your terminal, used once, and thrown away. Since `awk' programs are interpreted, you can avoid the usually lengthy edit-compile-test-debug cycle of software development. Complex programs have been written in `awk', including a complete retargetable assembler for 8-bit microprocessors (*note Glossary::., for more information) and a microcode assembler for a special purpose Prolog computer. However, `awk''s capabilities are strained by tasks of such complexity. If you find yourself writing `awk' scripts of more than, say, a few hundred lines, you might consider using a different programming language. Emacs Lisp is a good choice if you need sophisticated string or pattern matching capabilities. The shell is also good at string and pattern matching; in addition, it allows powerful use of the system utilities. More conventional languages, such as C, C++, and Lisp, offer better facilities for system programming and for managing the complexity of large programs. Programs in these languages may require more lines of source code than the equivalent `awk' programs, but they are easier to maintain and usually run more efficiently. File: gawk.info, Node: Reading Files, Next: Printing, Prev: Getting Started, Up: Top Reading Input Files ******************* In the typical `awk' program, all input is read either from the standard input (by default the keyboard, but often a pipe from another command) or from files whose names you specify on the `awk' command line. If you specify input files, `awk' reads them in order, reading all the data from one before going on to the next. The name of the current input file can be found in the built-in variable `FILENAME' (*note Built-in Variables::.). The input is read in units called records, and processed by the rules one record at a time. By default, each record is one line. Each record is split automatically into fields, to make it more convenient for a rule to work on its parts. On rare occasions you will need to use the `getline' command, which can do explicit input from any number of files (*note Explicit Input with `getline': Getline.). * Menu: * Records:: Controlling how data is split into records. * Fields:: An introduction to fields. * Non-Constant Fields:: Non-constant Field Numbers. * Changing Fields:: Changing the Contents of a Field. * Field Separators:: The field separator and how to change it. * Constant Size:: Reading constant width data. * Multiple Line:: Reading multi-line records. * Getline:: Reading files under explicit program control using the `getline' function. * Close Input:: Closing an input file (so you can read from the beginning once more). File: gawk.info, Node: Records, Next: Fields, Prev: Reading Files, Up: Reading Files How Input is Split into Records =============================== The `awk' language divides its input into records and fields. Records are separated by a character called the "record separator". By default, the record separator is the newline character, defining a record to be a single line of text. Sometimes you may want to use a different character to separate your records. You can use a different character by changing the built-in variable `RS'. The value of `RS' is a string that says how to separate records; the default value is `"\n"', the string containing just a newline character. This is why records are, by default, single lines. `RS' can have any string as its value, but only the first character of the string is used as the record separator. The other characters are ignored. `RS' is exceptional in this regard; `awk' uses the full value of all its other built-in variables. You can change the value of `RS' in the `awk' program with the assignment operator, `=' (*note Assignment Expressions: Assignment Ops.). The new record-separator character should be enclosed in quotation marks to make a string constant. Often the right time to do this is at the beginning of execution, before any input has been processed, so that the very first record will be read with the proper separator. To do this, use the special `BEGIN' pattern (*note `BEGIN' and `END' Special Patterns: BEGIN/END.). For example: awk 'BEGIN { RS = "/" } ; { print $0 }' BBS-list changes the value of `RS' to `"/"', before reading any input. This is a string whose first character is a slash; as a result, records are separated by slashes. Then the input file is read, and the second rule in the `awk' program (the action with no pattern) prints each record. Since each `print' statement adds a newline at the end of its output, the effect of this `awk' program is to copy the input with each slash changed to a newline. Another way to change the record separator is on the command line, using the variable-assignment feature (*note Invoking `awk': Command Line.). awk '{ print $0 }' RS="/" BBS-list This sets `RS' to `/' before processing `BBS-list'. Reaching the end of an input file terminates the current input record, even if the last character in the file is not the character in `RS'. The empty string, `""' (a string of no characters), has a special meaning as the value of `RS': it means that records are separated only by blank lines. *Note Multiple-Line Records: Multiple Line, for more details. The `awk' utility keeps track of the number of records that have been read so far from the current input file. This value is stored in a built-in variable called `FNR'. It is reset to zero when a new file is started. Another built-in variable, `NR', is the total number of input records read so far from all files. It starts at zero but is never automatically reset to zero. If you change the value of `RS' in the middle of an `awk' run, the new value is used to delimit subsequent records, but the record currently being processed (and records already processed) are not affected. File: gawk.info, Node: Fields, Next: Non-Constant Fields, Prev: Records, Up: Reading Files Examining Fields ================ When `awk' reads an input record, the record is automatically separated or "parsed" by the interpreter into chunks called "fields". By default, fields are separated by whitespace, like words in a line. Whitespace in `awk' means any string of one or more spaces and/or tabs; other characters such as newline, formfeed, and so on, that are considered whitespace by other languages are *not* considered whitespace by `awk'. The purpose of fields is to make it more convenient for you to refer to these pieces of the record. You don't have to use them--you can operate on the whole record if you wish--but fields are what make simple `awk' programs so powerful. To refer to a field in an `awk' program, you use a dollar-sign, `$', followed by the number of the field you want. Thus, `$1' refers to the first field, `$2' to the second, and so on. For example, suppose the following is a line of input: This seems like a pretty nice example. Here the first field, or `$1', is `This'; the second field, or `$2', is `seems'; and so on. Note that the last field, `$7', is `example.'. Because there is no space between the `e' and the `.', the period is considered part of the seventh field. No matter how many fields there are, the last field in a record can be represented by `$NF'. So, in the example above, `$NF' would be the same as `$7', which is `example.'. Why this works is explained below (*note Non-constant Field Numbers: Non-Constant Fields.). If you try to refer to a field beyond the last one, such as `$8' when the record has only 7 fields, you get the empty string. Plain `NF', with no `$', is a built-in variable whose value is the number of fields in the current record. `$0', which looks like an attempt to refer to the zeroth field, is a special case: it represents the whole input record. This is what you would use if you weren't interested in fields. Here are some more examples: awk '$1 ~ /foo/ { print $0 }' BBS-list This example prints each record in the file `BBS-list' whose first field contains the string `foo'. The operator `~' is called a "matching operator" (*note Comparison Expressions: Comparison Ops.); it tests whether a string (here, the field `$1') matches a given regular expression. By contrast, the following example: awk '/foo/ { print $1, $NF }' BBS-list looks for `foo' in *the entire record* and prints the first field and the last field for each input record containing a match. File: gawk.info, Node: Non-Constant Fields, Next: Changing Fields, Prev: Fields, Up: Reading Files Non-constant Field Numbers ========================== The number of a field does not need to be a constant. Any expression in the `awk' language can be used after a `$' to refer to a field. The value of the expression specifies the field number. If the value is a string, rather than a number, it is converted to a number. Consider this example: awk '{ print $NR }' Recall that `NR' is the number of records read so far: 1 in the first record, 2 in the second, etc. So this example prints the first field of the first record, the second field of the second record, and so on. For the twentieth record, field number 20 is printed; most likely, the record has fewer than 20 fields, so this prints a blank line. Here is another example of using expressions as field numbers: awk '{ print $(2*2) }' BBS-list The `awk' language must evaluate the expression `(2*2)' and use its value as the number of the field to print. The `*' sign represents multiplication, so the expression `2*2' evaluates to 4. The parentheses are used so that the multiplication is done before the `$' operation; they are necessary whenever there is a binary operator in the field-number expression. This example, then, prints the hours of operation (the fourth field) for every line of the file `BBS-list'. If the field number you compute is zero, you get the entire record. Thus, `$(2-2)' has the same value as `$0'. Negative field numbers are not allowed. The number of fields in the current record is stored in the built-in variable `NF' (*note Built-in Variables::.). The expression `$NF' is not a special feature: it is the direct consequence of evaluating `NF' and using its value as a field number. File: gawk.info, Node: Changing Fields, Next: Field Separators, Prev: Non-Constant Fields, Up: Reading Files Changing the Contents of a Field ================================ You can change the contents of a field as seen by `awk' within an `awk' program; this changes what `awk' perceives as the current input record. (The actual input is untouched: `awk' never modifies the input file.) Consider this example: awk '{ $3 = $2 - 10; print $2, $3 }' inventory-shipped The `-' sign represents subtraction, so this program reassigns field three, `$3', to be the value of field two minus ten, `$2 - 10'. (*Note Arithmetic Operators: Arithmetic Ops.) Then field two, and the new value for field three, are printed. In order for this to work, the text in field `$2' must make sense as a number; the string of characters must be converted to a number in order for the computer to do arithmetic on it. The number resulting from the subtraction is converted back to a string of characters which then becomes field three. *Note Conversion of Strings and Numbers: Conversion. When you change the value of a field (as perceived by `awk'), the text of the input record is recalculated to contain the new field where the old one was. Therefore, `$0' changes to reflect the altered field. Thus, awk '{ $2 = $2 - 10; print $0 }' inventory-shipped prints a copy of the input file, with 10 subtracted from the second field of each line. You can also assign contents to fields that are out of range. For example: awk '{ $6 = ($5 + $4 + $3 + $2) ; print $6 }' inventory-shipped We've just created `$6', whose value is the sum of fields `$2', `$3', `$4', and `$5'. The `+' sign represents addition. For the file `inventory-shipped', `$6' represents the total number of parcels shipped for a particular month. Creating a new field changes the internal `awk' copy of the current input record--the value of `$0'. Thus, if you do `print $0' after adding a field, the record printed includes the new field, with the appropriate number of field separators between it and the previously existing fields. This recomputation affects and is affected by several features not yet discussed, in particular, the "output field separator", `OFS', which is used to separate the fields (*note Output Separators::.), and `NF' (the number of fields; *note Examining Fields: Fields.). For example, the value of `NF' is set to the number of the highest field you create. Note, however, that merely *referencing* an out-of-range field does *not* change the value of either `$0' or `NF'. Referencing an out-of-range field merely produces a null string. For example: if ($(NF+1) != "") print "can't happen" else print "everything is normal" should print `everything is normal', because `NF+1' is certain to be out of range. (*Note The `if' Statement: If Statement, for more information about `awk''s `if-else' statements.) It is important to note that assigning to a field will change the value of `$0', but will not change the value of `NF', even when you assign the null string to a field. For example: echo a b c d | awk '{ OFS = ":"; $2 = "" ; print ; print NF }' prints a::c:d 4 The field is still there, it just has an empty value. You can tell because there are two colons in a row. File: gawk.info, Node: Field Separators, Next: Constant Size, Prev: Changing Fields, Up: Reading Files Specifying how Fields are Separated =================================== (This section is rather long; it describes one of the most fundamental operations in `awk'. If you are a novice with `awk', we recommend that you re-read this section after you have studied the section on regular expressions, *Note Regular Expressions as Patterns: Regexp.) The way `awk' splits an input record into fields is controlled by the "field separator", which is a single character or a regular expression. `awk' scans the input record for matches for the separator; the fields themselves are the text between the matches. For example, if the field separator is `oo', then the following line: moo goo gai pan would be split into three fields: `m', ` g' and ` gai pan'. The field separator is represented by the built-in variable `FS'. Shell programmers take note! `awk' does not use the name `IFS' which is used by the shell. You can change the value of `FS' in the `awk' program with the assignment operator, `=' (*note Assignment Expressions: Assignment Ops.). Often the right time to do this is at the beginning of execution, before any input has been processed, so that the very first record will be read with the proper separator. To do this, use the special `BEGIN' pattern (*note `BEGIN' and `END' Special Patterns: BEGIN/END.). For example, here we set the value of `FS' to the string `","': awk 'BEGIN { FS = "," } ; { print $2 }' Given the input line, John Q. Smith, 29 Oak St., Walamazoo, MI 42139 this `awk' program extracts the string ` 29 Oak St.'. Sometimes your input data will contain separator characters that don't separate fields the way you thought they would. For instance, the person's name in the example we've been using might have a title or suffix attached, such as `John Q. Smith, LXIX'. From input containing such a name: John Q. Smith, LXIX, 29 Oak St., Walamazoo, MI 42139 the previous sample program would extract ` LXIX', instead of ` 29 Oak St.'. If you were expecting the program to print the address, you would be surprised. So choose your data layout and separator characters carefully to prevent such problems. As you know, by default, fields are separated by whitespace sequences (spaces and tabs), not by single spaces: two spaces in a row do not delimit an empty field. The default value of the field separator is a string `" "' containing a single space. If this value were interpreted in the usual way, each space character would separate fields, so two spaces in a row would make an empty field between them. The reason this does not happen is that a single space as the value of `FS' is a special case: it is taken to specify the default manner of delimiting fields. If `FS' is any other single character, such as `","', then each occurrence of that character separates two fields. Two consecutive occurrences delimit an empty field. If the character occurs at the beginning or the end of the line, that too delimits an empty field. The space character is the only single character which does not follow these rules. More generally, the value of `FS' may be a string containing any regular expression. Then each match in the record for the regular expression separates fields. For example, the assignment: FS = ", \t" makes every area of an input line that consists of a comma followed by a space and a tab, into a field separator. (`\t' stands for a tab.) For a less trivial example of a regular expression, suppose you want single spaces to separate fields the way single commas were used above. You can set `FS' to `"[ ]"'. This regular expression matches a single space and nothing else. `FS' can be set on the command line. You use the `-F' argument to do so. For example: awk -F, 'PROGRAM' INPUT-FILES sets `FS' to be the `,' character. Notice that the argument uses a capital `F'. Contrast this with `-f', which specifies a file containing an `awk' program. Case is significant in command options: the `-F' and `-f' options have nothing to do with each other. You can use both options at the same time to set the `FS' argument *and* get an `awk' program from a file. The value used for the argument to `-F' is processed in exactly the same way as assignments to the built-in variable `FS'. This means that if the field separator contains special characters, they must be escaped appropriately. For example, to use a `\' as the field separator, you would have to type: # same as FS = "\\" awk -F\\\\ '...' files ... Since `\' is used for quoting in the shell, `awk' will see `-F\\'. Then `awk' processes the `\\' for escape characters (*note Constant Expressions: Constants.), finally yielding a single `\' to be used for the field separator. As a special case, in compatibility mode (*note Invoking `awk': Command Line.), if the argument to `-F' is `t', then `FS' is set to the tab character. (This is because if you type `-F\t', without the quotes, at the shell, the `\' gets deleted, so `awk' figures that you really want your fields to be separated with tabs, and not `t's. Use `-v FS="t"' on the command line if you really do want to separate your fields with `t's.) For example, let's use an `awk' program file called `baud.awk' that contains the pattern `/300/', and the action `print $1'. Here is the program: /300/ { print $1 } Let's also set `FS' to be the `-' character, and run the program on the file `BBS-list'. The following command prints a list of the names of the bulletin boards that operate at 300 baud and the first three digits of their phone numbers: awk -F- -f baud.awk BBS-list It produces this output: aardvark 555 alpo barfly 555 bites 555 camelot 555 core 555 fooey 555 foot 555 macfoo 555 sdace 555 sabafoo 555 Note the second line of output. If you check the original file, you will see that the second line looked like this: alpo-net 555-3412 2400/1200/300 A The `-' as part of the system's name was used as the field separator, instead of the `-' in the phone number that was originally intended. This demonstrates why you have to be careful in choosing your field and record separators. The following program searches the system password file, and prints the entries for users who have no password: awk -F: '$2 == ""' /etc/passwd Here we use the `-F' option on the command line to set the field separator. Note that fields in `/etc/passwd' are separated by colons. The second field represents a user's encrypted password, but if the field is empty, that user has no password. According to the POSIX standard, `awk' is supposed to behave as if each record is split into fields at the time that it is read. In particular, this means that you can change the value of `FS' after a record is read, but before any of the fields are referenced. The value of the fields (i.e. how they were split) should reflect the old value of `FS', not the new one. However, many implementations of `awk' do not do this. Instead, they defer splitting the fields until a field reference actually happens, using the *current* value of `FS'! This behavior can be difficult to diagnose. The following example illustrates the results of the two methods. (The `sed' command prints just the first line of `/etc/passwd'.) sed 1q /etc/passwd | awk '{ FS = ":" ; print $1 }' will usually print root on an incorrect implementation of `awk', while `gawk' will print something like root:nSijPlPhZZwgE:0:0:Root:/: There is an important difference between the two cases of `FS = " "' (a single blank) and `FS = "[ \t]+"' (which is a regular expression matching one or more blanks or tabs). For both values of `FS', fields are separated by runs of blanks and/or tabs. However, when the value of `FS' is `" "', `awk' will strip leading and trailing whitespace from the record, and then decide where the fields are. For example, the following expression prints `b': echo ' a b c d ' | awk '{ print $2 }' However, the following prints `a': echo ' a b c d ' | awk 'BEGIN { FS = "[ \t]+" } ; { print $2 }' In this case, the first field is null. The stripping of leading and trailing whitespace also comes into play whenever `$0' is recomputed. For instance, this pipeline echo ' a b c d' | awk '{ print; $2 = $2; print }' produces this output: a b c d a b c d The first `print' statement prints the record as it was read, with leading whitespace intact. The assignment to `$2' rebuilds `$0' by concatenating `$1' through `$NF' together, separated by the value of `OFS'. Since the leading whitespace was ignored when finding `$1', it is not part of the new `$0'. Finally, the last `print' statement prints the new `$0'. The following table summarizes how fields are split, based on the value of `FS'. `FS == " "' Fields are separated by runs of whitespace. Leading and trailing whitespace are ignored. This is the default. `FS == ANY SINGLE CHARACTER' Fields are separated by each occurrence of the character. Multiple successive occurrences delimit empty fields, as do leading and trailing occurrences. `FS == REGEXP' Fields are separated by occurrences of characters that match REGEXP. Leading and trailing matches of REGEXP delimit empty fields. File: gawk.info, Node: Constant Size, Next: Multiple Line, Prev: Field Separators, Up: Reading Files Reading Fixed-width Data ======================== (This section discusses an advanced, experimental feature. If you are a novice `awk' user, you may wish to skip it on the first reading.) `gawk' 2.13 introduced a new facility for dealing with fixed-width fields with no distinctive field separator. Data of this nature arises typically in one of at least two ways: the input for old FORTRAN programs where numbers are run together, and the output of programs that did not anticipate the use of their output as input for other programs. An example of the latter is a table where all the columns are lined up by the use of a variable number of spaces and *empty fields are just spaces*. Clearly, `awk''s normal field splitting based on `FS' will not work well in this case. (Although a portable `awk' program can use a series of `substr' calls on `$0', this is awkward and inefficient for a large number of fields.) The splitting of an input record into fixed-width fields is specified by assigning a string containing space-separated numbers to the built-in variable `FIELDWIDTHS'. Each number specifies the width of the field *including* columns between fields. If you want to ignore the columns between fields, you can specify the width as a separate field that is subsequently ignored. The following data is the output of the `w' utility. It is useful to illustrate the use of `FIELDWIDTHS'. 10:06pm up 21 days, 14:04, 23 users User tty login idle JCPU PCPU what hzuo ttyV0 8:58pm 9 5 vi p24.tex hzang ttyV3 6:37pm 50 -csh eklye ttyV5 9:53pm 7 1 em thes.tex dportein ttyV6 8:17pm 1:47 -csh gierd ttyD3 10:00pm 1 elm dave ttyD4 9:47pm 4 4 w brent ttyp0 26Jun91 4:46 26:46 4:41 bash dave ttyq4 26Jun9115days 46 46 wnewmail The following program takes the above input, converts the idle time to number of seconds and prints out the first two fields and the calculated idle time. (This program uses a number of `awk' features that haven't been introduced yet.) BEGIN { FIELDWIDTHS = "9 6 10 6 7 7 35" } NR > 2 { idle = $4 sub(/^ */, "", idle) # strip leading spaces if (idle == "") idle = 0 if (idle ~ /:/) { split(idle, t, ":"); idle = t[1] * 60 + t[2] } if (idle ~ /days/) { idle *= 24 * 60 * 60 } print $1, $2, idle } Here is the result of running the program on the data: hzuo ttyV0 0 hzang ttyV3 50 eklye ttyV5 0 dportein ttyV6 107 gierd ttyD3 1 dave ttyD4 0 brent ttyp0 286 dave ttyq4 1296000 Another (possibly more practical) example of fixed-width input data would be the input from a deck of balloting cards. In some parts of the United States, voters make their choices by punching holes in computer cards. These cards are then processed to count the votes for any particular candidate or on any particular issue. Since a voter may choose not to vote on some issue, any column on the card may be empty. An `awk' program for processing such data could use the `FIELDWIDTHS' feature to simplify reading the data. This feature is still experimental, and will likely evolve over time. File: gawk.info, Node: Multiple Line, Next: Getline, Prev: Constant Size, Up: Reading Files Multiple-Line Records ===================== In some data bases, a single line cannot conveniently hold all the information in one entry. In such cases, you can use multi-line records. The first step in doing this is to choose your data format: when records are not defined as single lines, how do you want to define them? What should separate records? One technique is to use an unusual character or string to separate records. For example, you could use the formfeed character (written `\f' in `awk', as in C) to separate them, making each record a page of the file. To do this, just set the variable `RS' to `"\f"' (a string containing the formfeed character). Any other character could equally well be used, as long as it won't be part of the data in a record. Another technique is to have blank lines separate records. By a special dispensation, a null string as the value of `RS' indicates that records are separated by one or more blank lines. If you set `RS' to the null string, a record always ends at the first blank line encountered. And the next record doesn't start until the first nonblank line that follows--no matter how many blank lines appear in a row, they are considered one record-separator. (End of file is also considered a record separator.) The second step is to separate the fields in the record. One way to do this is to put each field on a separate line: to do this, just set the variable `FS' to the string `"\n"'. (This simple regular expression matches a single newline.) Another way to separate fields is to divide each of the lines into fields in the normal manner. This happens by default as a result of a special feature: when `RS' is set to the null string, the newline character *always* acts as a field separator. This is in addition to whatever field separations result from `FS'. The original motivation for this special exception was probably so that you get useful behavior in the default case (i.e., `FS == " "'). This feature can be a problem if you really don't want the newline character to separate fields, since there is no way to prevent it. However, you can work around this by using the `split' function to break up the record manually (*note Built-in Functions for String Manipulation: String Functions.). File: gawk.info, Node: Getline, Next: Close Input, Prev: Multiple Line, Up: Reading Files Explicit Input with `getline' ============================= So far we have been getting our input files from `awk''s main input stream--either the standard input (usually your terminal) or the files specified on the command line. The `awk' language has a special built-in command called `getline' that can be used to read input under your explicit control. This command is quite complex and should *not* be used by beginners. It is covered here because this is the chapter on input. The examples that follow the explanation of the `getline' command include material that has not been covered yet. Therefore, come back and study the `getline' command *after* you have reviewed the rest of this manual and have a good knowledge of how `awk' works. `getline' returns 1 if it finds a record, and 0 if the end of the file is encountered. If there is some error in getting a record, such as a file that cannot be opened, then `getline' returns -1. In this case, `gawk' sets the variable `ERRNO' to a string describing the error that occurred. In the following examples, COMMAND stands for a string value that represents a shell command. `getline' The `getline' command can be used without arguments to read input from the current input file. All it does in this case is read the next input record and split it up into fields. This is useful if you've finished processing the current record, but you want to do some special processing *right now* on the next record. Here's an example: awk '{ if (t = index($0, "/*")) { if (t > 1) tmp = substr($0, 1, t - 1) else tmp = "" u = index(substr($0, t + 2), "*/") while (u == 0) { getline t = -1 u = index($0, "*/") } if (u <= length($0) - 2) $0 = tmp substr($0, t + u + 3) else $0 = tmp } print $0 }' This `awk' program deletes all C-style comments, `/* ... */', from the input. By replacing the `print $0' with other statements, you could perform more complicated processing on the decommented input, like searching for matches of a regular expression. (This program has a subtle problem--can you spot it?) This form of the `getline' command sets `NF' (the number of fields; *note Examining Fields: Fields.), `NR' (the number of records read so far; *note How Input is Split into Records: Records.), `FNR' (the number of records read from this input file), and the value of `$0'. *Note:* the new value of `$0' is used in testing the patterns of any subsequent rules. The original value of `$0' that triggered the rule which executed `getline' is lost. By contrast, the `next' statement reads a new record but immediately begins processing it normally, starting with the first rule in the program. *Note The `next' Statement: Next Statement. `getline VAR' This form of `getline' reads a record into the variable VAR. This is useful when you want your program to read the next record from the current input file, but you don't want to subject the record to the normal input processing. For example, suppose the next line is a comment, or a special string, and you want to read it, but you must make certain that it won't trigger any rules. This version of `getline' allows you to read that line and store it in a variable so that the main read-a-line-and-check-each-rule loop of `awk' never sees it. The following example swaps every two lines of input. For example, given: wan tew free phore it outputs: tew wan phore free Here's the program: awk '{ if ((getline tmp) > 0) { print tmp print $0 } else print $0 }' The `getline' function used in this way sets only the variables `NR' and `FNR' (and of course, VAR). The record is not split into fields, so the values of the fields (including `$0') and the value of `NF' do not change. `getline < FILE' This form of the `getline' function takes its input from the file FILE. Here FILE is a string-valued expression that specifies the file name. `< FILE' is called a "redirection" since it directs input to come from a different place. This form is useful if you want to read your input from a particular file, instead of from the main input stream. For example, the following program reads its input record from the file `foo.input' when it encounters a first field with a value equal to 10 in the current input file. awk '{ if ($1 == 10) { getline < "foo.input" print } else print }' Since the main input stream is not used, the values of `NR' and `FNR' are not changed. But the record read is split into fields in the normal manner, so the values of `$0' and other fields are changed. So is the value of `NF'. This does not cause the record to be tested against all the patterns in the `awk' program, in the way that would happen if the record were read normally by the main processing loop of `awk'. However the new record is tested against any subsequent rules, just as when `getline' is used without a redirection. `getline VAR < FILE' This form of the `getline' function takes its input from the file FILE and puts it in the variable VAR. As above, FILE is a string-valued expression that specifies the file from which to read. In this version of `getline', none of the built-in variables are changed, and the record is not split into fields. The only variable changed is VAR. For example, the following program copies all the input files to the output, except for records that say `@include FILENAME'. Such a record is replaced by the contents of the file FILENAME. awk '{ if (NF == 2 && $1 == "@include") { while ((getline line < $2) > 0) print line close($2) } else print }' Note here how the name of the extra input file is not built into the program; it is taken from the data, from the second field on the `@include' line. The `close' function is called to ensure that if two identical `@include' lines appear in the input, the entire specified file is included twice. *Note Closing Input Files and Pipes: Close Input. One deficiency of this program is that it does not process nested `@include' statements the way a true macro preprocessor would. `COMMAND | getline' You can "pipe" the output of a command into `getline'. A pipe is simply a way to link the output of one program to the input of another. In this case, the string COMMAND is run as a shell command and its output is piped into `awk' to be used as input. This form of `getline' reads one record from the pipe. For example, the following program copies input to output, except for lines that begin with `@execute', which are replaced by the output produced by running the rest of the line as a shell command: awk '{ if ($1 == "@execute") { tmp = substr($0, 10) while ((tmp | getline) > 0) print close(tmp) } else print }' The `close' function is called to ensure that if two identical `@execute' lines appear in the input, the command is run for each one. *Note Closing Input Files and Pipes: Close Input. Given the input: foo bar baz @execute who bletch the program might produce: foo bar baz hack ttyv0 Jul 13 14:22 hack ttyp0 Jul 13 14:23 (gnu:0) hack ttyp1 Jul 13 14:23 (gnu:0) hack ttyp2 Jul 13 14:23 (gnu:0) hack ttyp3 Jul 13 14:23 (gnu:0) bletch Notice that this program ran the command `who' and printed the result. (If you try this program yourself, you will get different results, showing you who is logged in on your system.) This variation of `getline' splits the record into fields, sets the value of `NF' and recomputes the value of `$0'. The values of `NR' and `FNR' are not changed. `COMMAND | getline VAR' The output of the command COMMAND is sent through a pipe to `getline' and into the variable VAR. For example, the following program reads the current date and time into the variable `current_time', using the `date' utility, and then prints it. awk 'BEGIN { "date" | getline current_time close("date") print "Report printed on " current_time }' In this version of `getline', none of the built-in variables are changed, and the record is not split into fields. File: gawk.info, Node: Close Input, Prev: Getline, Up: Reading Files Closing Input Files and Pipes ============================= If the same file name or the same shell command is used with `getline' more than once during the execution of an `awk' program, the file is opened (or the command is executed) only the first time. At that time, the first record of input is read from that file or command. The next time the same file or command is used in `getline', another record is read from it, and so on. This implies that if you want to start reading the same file again from the beginning, or if you want to rerun a shell command (rather than reading more output from the command), you must take special steps. What you must do is use the `close' function, as follows: close(FILENAME) close(COMMAND) The argument FILENAME or COMMAND can be any expression. Its value must exactly equal the string that was used to open the file or start the command--for example, if you open a pipe with this: "sort -r names" | getline foo then you must close it with this: close("sort -r names") Once this function call is executed, the next `getline' from that file or command will reopen the file or rerun the command. `close' returns a value of zero if the close succeeded. Otherwise, the value will be non-zero. In this case, `gawk' sets the variable `ERRNO' to a string describing the error that occurred. File: gawk.info, Node: Printing, Next: One-liners, Prev: Reading Files, Up: Top Printing Output *************** One of the most common things that actions do is to output or "print" some or all of the input. For simple output, use the `print' statement. For fancier formatting use the `printf' statement. Both are described in this chapter. * Menu: * Print:: The `print' statement. * Print Examples:: Simple examples of `print' statements. * Output Separators:: The output separators and how to change them. * OFMT:: Controlling Numeric Output With `print'. * Printf:: The `printf' statement. * Redirection:: How to redirect output to multiple files and pipes. * Special Files:: File name interpretation in `gawk'. `gawk' allows access to inherited file descriptors. File: gawk.info, Node: Print, Next: Print Examples, Prev: Printing, Up: Printing The `print' Statement ===================== The `print' statement does output with simple, standardized formatting. You specify only the strings or numbers to be printed, in a list separated by commas. They are output, separated by single spaces, followed by a newline. The statement looks like this: print ITEM1, ITEM2, ... The entire list of items may optionally be enclosed in parentheses. The parentheses are necessary if any of the item expressions uses a relational operator; otherwise it could be confused with a redirection (*note Redirecting Output of `print' and `printf': Redirection.). The relational operators are `==', `!=', `<', `>', `>=', `<=', `~' and `!~' (*note Comparison Expressions: Comparison Ops.). The items printed can be constant strings or numbers, fields of the current record (such as `$1'), variables, or any `awk' expressions. The `print' statement is completely general for computing *what* values to print. With two exceptions, you cannot specify *how* to print them--how many columns, whether to use exponential notation or not, and so on. (*Note Output Separators::, and *Note Controlling Numeric Output with `print': OFMT.) For that, you need the `printf' statement (*note Using `printf' Statements for Fancier Printing: Printf.). The simple statement `print' with no items is equivalent to `print $0': it prints the entire current record. To print a blank line, use `print ""', where `""' is the null, or empty, string. To print a fixed piece of text, use a string constant such as `"Hello there"' as one item. If you forget to use the double-quote characters, your text will be taken as an `awk' expression, and you will probably get an error. Keep in mind that a space is printed between any two items. Most often, each `print' statement makes one line of output. But it isn't limited to one line. If an item value is a string that contains a newline, the newline is output along with the rest of the string. A single `print' can make any number of lines this way. File: gawk.info, Node: Print Examples, Next: Output Separators, Prev: Print, Up: Printing Examples of `print' Statements ============================== Here is an example of printing a string that contains embedded newlines: awk 'BEGIN { print "line one\nline two\nline three" }' produces output like this: line one line two line three Here is an example that prints the first two fields of each input record, with a space between them: awk '{ print $1, $2 }' inventory-shipped Its output looks like this: Jan 13 Feb 15 Mar 15 ... A common mistake in using the `print' statement is to omit the comma between two items. This often has the effect of making the items run together in the output, with no space. The reason for this is that juxtaposing two string expressions in `awk' means to concatenate them. For example, without the comma: awk '{ print $1 $2 }' inventory-shipped prints: Jan13 Feb15 Mar15 ... Neither example's output makes much sense to someone unfamiliar with the file `inventory-shipped'. A heading line at the beginning would make it clearer. Let's add some headings to our table of months (`$1') and green crates shipped (`$2'). We do this using the `BEGIN' pattern (*note `BEGIN' and `END' Special Patterns: BEGIN/END.) to force the headings to be printed only once: awk 'BEGIN { print "Month Crates" print "----- ------" } { print $1, $2 }' inventory-shipped Did you already guess what happens? This program prints the following: Month Crates ----- ------ Jan 13 Feb 15 Mar 15 ... The headings and the table data don't line up! We can fix this by printing some spaces between the two fields: awk 'BEGIN { print "Month Crates" print "----- ------" } { print $1, " ", $2 }' inventory-shipped You can imagine that this way of lining up columns can get pretty complicated when you have many columns to fix. Counting spaces for two or three columns can be simple, but more than this and you can get "lost" quite easily. This is why the `printf' statement was created (*note Using `printf' Statements for Fancier Printing: Printf.); one of its specialties is lining up columns of data.