Use `getline < file` to play the next record of the file. Here, file is a string value expression that specifies the name of the file. "< file" is called redirection because the input is routed from another location. For example, the following program reads its input record from the secondary.input file when it encounters a first field in the current input file with a value equal to 10: Table 4-1 summarizes the eight variants of getline and lists the predefined variables defined by each of them and whether the variant is a standard or gawk extension. Note: For each variant, gawk sets the predefined variable RT. If this variable is set, GAWK is not case sensitive. The following example illustrates this − The use of RS as a regular expression and the VARIABLE RT are gawk extensions; they are not available in compatibility mode (see Command-line options). In compatibility mode, only the first character of the RS value determines the end of the record. The value used for the -F argument is treated in the same way as mappings to the predefined variable FS. All special characters in the field separator must be escaped accordingly. For example, to use a „ as a field separator on the command line, you would type the following: However, this does not set group to the token value because awk treats the right side of the assignment as a literal in awk syntax. For example, if the value of TOKEN is the 7-digit string foobar, the variable group awk is set to the 6-digit fooaar string, where â is a rollback character (byte value 8). AWK offers several built-in variables.
They play an important role in writing AWK scripts. This chapter illustrates the use of built-in variables. Once the entry is divided into records, awk further divides the records into individual fields named $1, $2, etc. $0 is the entire record, and NF indicates the number of fields there are. The default method for dividing fields is between spaces. Note that the entry for the name `Bill` is not divided. In the original data file (see Data files for examples), the line looks like this: The easiest way to pass a variable to an awk script is to export it to the environment and use it through the ENVIRON table. A variable name is a valid expression in itself.
It represents the current value of the variable. Variables receive new values with mapping operators, incremental operators, and decrement multiples (see Mapping expressions). In addition, the sub() and gsub() functions can change the value of a variable, and the match(), split(), and patsplit() functions can change the contents of their array parameters (see String manipulation functions). Also, you do not use the variable group anywhere in the awk script. Regexp /group/ is any string that contains the 5-digit string group. If you want to check if the field is exactly the value of the group (so that, for example, if the value of TOKEN is GROUP2, then a field containing GROUP24 is not mapped), use the equality == operator. One technique is to use an unusual character or string to separate records. For example, you can use the Formfeed character (written `f` in awk, as in C) to separate them and make each record a page of the file. To do this, simply set the RS variable to „f“ (a string that contains the Formfeed character). Any other character could just as easily be used as long as it is not part of the data in a dataset. Once the end of the data record is determined, gawk sets the RT variable to the text of the entry that corresponds to RS.
As mentioned in Field Review, awk stores the number of fields in the active record in the built-in NF variable (see also Predefined Variables). The expression $NF is therefore not a peculiarity – it is the direct consequence of the evaluation of NF and the use of its value as a field number. FS can be set from the command line with the -F option. This can also be done by using command-line variable assignment. AWK divides your program`s input into records and fields. It tracks the number of records that have been read from the current input file so far. This value is stored in a predefined variable called FNR, which is reset each time a new file is started. Another predefined variable, NR, records the total number of input records that have been read from all data files so far. It starts from scratch, but never automatically resets to zero.
An example of the latter is a table in which all columns are aligned using a variable number of spaces, and empty fields are just spaces. Obviously, the normal field division of the FS-based awk does not work well in this case. Although a portable awk program can use a series of calls substr() to $0 (see String manipulation functions), this is cumbersome and inefficient for a large number of fields. The field separator is represented by the predefined variable FS. Note for shell programmers: awk does not use the IFS name, which is used by POSIX-compatible shells (for example, Unix Bourne shell, sh, or Bash). The IGNORECASE variable (see Built-in variables that control awk) affects field splitting only if the value of FS is a regexp. This has no effect if FS is a single character, even if that character is a letter. Therefore, in the following code: If one of the –posix or –traditional options is specified, gawk treats a directory on the command line as a fatal error. It provides dynamic control of the –lint option of the GAWK program.
If this variable is set, GAWK issues lint warnings. When the string value is assigned fatally, lint warnings become fatal errors, just like –lint=fatal. Like any other variable, the value of RS in the awk program can be changed using the mapping operator `=` (see Mapping expressions). The new record separator must be enclosed in quotation marks that indicate a string constant. Often, the right time to do this is at the beginning of the execution before an entry is processed, so that the very first record is read with the correct delimiter. To do this, use the special begin model (see Special BEGIN and END models). For example, the „-“ in the person`s name was used as a field separator, instead of the „-“ in the phone number originally intended. This shows why you need to be careful when choosing your field and saving the delimiters. Fields can be referenced as in $NF with a variable. Values can also be assigned to fields, resulting in the $0 value being recalculated when referenced later.
Assigning a field with a number greater than NF creates the field and recreates the record, using OFS to separate the fields. Incrementing NF does the same. Reducing NF throws away the fields and recreates the record. If the affected variable is an expression with side effects, different versions of awk behave differently when they encounter the end of the file. Some versions do not evaluate the expression. Many versions (including GAWK) do this. Here`s an example, courtesy of Duncan Moore: Another way to separate fields is to place each field on a separate line: just set the FS variable to the string „n“. (This delimiter corresponds to a single line break.) A practical example of a data file organized in this way could be a mailing list where empty lines separate the entries.
Imagine a mailing list in a file called addresses that looks like this: In this version of getline, none of the predefined variables are changed and the record is not divided into fields. However, RT is fixed. Note here that the name of the additional input file is not embedded in the program. It is taken directly from the data, especially from the second field in row @include. If the PROCINFO element does not exist and the environment variable GAWK_READ_TIMEOUT exists, gawk uses its value to initialize the timeout value. The disadvantage of using the environment variable exclusively to specify the timeout is that it cannot be controlled by command or connection. The -v argument requires name=value. So you need to write -v lengthA = $3 The getline command used in this way only defines the variables NR, FNR, and RT (and of course var).
The record is not divided into fields, so the values of the fields (including $0) and the value of NF do not change. The division of an input record into fixed-width fields is determined by assigning a string of numbers separated by spaces to the built-in VARIABLE FIELD WIDTHS. Each number specifies the width of the field, including the columns between the fields.
