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4.5.5 Field-Splitting Summary

It is important to remember that when you assign a string constant as the value of FS, it undergoes normal awk string processing. For example, with Unix awk and gawk, the assignment ‘FS = "\.."’ assigns the character string ".." to FS (the backslash is stripped). This creates a regexp meaning “fields are separated by occurrences of any two characters.” If instead you want fields to be separated by a literal period followed by any single character, use ‘FS = "\\.."’.

The following table summarizes how fields are split, based on the value of FS (‘==’ means “is equal to”):

FS == " "
Fields are separated by runs of whitespace. Leading and trailing whitespace are ignored. This is the default.
FS == any other single character
Fields are separated by each occurrence of the character. Multiple successive occurrences delimit empty fields, as do leading and trailing occurrences. The character can even be a regexp metacharacter; it does not need to be escaped.
FS == regexp
Fields are separated by occurrences of characters that match regexp. Leading and trailing matches of regexp delimit empty fields.
FS == ""
Each individual character in the record becomes a separate field. (This is a gawk extension; it is not specified by the POSIX standard.)

Changing FS Does Not Affect the Fields

According to the POSIX standard, awk is supposed to behave as if each record is split into fields at the time it is read. In particular, this means that if you change the value of FS after a record is read, the value of the fields (i.e., how they were split) should reflect the old value of FS, not the new one.

However, many older implementations of awk do not work this way. Instead, they defer splitting the fields until a field is actually referenced. The fields are split using the current value of FS! (d.c.) This behavior can be difficult to diagnose. The following example illustrates the difference between the two methods. (The sed1 command prints just the first line of /etc/passwd.)

     sed 1q /etc/passwd | awk '{ FS = ":" ; print $1 }'

which usually prints:

     root

on an incorrect implementation of awk, while gawk prints something like:

     root:nSijPlPhZZwgE:0:0:Root:/:

FS and IGNORECASE

The IGNORECASE variable (see User-modified) affects field splitting only when the value of FS is a regexp. It has no effect when FS is a single character, even if that character is a letter. Thus, in the following code:

     FS = "c"
     IGNORECASE = 1
     $0 = "aCa"
     print $1

The output is ‘aCa’. If you really want to split fields on an alphabetic character while ignoring case, use a regexp that will do it for you. E.g., ‘FS = "[c]"’. In this case, IGNORECASE will take effect.


Footnotes

[1] The sed utility is a “stream editor.” Its behavior is also defined by the POSIX standard.