Classic Shell Scripting

String Functions

In Section 9.3.2 we introduced the length( string ) function, which returns the length of a string string. Other common string operations include concatenation, data formatting, lettercase conversion, matching, searching, splitting, string substitution, and substring extraction.

Substring Extraction

The substring function, substr( string, start, len ), returns a copy of the substring of len characters from string starting from character start. Character positions are numbered starting from one: substr("abcde", 2, 3) returns "bcd". The len argument can be omitted, in which case, it defaults to length( string ) - start + 1, selecting the remainder of the string.

It is not an error for the arguments of substr( ) to be out of bounds, but the result may be implementation-dependent. For example, nawk and gawk evaluate substr("ABC", -3, 2) as "AB", whereas mawk produces the empty string "". All of them produce an empty string for substr("ABC", 4, 2) and for substr("ABC", 1, 0). gawk's —lint option diagnoses out-of-bounds arguments in substr( ) calls.

Lettercase Conversion

Some alphabets have uppercase and lowercase forms of each letter, and in string searching and matching, it is often desirable to ignore case differences. awk provides two functions for this purpose: tolower( string ) returns a copy of string with all characters replaced by their lowercase equivalents, and toupper( string ) returns a copy with uppercase equivalents. Thus, tolower("aBcDeF123") returns ...