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Full Discussion: Understanding Xargs
Top Forums Shell Programming and Scripting Understanding Xargs Post 303010124 by RudiC on Wednesday 27th of December 2017 05:24:23 AM
Old 12-27-2017
Now, these are very basic questions from quite broad a range of IT, and I'm not sure I can cover that to satisfaction. On top, there may be a language barrier, e.g with "occurrence". I'd advise to use a dictionary, as - to me - the meaning got immediately clear when seeing the translations. Please also consult introductory text books and / or man pages.

It might be worthwhile to internalize the concept of a string which you will encounter everywhere in IT (tools, databases, documents, files, ...) when needing to represent text. It can come in a variety of shapes, like fixed or varying length strings, zero terminated or with a leading length indicator, string constants, substrings, string concatenations, and what have you, and there are many tools, libraries, functions to handle them. Different digital items (numbers, logical values) can be output to screen as readable text representations only, not as the individual items themselves.

Then, there are text files, a loosely structured collection of (mostly) printable characters. In *nix systems, those consist of lines of characters terminated by a <new line> (\n, ^J, 0x0A) character. But this is not the only possible text representation. When reading a line from a file, you can put it into a single string variable, or split it into several substrings. If you do so by applying spaces and / or punctuation chars for separating, the substrings will be words. But any other separation is possible albeit not necessarily sensible. So, a line is sort of a superset of (a group of) strings.

A "command line" specifies a collection of a command name (perhaps including a path), zero or more options (with possible arguments), and zero or more parameters. Any of those is a (possibly one character) string, analysed by the command interpreter, and then supplied to the program being executed. Please be aware that the terms "argument" and "parameter" are not strictly distinguished between and both are loosely and interchangably used. (I neglected possible local variable assingments and redirections to avoid overcomplicaton.)

Last edited by RudiC; 12-27-2017 at 10:13 AM.. Reason: Some typos
 

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textutil(n)						      Texts and strings utils						       textutil(n)

NAME
textutil - Procedures to manipulate texts and strings. SYNOPSIS
package require Tcl 8.2 package require textutil ?0.5? textutil::adjust string args textutil::splitx string ?regexp? textutil::tabify string ?num? textutil::tabify2 string ?num? textutil::trim string ?regexp? textutil::trimleft string ?regexp? textutil::trimright string ?regexp? textutil::untabify string ?num? textutil::untabify2 string ?num? textutil::strRepeat text num DESCRIPTION
The textutil package provides commands that manipulate strings or texts (a.k.a. long strings or string with embedded newlines or para- graphs). The complete set of procedures is described below. textutil::adjust string args Do a justification on the string according to args. The string is taken as one big paragraph, ignoring any newlines. Then the line is formatted according to the options used, and the command return a new string with enough lines to contain all the printable chars in the input string. A line is a set of chars between the beginning of the string and a newline, or between 2 newlines, or between a newline and the end of the string. If the input string is small enough, the returned string won't contain any newlines. By default, any occurrence of spaces characters or tabulation are replaced by a single space so each word in a line is separated from the next one by exactly one space char, and this forms a real line. Each real line is placed in a logical line, which have exactly a given length (see -length option below). The real line may have a lesser length. Again by default, any trailing spaces are ignored before returning the string (see -full option below). The following options may be used after the string parameter, and change the way the command place a real line in a logical line. -full boolean If set to false, any trailing space chars are deleted before returning the string. If set to true, any trailing space chars are left in the string. Default to false. -justify (center|left|plain|right) Set the justification of the returned string to center, left, plain or right. By default, it is set to left. The justifica- tion means that any line in the returned string but the last one is build according to the value. If the justification is set to plain and the number of printable chars in the last line is less than 90% of the length of a line (see -length), then this line is justified with the left value, avoiding the expansion of this line when it is too small. The meaning of each value is: center The real line is centered in the logical line. If needed, a set of space char are added at the beginning (half of the needed set) and at the end (half of the needed set) of the line if required (see the option -full). left The real line is set on the left of the logical line. It means that there are no space chars at the beginning of this line. If required, all needed space chars are added at the end of the line (see the option -full). plain The real line is exactly set in the logical line. It means that there are no leading or trailing space chars. All the needed space chars are added in the real line, between 2 (or more) words. right The real line is set on the right of the logical line. It means that there are no space chars at the end of this line, and there may be some space chars at the beginning, despite of the -full option. -length integer Set the length of the logical line in the string to integer. integer must be a positive integer value. Defaults to 72. -strictlength boolean If set to false, a line can exceed the specified -length if a single word is longer than -length. If set to true, words that are longer than -length are split so that no line exceeds the specified -length. Defaults to false. textutil::splitx string ?regexp? Split the string and return a list. The string is split according to the regular expression regexp instead of a simple list of chars. Note that if you add parenthesis into the regexp, the parentheses part of separator would be added into list as additional element. If the string is empty the result is the empty list, like for split. If regexp is empty the string is split at every char- acter, like split does. The regular expression regexp defaults to "[ ]+". textutil::tabify string ?num?] Tabify the string by replacing any substring of num space chars by a tabulation and return the result as a new string. num defaults to 8. textutil::tabify2 string ?num?] Similar to textutil::tabify this command tabifies the string and returns the result as a new string. A different algorithm is used however. Instead of replacing any substring of num spaces this comand works more like an editor. num defaults to 8. Each line of the text in string is treated as if there are tabstops every num columns. Only sequences of space characters containing more than one space character and found immediately before a tabstop are replaced with tabs. textutil::trim string ?regexp?] Remove in string any leading and trailing substring according to the regular expression regexp and return the result as a new string. This apply on any line in the string, that is any substring between 2 newline chars, or between the beginning of the string and a newline, or between a newline and the end of the string, or, if the string contain no newline, between the beginning and the end of the string. The regular expression regexp defaults to "[ ]+". textutil::trimleft string ?regexp?] Remove in string any leading substring according to the regular expression regexp and return the result as a new string. This apply on any line in the string, that is any substring between 2 newline chars, or between the beginning of the string and a newline, or between a newline and the end of the string, or, if the string contain no newline, between the beginning and the end of the string. The regular expression regexp defaults to "[ ]+". textutil::trimright string ?regexp?] Remove in string any trailing substring according to the regular expression regexp and return the result as a new string. This apply on any line in the string, that is any substring between 2 newline chars, or between the beginning of the string and a newline, or between a newline and the end of the string, or, if the string contain no newline, between the beginning and the end of the string. The regular expression regexp defaults to "[ ]+". textutil::untabify string ?num?] Untabify the string by replacing any tabulation char by a substring of num space chars and return the result as a new string. num defaults to 8. textutil::untabify2 string ?num?] Untabify the string by replacing any tabulation char by a substring of at most num space chars and return the result as a new string. Unlike textutil::untabify each tab is not replaced by a fixed number of space characters. The command overlays each line in the string with tabstops every num columns instead and replaces tabs with just enough space characters to reach the next tabstop. This is the complement of the actions taken by textutil::tabify2. num defaults to 8. There is one asymmetry though: A tab can be replaced with a single space, but not the other way around. textutil::strRepeat text num The implementation depends on the core executing the package. Used string repeat if it is present, or a fast tcl implementation if it is not. Returns a string containing the text repeated num times. The repetitions are joined without characters between them. A value of num <= 0 causes the command to return an empty string. SEE ALSO
regexp(n), split(n), string(n) KEYWORDS
string, regular expression textutil 0.5 textutil(n)
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