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Full Discussion: Scripts dont give a consistent output..
Top Forums UNIX for Beginners Questions & Answers Scripts dont give a consistent output.. Post 302981683 by Scrutinizer on Saturday 17th of September 2016 10:44:29 AM
Old 09-17-2016
Quote:
Originally Posted by bakunin
The "internal field separator" is a shell variable which tells the shell how separate "words" (=input fields) are separated from each other. Consider this command:

Code:
command arg1 arg2 arg3

Somehow the shell has to know why this is to parse as a call to "command" with three different arguments, "arg1", "arg2" and "arg3". The reason is that in fact the command looks (for the shells parser) like this:

Code:
<string1><IFS><string2><IFS><string3><IFS><string4>

and because the default value for IFS is a space the shell sees four strings, of which the first is a command and the others are arguments. This is why you have to quote "words" (commands, arguments) containing spaces: quotes turn off the mechanism inside them.
[..]
Not quite. In the example given, this is always a sequence of class [[:blank:]] characters, irrespective of the value of IFS :
Code:
<string1>[[:blank:]]+<string2>[[:blank:]]+<string3>[[:blank:]]+<string4>

Where [[:blank:]]+ means one or more characters from the [[:blank:]] character class (i.e. space or TAB).
When a line is read, this is determined during token recognition where it is determined that these are word tokens.

Then the shell grammar is used to determine that this is a simple command where the first field is the command name and remaining fields are the arguments for the command.

---
IFS is only used in the following cases:

To illustrate the difference:
Code:
$ bla=1,2,3
$ IFS=,
$ printf "%s\n" 1,2,3  4     # No field splitting, IFS is not used, besides %s\n there are 2 fields that were determined as words during token recognition: 
1,2,3
4
$ printf "%s\n" $bla  4      # Field splitting after variable expansion, IFS is used. Besides %s\n there are now 4 fields, one determined as word during token recognition, the other field is split into 3 further fields
1
2
3
4
$ printf "%s\n" "$bla"  4    # Variable expansion between double quotes, so no field splitting, IFS is not used, besides %s\n  there are 2 argument fields that were determined as words during token recognition: 
1,2,3
4
$ printf "%s\n" "$bla  4"    # Variable expansion between double quotes, so no field splitting, IFS is not used, besides %s\n there is one argument field after quote removal
1,2,3  4
$ IFS=$' \t\n'
$ printf "%s\n" $bla  4      # Field splitting after variable expansion, IFS is used, but the comma is not part of IFS, so no split is performed, besides %s\n there are 2 argument fields that were determined as words during token recognition: 
1,2,3
4

Last edited by Scrutinizer; 09-17-2016 at 12:33 PM..
These 3 Users Gave Thanks to Scrutinizer For This Post:
greet_sed RavinderSingh13 wisecracker
 

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