Input Field Separators
For many command line interpreters (“shell”) of Unix operating systems, the input field separators or internal field separators or Template:Tt shell variable holds characters used to separate text into tokens.
The value of Template:Mono, (in the bash shell) typically includes the space, tab, and the newline characters by default. These whitespace characters can be visualized by issuing the "declare" command in the bash shell or printing Template:Mono with commands like printf %s "$IFS" | od -c, printf "%q\n" "$IFS" or printf %s "$IFS" | cat -A (the latter two commands being only available in some shells and on some systems).
From the Bash, version 4 man page:
The shell treats each character of Template:Mono as a delimiter, and splits the results of the other expansions into words on these characters. If Template:Mono is unset, or its value is exactly Template:Mono, the default, then sequences of Template:Mono, Template:Mono, and Template:Mono at the beginning and end of the results of the previous expansions are ignored, and any sequence of Template:Mono characters not at the beginning or end serves to delimit words. If Template:Mono has a value other than the default, then sequences of the whitespace characters Template:Mono and Template:Mono are ignored at the beginning and end of the word, as long as the whitespace character is in the value of Template:Mono (an Template:Mono whitespace character). Any character in Template:Mono that is not Template:Mono whitespace, along with any adjacent Template:Mono whitespace characters, delimits a field. A sequence of Template:Mono whitespace characters is also treated as a delimiter. If the value of Template:Mono is null, no word splitting occurs.
IFS abbreviation
According to the Open Group Base Specifications, Template:Tt is an abbreviation for "input field separators."[1] A newer version of this specification mentions that "this name is misleading as the IFS characters are actually used as field terminators."[2] However Template:Tt is often referred to as "internal field separators."
Exploits
IFS was usable as an exploit in some versions of Unix. A program with root permissions could be fooled into executing user-supplied code if it ran (for instance) system("/bin/mail") and was called with Template:Mono set to Template:Mono, in which case it would run the program "Template:Mono" (in the current directory and thus writable by the user) with root permissions.[3] This has been fixed by making the shells not inherit the IFS variable.
References
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