[ это синоним для test. Обрабатывается как и любая другая команда с соответствующими ограничениями. [[ — это расширение Bash (не совместимо с другими оболочками) и так как это внутренняя команда, то имеет несколько больше возможностей. В частности, нет необходимости заключать переменные в кавычки, можно использовать &&, ||, < и > и есть поддержка регулярных выражений через оператор =~. Для подробностей смотрите man bash. $IFS.IFS=$'\n'
for addr in $(cat filename); do
IFS=' '
telnet $addr
doneWord Splitting
The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting.
The shell treats each character of IFS as a delimiter, and splits the results of the other expansions into words using these characters as field terminators. If IFS is unset, or its value is exactly , the default, then sequences of , , and at the beginning and end of the results of the previous expansions are ignored, and any sequence of IFS characters not at the beginning or end serves to delimit words. If IFS has a value other than the default, then sequences of the whitespace characters space, tab, and newline are ignored at the beginning and end of the word, as long as the whitespace character is in the value of IFS (an IFS whitespace character). Any character in IFS that is not IFS whitespace, along with any adjacent IFS whitespace characters, delimits a field. A sequence of IFS whitespace characters is also treated as a delimiter. If the value of IFS is null, no word splitting occurs.
Explicit null arguments ("" or '') are retained and passed to commands as empty strings. Unquoted implicit null arguments, resulting from the expansion of parameters that have no values, are removed. If a parameter with no value is expanded within double quotes, a null argument results and is retained and passed to a command as an empty string. When a quoted null argument appears as part of a word whose expansion is non-null, the null argument is removed. That is, the word -d'' becomes -d after word splitting and null argument removal.
Note that if no expansion occurs, no splitting is performed.