Shell Scripting – Why Use ‘while IFS= read’ Instead of ‘IFS=; while read’?

(Sorry, long explanation)

Yes, the IFS variable in while IFS=" " read; do … has no effect on the rest of the code.

Let's first precise that the shell command line features two different kinds of variables:

• shell variables (which only exist within a shell, and are local to the shell)
• environment variables, which exist for every process. Those are usually preserved upon fork() and exec(), so child processes inherit them.

When you call a command with:

  A=foo B=bar command

the command is executed in within an environment where (environment) variable A is set to foo and B is set to bar. But with this command line, the current shell variables A and B are are left unchanged.

This is different from:

A=foo; B=bar; command

Here, shell variables A and B are defined and the command is run without environment variables A and B defined. Values of A and B are unaccessible from command.

However, if some shell variables are export-ed, the corresponding environment variables are synchronized with their respective shell variables. Example:

export A
export B
A=foo; B=bar; command

With this code, both shell variables and the shell environment variables are set to foo and bar. Since environment variables are inherited by sub-processes, command will be able to access their values.

To jump back to your original question, in:

IFS='a' read

only read is affected. And in fact, in this case, read doesn't care about the value of the IFS variable. It uses IFS only when you ask the line to be split (and stored in several variables), like in:

echo "a :  b :    c" | IFS=":" read i j k; \
    printf "i is '%s', j is '%s', k is '%s'" "$i" "$j" "$k"

IFS is not used by read unless it is called with arguments. (Edit: This is not exactly true: whitespace characters, i.e. space and tab, present in IFS are always ignored at the beginning/end of the input line. )

I find very useful the following readline commands

history-search-backward,
history-search-forward

(be aware they are different from the usual reverse-search-history, forward-search-history, tied to Ctrl-R, Ctrl-S).

I have these commands associated to Ctrl-Up and Ctrl-Down putting the following lines into ~/.inputrc:

"\e[1;5A": history-search-backward
"\e[1;5B": history-search-forward

How they work: write few chars of the beginning of the command, press Ctrl-Up and the next older command starting with that prefix will be shown, press again to see the next, and so on. When you are satisfied, after possibly modifying the command, press Enter to execute.