UNIX for Dummies Questions & Answers

Can someone please explain when input redirection is necessary?

For example, "cat filename" and "cat< filename" produce the same result. I was told that if I need to bunzip a file that I should type "bunzip2<filename.bz2." However, if I omit the "<" I still get the same result. Can someone please give me a situation or command which requires input redirection?

Try the following:

Code:

tr -d " " < file

or:

Code:

i=0
while read x1 x2 x3
do      printf "Line%2d: %s\n" $((++i)) "$x2"
done < file

when file contains:

Code:

abc def ghi jkl
123 456 7
Last input line

$ echo hi
--- it just prints "hi" on the screen.
if i want to send the output to a file, then i will use output redirection.
$ echo hi > test-hi

in the same, input redirection is just the same.

Input redirection can be useful if you have written a program which expects input from the terminal and you want to provide it from a file. In the following example, myprog, which was written to read standard input and write standard output, is redirected to read myin.

$ myprog < myin

Some commands use arguments to specify a filename from which they acquire their input.

Code:

command /file/to/process

Some other commands get their input via stdin.

Code:

process-generating-output | command
command < /file/to/process

Some commands can do both. For this last group there is no difference in results if you use a file as input (the first method) or get the input via stdin (the second method).

You cited as examples only programs/commands from this third group, which is why you do not see the difference. The "tr" command Don Cragun mentioned belongs to the second group, so it wouldn't work with the first method while it would do so with the second.

I hope this helps.

bakunin

There are also commands that accept one or more pathname operands or read from standard input, but produce different output when no pathname operands are given. For example, if the command:

Code:

wc -l filename

produces the output:

Code:

    4917 filename

the command:

Code:

wc -l < filename

would produce the output:

Code:

    4917

Compare on your own the output produced by:

Code:

wc -l filename1 filename2
    and
cat filename1 filename2 | wc -l

for any two text files you may have lying around.