UNIX for Dummies Questions & Answers

Hi all,
What is the difference between these two comands?
sed s/a/b/ <f1 >f2
sed s/a/b/ f1 >f2

Best,
santiagorf

Standard input (stdin) normally comes from your keyboard. Many programs ignore stdin.

E.g. cat filename never reads its standard input; it reads the files directly. But without file names on the command line, UNIX commands that need input will usually read stdin.

Standard input normally comes from your keyboard, but the shell can redirect stdin from a file. This is handy for UNIX commands that can't open files directly.

E.g. mail user@domain.com < filename. To mail a file use < filename to tell the shell to attach the file, instead of your keyboard, to mail's standard input.

sed can read file directly, so no need to redirect stdin from a file. I hope this helps.

The results from those two commands should be the same.

The difference between them is that in the first case, the shell opens f1 as standard input to be read by sed. In the second case, sed opens and reads f1 using a file descriptor that might or might not (but probably won't) be the standard input file descriptor for sed.

thank you!!