Standard input is conventionally file descriptor 0."} ::option[`1`]{#stdin-fd-one explanation="File descriptor 1 conventionally represents standard output, the stream for regular results."} ::option[`2`]{#stdin-fd-two explanation="File descriptor 2 conventionally represents standard error, not standard input."} ::: ## Redirecting a File into stdin The `<` operator tells Bash to open a file for reading and connect it to the command's stdin: ```bash $ cat < peanuts.txt Hello World ``` The shell handles `< peanuts.txt`; `cat` simply reads file descriptor 0. The pathname is not passed to `cat` as a normal file operand. If the input file does not exist or cannot be opened, the shell reports the redirection error and does not start the command with that input. :::single-choice{#stdin-from-file} Which command makes `sort` read its standard input from `names.txt`? ::option[`sort < names.txt`]{#sort-stdin-file .correct explanation="Bash opens `names.txt` for reading and connects it to `sort` on file descriptor 0."} ::option[`sort > names.txt`]{#stdout-to-names explanation="A greater-than operator redirects stdout to the file and can truncate it. It does not supply the file as input."} ::option[`sort names.txt >`]{#incomplete-sort-output explanation="This includes an incomplete output redirection. It does not express the requested stdin connection."} ::: ## File Operand versus Input Redirection Some commands accept either a filename operand or stdin, but the results can differ slightly. For example: ```bash $ wc -l peanuts.txt 1 peanuts.txt $ wc -l < peanuts.txt 1 ``` Both forms count lines in the same data. In the first form, `wc` knows the filename because it received it as an argument. In the second, it only receives a stream on stdin, so it has no filename to print. :::single-choice{#stdin-not-command-argument} Why does `wc -l < peanuts.txt` normally omit `peanuts.txt` from its output? ::option[`wc` deletes the filename after it finishes counting lines.]{#stdin-delete-name explanation="The command does not rename or delete the source file. Only its input connection differs."} ::option[The `<` operator hides every word printed by the command.]{#stdin-hide-words explanation="Input redirection does not filter stdout. The filename is absent because `wc` never received it as an argument."} ::option[Bash provides the file as stdin rather than as a filename argument.]{#stdin-no-filename .correct explanation="The shell consumes the redirection and connects the file to descriptor 0, so `wc` is not given the pathname as an operand."} ::: ## Combining Input and Output Redirection One command line can redirect more than one stream: ```bash $ cat < peanuts.txt > banana.txt ``` The shell performs two independent connections: 1. `< peanuts.txt` opens `peanuts.txt` as `cat`'s stdin. 2. `> banana.txt` creates or truncates `banana.txt` and connects it to `cat`'s stdout. `cat` reads bytes from stdin and writes them to stdout, so `banana.txt` receives the source content. For ordinary file copying, `cp peanuts.txt banana.txt` communicates the intent more directly; this example is about stream connections. :::single-choice{#stdin-and-stdout-files} In `cat < input.txt > output.txt`, which file supplies stdin and which receives stdout? ::option[`output.txt` supplies stdin; `input.txt` receives stdout.]{#stdin-output-stdout-input explanation="This reverses the meanings of the redirection operators. The arrows point toward the command for input and toward the file for output."} ::option[`input.txt` supplies stdin; `output.txt` receives stdout.]{#stdin-input-stdout-output .correct explanation="The `<` redirection opens `input.txt` for descriptor 0, and `>` opens `output.txt` for descriptor 1."} ::option[Both files supply stdin, and stdout stays on the terminal.]{#both-stdin explanation="The two operators affect different standard streams. `>` redirects stdout away from the terminal."} ::: To practice input and output redirection, try these hands-on labs: 1. **[Redirecting Input and Output in Linux](https://labex.io/labs/comptia-redirecting-input-and-output-in-linux-590840)** - Practice controlling data flow from commands by manipulating standard output (stdout), standard error (stderr), and standard input (stdin) using operators like >, >>, 2>, and the tee command. 2. **[Data Stream Redirection](https://labex.io/labs/linux-data-stream-redirection-17995)** - Learn the art of Linux stream redirection. Manipulate standard input, output, and error streams, combine outputs, and utilize /dev/null for advanced file operations. ## Summary You can now connect a command's standard input to a file through the shell. 1. Recognize stdin as file descriptor 0. 2. Redirect a readable file with `<`. 3. Distinguish a filename operand from redirected input. 4. Combine stdin and stdout redirections deliberately.

stdin (Standard In)