Standard Input, Output, and Error

Every Linux program interacts with three primary input/output streams: Standard Input (stdin) – By default, takes input from the keyboard. Standard Output (stdout) – Displays results on the screen unless redirected. Standard Error (stderr) – Shows error messages separately from standard output.
Understanding these streams enables users to redirect and manipulate input and output efficiently using various I/O redirection techniques.
Redirecting Standard Output (stdout) By default, the output of a command is displayed on the screen, but we can redirect it to a file using >: This saves the directory listing of /usr/bin in ls-output.txt instead of printing it on the terminal.
Checking the Output File If successful, the file contains the command output and its size reflects the generated data. To view the contents:
Handling Errors with Standard Error (stderr) Not all output is standard output. Programs like ls send error messages to stderr, meaning they will still appear on-screen even if stdout is redirected.
Example: Redirecting Output with an Invalid Directory If /bin/usr does not exist, an error message appears on the screen instead of in ls-output.txt. To verify the file size: Output shows: bash -rw-rw-r-- 1 user user 0 ls-output.txt The file is empty because stdout was redirected while stderr remained on-screen.
Appending Output Instead of Overwriting By default, > overwrites existing files. If we want to append output instead, use >>: Using >>, the output adds to the file rather than replacing its contents.
Example: Appending Multiple Times Now, ls-output.txt grows in size after each execution.

In Linux, commands send their main output to standard output (stdout) and error messages to standard error (stderr). By default, both streams are displayed on the screen, but users can redirect them to files or suppress them entirely.
This article explains how to redirect standard error (stderr), capture both output streams together, and dispose of unwanted output using /dev/null.
Redirecting Standard Error (stderr) Since standard error is file descriptor 2, we use 2> to redirect error messages to a file: This redirects only error messages (if any) to ls-error.txt while keeping standard output on the screen.
Redirecting Both Standard Output (stdout) and Standard Error (stderr) Together Sometimes, users want both outputs to be saved in one file. There are two methods to achieve this:
Traditional Method (Compatible with Older Shells) Explanation: Redirects stdout to ls-output.txt. Redirects stderr (file descriptor 2) to match stdout (file descriptor 1) using 2>&1.
Newer Bash Method (Simpler Syntax) The &> notation combines both streams into one file without needing 2>&1.
Appending Instead of Overwriting To append output to an existing file rather than overwriting, use >>: Now, each execution adds the new output instead of replacing the existing data.
Disposing of Unwanted Output with /dev/null Sometimes, users want to suppress error messages entirely. The best way to discard unwanted output is by redirecting it to /dev/null, known as the bit bucket: Here, all error messages are discarded, and nothing is saved.
Unix Culture: /dev/null The bit bucket (/dev/null) is an old Unix concept used to represent complete disposal of data. In tech communities, saying "Your request was sent to /dev/null" humorously means it was ignored entirely.

Standard input (stdin) is one of the three fundamental I/O streams in Linux. It allows programs to receive input, typically from the keyboard, but can be redirected from files or other sources.
This article explores how to redirect standard input, using the cat command as an example, and demonstrates how input redirection improves efficiency in text processing.
Using cat to Read Files The cat command copies the contents of a file to standard output (stdout): For example, displaying the contents of ls-output.txt: This prints the file contents without paging.
Joining Multiple Files with cat If a large file has been split into parts, cat can rejoin them: Since wildcards sort files automatically, the parts are joined in the correct order.
Using cat with Standard Input (stdin) When cat runs without arguments, it waits for input from the keyboard: It behaves as an interactive prompt, allowing users to type text.
Ending Input with Ctrl+D To signal the end of input (EOF), press Ctrl+D: Pressing Ctrl+D submits the input and displays the text.
Creating a File Using Input Redirection The > operator allows users to save standard input into a file: This stores the text in lazy_dog.txt.
Verifying the File Contents To confirm the contents: Output: bash The quick brown fox jumped over the lazy dog.
Redirecting Standard Input (<) from a File Instead of typing text, users can redirect a file as input: This reads lazy_dog.txt and behaves identically to passing it as an argument. While not particularly useful for cat, other commands make better use of input redirection, as we will see later.
Pipelines allow Linux commands to process data efficiently by passing standard output (stdout) from one command into standard input (stdin) of another using the pipe operator |. By chaining multiple commands, users can filter, manipulate, and analyze data dynamically.
This article explores how to use pipelines, apply filters like sort, uniq, grep, and utilize tee for saving pipeline output.
Basic Pipe Usage (|) A pipeline connects two or more commands: Example: Display paged output using less: Here, ls -l /usr/bin lists files, and less allows scrolling through results page by page.
Sorting Output with sort To list all executable programs in /bin and /usr/bin, sort them, and view results: Using sort, the output is now merged into a single sorted list.
Understanding > vs. | While > redirects output to a file, | passes output to another command: Using > incorrectly can overwrite system files, causing severe issues.
Removing Duplicates with uniq uniq filters duplicate entries when used after sorting: To view only duplicates, use -d:
Counting Output Lines with wc To count items in a sorted list, use wc -l: Output: bash 2728 This confirms 2,728 unique items exist in the directories.
Searching Text Patterns with grep grep finds specific patterns in text. Example: Listing compression-related tools using keyword zip: Results: bash bunzip2 bzip2 gunzip gzip unzip zip zipcloak zipgrep zipinfo zipnote zipsplit Additional grep options: -i – Ignore case sensitivity. -v – Show lines that do not match the pattern.
Displaying First or Last Lines with head and tail head shows first lines; tail shows last lines: Useful inside pipelines: Example monitoring log files in real-time using tail -f: To stop monitoring, press Ctrl+C.
Saving Pipeline Output with tee tee allows saving pipeline data while still passing it forward: Now, ls.txt stores the full list, while grep zip extracts relevant results.