cli-components.md

CLI Components

Use this page as a quick reference for shell operators, streams, and redirection.

Common Operators

• && - Run the second command only if the first command succeeds.
  • Example: mkdir reports && cd reports
• || - Run the second command only if the first command fails.
  • Example: grep "needle" file.txt || echo "not found"
• ; - Run the next command regardless of whether the previous command succeeds.
  • Example: whoami; hostname
• | - Pipe standard output from one command into the next command.
  • Example: ps -ef | grep ssh
• & - Run the command in the background.
  • Example: sleep 60 &
• > - Redirect standard output to a file, overwriting the file.
  • Example: date > run.log
• >> - Append standard output to a file.
  • Example: date >> run.log
• < - Read command input from a file.
  • Example: sort < names.txt

Streams

• stdin - Standard input, usually data going into the program.
• stdout - Standard output, usually normal command output.
• stderr - Standard error, usually errors and diagnostic messages.

Stream Redirection

• 2> - Redirect errors to a file.
  • Example: grep needle missing.txt 2> errors.log
• 2>&1 - Send errors to the same place as standard output.
  • Example: command > output.log 2>&1
• /dev/null - Discard output.
  • Example: command > /dev/null 2>&1