date
# Fri May 17 11:41:31 MST 2024Commands
Commands are the first part of a command line instruction, specifying the program or built-in functionality to be executed. In Unix/Linux, several commands can operate without any options or arguments, performing their basic functions in their simplest form.
We’re going to start with a basic command: date:
The command date returns the current date and time. What we’ve just done is referred to as the read–eval–print loop, or REPL, and it’s the underlying process of the command-line.
REPL
The REPL in Bash exemplifies a powerful and flexible interface for interacting with the system, running commands, and developing scripts, providing both novice and experienced users with an efficient way to manage their computing environment.
Here is how it works:
1. Read
In the Bash, the “Read” step occurs when the shell waits for input from the user. This is typically represented by the shell prompt, where we can type commands.
The prompt might display useful information, such as the current user (username), hostname (hostname), and working directory (current_directory), depending on its configuration.
user@host:directory$
2. Eval
Once a command is entered, Bash “evaluates” it. This step involves parsing the command and its arguments, checking for syntax correctness, and then executing it.
user@host:directory$ date
Commands can be simple, such as listing the current date and time, or complex scripts involving loops, conditionals, and functions.
3. Print
After evaluating the command, Bash “prints” the output or the result of the command execution to the screen (stdout or standard output) or another specified location.
# Wed Apr 10 02:55:23 MST 2024
Loop
After executing a command and returning the output, Bash immediately returns to the “read” step, displaying the prompt and waiting for new user input.
user@host:directory$
user@host:directory$ date
#> Wed Apr 10 02:55:23 MST 2024
user@host:directory$This cycle repeats indefinitely until the user exits the REPL environment, typically with an exit command or by pressing Ctrl + D.
Commands
In Unix, several commands can operate without any options or arguments, performing their basic functions in their simplest form. Below are some of these commands:
who
who shows who is logged on the system.
who
# username console Apr 14 13:45
# username ttys000 Apr 14 13:45
# username ttys001 Apr 14 13:45
# username ttys003 Apr 16 05:56who by itself, without options or arguments, lists the users currently logged into the system.
whoami
whoami shows the username of the user currently logged into the system.
whoami
# usernamehostname
hostname displays the system’s network name.
hostname
# Users-MacBook-Pro.localcal
cal displays a calender of the current month.
cal
# April 2024
# Su Mo Tu We Th Fr Sa
# 1 2 3 4 5 6
# 7 8 9 10 11 12 13
# 14 15 16 17 18 19 20
# 21 22 23 24 25 26 27
# 28 29 30
# uptime
uptime shows how long the system has been running.
uptime
# 11:39 up 15:05, 4 users, load averages: 3.85 3.08 2.93clear
clear clears the terminal screen and doesn’t print any return values.
clearclear does its job without the need for additional input.
exit
exit exits the shell or your current session.
exitexit requires no arguments or options to execute this action, and doesn’t print any return values.
Recap
Each of these commands performs a specific and often utilized function within the Unix environment, embodying the Unix philosophy of doing one thing well.