231 lines
10 KiB
Markdown
231 lines
10 KiB
Markdown
# Shell Cheatsheet
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## Introduction
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I am only going over POSIX shell things here as there are a bunch of extensions
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found in some shells / on some systems but not others.
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The sections below should apply to any UNIX-like system.
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This cheatsheet is by no means a comprehensive guide to UNIX shell scripting
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(it’s a cheatsheet, not a handbook, and I don’t know everything) but you can usually
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find out more information about any given command right there on the shell.
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Many systems come with manuals, try `man COMMAND` or
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`info COMMAND` to find out more about most utilities
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(try `man man`, for example).
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Alternatively, if no manual is available,
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passing the flag `-h` or `--help` will usually print some information.
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I find myself consulting these constantly because nobody is expected
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to remember everything beyond the basics and it’s usually faster than googling it.
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That said, googling for a solution is generally also a viable option.
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If you are trying to learn how to use the shell, I recommend you start
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with the examples section at the end and work your way backwards from there
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to figure out what they do.
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I also highly recommend you check the manuals or help information for
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any command you want to use - especially if it was given to you
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by a stranger on the internet.
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## Shell Features
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The shell’s main tasks are providing a text based user interface to an
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operating system as well as running scripts.
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To facilitate this, the shell runs the programs the user/script specifies
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and deals with the output.
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**Working Directory**
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A shell (or any process for that matter) is always running inside a specific directory
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on the file system, the so-called working directory.
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It is relevant when determining where files specified by relative path are located.
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There are two kinds of file paths in the UNIX world: relative and absolute.
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Absolute paths refer to a fixed location on the file system and start with a `/`.
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Relative paths refer to a location relative to the current working directory and cannot start with a `/`.
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Relative paths are just the equivalent of an absolute path with the working directory cut off.
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In many cases, just the name of a file or subdirectory inside the working directory is used.
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You can change to another working directory using `cd` (change directory) like so: `cd PATH_TO_DIRECTORY`.
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When no path is specified, `cd` takes you to the current user’s home directory (usually...).
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**Streams**
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Normally, there are two output streams and one input stream for a running program:
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Standard output (stdout), standard error (stderr), and standard input (stdin).
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By default, they are passed to the stdout, stderr, and stdin of the shell,
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which end up in your terminal when you run a shell interactively.
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There are two notable exceptions to this which you can specify:
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**Redirects**: You can send the output of a command to a file instead of
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the standard output/error streams.
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- `COMMAND > FILE` redirects stdout of COMMAND into FILE.
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- `COMMAND 2>FILE` redirects stderr into FILE.
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- `COMMAND 2>&1` redirects stderr into stdout.
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- `COMMAND < FILE` puts the contents of FILE into stdin.
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You can have multiple redirects per command though more than three usually don’t make sense.
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The most common example of this is redirecting stderr to stdout and
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the combined stdout into a file like so: `COMMAND > FILE 2>&1`.
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**Pipes**: The output of one command can be used as the input of another:
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`COMMAND | OTHER_COMMAND`
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**Variables**
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There are two kinds of variables that you can deal with on the shell, let’s call them
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shell variables and environment variables. They behave the same when interacting with them
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on the shell but shell variables are only available in the shell whereas environment variables
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are handed to a new process when it is spawned.
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The shell can set environment variables for its child processes and it itself has variables
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from the environment it launched with. The environment it launched with is passed on
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to the processes it spawns.
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Shell variables can be set using `VARIABLE=VALUE`.
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They can be added to the environment of subsequent commands using `export VARIABLE`.
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To alter the environment for just one command, prepend it with the variable like so: `VARIABLE=VALUE COMMAND`.
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Variables can be unset using `unset VARIABLE`.
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Unknown or unset variables are ignored.
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**Control Flow**
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When a command finishes running, it produces an exit code that gives some rudimentary status information
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Usually, an exit code of 0 indicates success and anything else an error of some kind.
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This can be used in control structures (while loops, conditions) or directly accessed using the special
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shell variable `$?`.
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If statements are built as follows:
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```sh
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if COMMAND; then
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OTHER
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COMMANDS
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HERE
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else
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MORE
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COMMANDS
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HERE
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fi
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```
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There are also two operators that can be used as short forms of if and else: `&&` and `||`.
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They can be combined and chained as needed.
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Commands that are chained using `&&` only run if the previous command exited with exit code 0.
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Commands that are chained using `||` run if the previous command *didn’t* exit with exit code 0.
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In many cases, this is used as follows: `COMMAND && THEN_COMMAND || ELSE_COMMAND`
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While loops are built as follows:
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```sh
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while COMMAND; do
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OTHER
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COMMANDS
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HERE
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done
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```
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The `[ ... ]` seen in many if statements and while loops is not actually part of the shell syntax
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but an alias to the `test` command (see below).
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For loops are different in that they act as "for each" instead of checking a condition.
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```sh
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for VARNAME in STRING; do
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COMMANDS
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USING $VARNAME
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HERE
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done
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```
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You can break out of a loop using `break`.
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You can end the shell session or script using `exit`. It optionally takes an exit code as argument.
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**Job Control**
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You can send a process to the background by adding an `&` at the end of the command.
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The `jobs` command shows programs that are currently running in the background.
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You can get a process from the background into the foreground using `fg`.
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Alternatively, if you just want to wait for all jobs to finish, `wait` does that.
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**Miscellaneous**
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Spaces separate arguments and newlines separate commands. To override this, use quoted strings.
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Single-quoted strings are taken verbatim, variables in double-quoted strings get replaced.
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The POSIX shell is case-sensitive but it’s not whitespace-sensitive meaning you can put
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multiple whitespace characters and they count as one or get ignored.
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You can put multiple commands on the same line using semicolons.
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This is commonly used for formatting as it makes scripts more readable.
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The built-in `read` command can be used in scripts to read input into a variable.
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Command line arguments given to a script end up in the variables `$0` through `$9`.
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They can be moved to the left using `shift`. `$0` starts out being the name of the script itself.
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You can send Ctrl+c to the terminal to send an interrupt to the process running inside it.
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Note that this is a feature of the terminal, not of the shell itself.
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Usually, this is used to stop whatever child process of the shell is running in the foreground.
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## Commands
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This section gives you a list of commonly used UNIX utilities.
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This does not include shell-builtins as I have covered them above already.
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Some shells have built-in versions of some of these commands but they should mostly work
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the same as their dedicated counterparts.
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- `sh` (shell) The shell itself, can also be used to invoke scripts
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- `echo` output text to standard output
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- `pwd` (print working directory)
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- `ls` (list) Get a listing of the specified directory. If none given, the current directory is used.
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- `cat` (concatenate) Originally created to concatenate files, it is most commonly used to print
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the contents of a file to the standard output.
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- `mkdir` (make directory) self-explanatory
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- `touch` Change the access and modification timestamp of a file. Also commonly used to create empty files.
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- `mv` (move) files or directories
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- `cp` (copy) files or directories
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- `rm` (remove) files or directories
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- `test` (more commonly known as `[ ... ]`) used to check conditions, result is passed back using exit code
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- `true` returns exit code 0
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- `false` returns non-zero exit code
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- `grep` search for regular expressions in text
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- `tr` (text replace) replace all occurrences of a character in text
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- `sed` (stream editor) edit text on the fly
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- `uname` (UNIX name) get information about the operating system
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- `id` (identity) get information about users
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- `su` (switch user)
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- `ps` (process status) get information about running processes
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- `kill` kill a given process
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- `find` find a file or directoy by specified criteria
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- `chmod` (change mode) change file permissions
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- `chown` (change owner) change file ownership
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- `df` (disk free) show available disk space
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- `du` (disk usage) show how much space a given file or directoy is using
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- `vi` unholy abommination of a text editor :)
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- `more` print only a screen worth of text at a time and wait so you have the time to read it
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## Commonly Used Special Files
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- `.` The working directory
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- `..` Parent directory of the working directory
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- `/dev/null` Your personal trash can to redirect all the output you don’t care about to.
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Also known as a black hole or the infinite void. :)
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- `/dev/urandom` Source of infinite random data
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- `/dev/zero` Source of infinite null bytes
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## Honorable Mentions
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The following things won’t be available on all systems, but are really handy when they are:
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- `less` better version of `more`
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- `nano` nice user-friendly terminal text editor available on many systems
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- `htop` nice user-friendly terminal task manager
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- `which` easy way to find out where the binary for a given command is located
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- `sudo` become root (or any other user) by authenticating the current user
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instead of the target user given the current user is permitted to do so - useful when root login is disabled
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- `free` shows information about used/free RAM on Linux systems
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- `/dev/stdin`, `/dev/stdout`, and `/dev/stderr` special files for you guessed what
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- `sl` a great way to infuriate anyone who happens to type too quickly
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# Examples
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Below are some examples that you can use to better understand above material.
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... to be finished another time ... |