Add 'posix_shell.md'
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#Shell Cheatsheet
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I am only going over POSIX shell things here as there are a bunch
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of extensions 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 will by no means be a comprehensive guide to UNIX
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(it’s a cheatsheet, not a handbook) but you can always find out
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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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If you are trying to learn how to use the shell, I encourage you to 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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**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), 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 will 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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1. Reditects: 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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You can have multiple redirects per command though more than two
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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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2. 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
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exit code
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if, esle
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while, for
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pipes, redirects
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ctrl-c
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*Builtin commands:*
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- `cd` [change directoy] A shell (or any process for that matter) is always working from a specific working directory. Pretty much self-explanatory what this does.
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- `exit` exit (optionally with an exit code though that feature is more useful for scripts)
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- `export` make the given shell variable an environment variable
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- `unset` unset a variable
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- `jobs` show programs that are currently running in the background
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- `fg` get a program from the background in the foreground
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__Basic commands:__
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- `pwd` [print working directory] self-explanatory
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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 the contents of a file to the standard output. It takes file paths as arguments.
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- `mv` [move] Move or rename a file or directory. It takes two arguments: Origin and destination.
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- `cp` [copy] Copy a file or directoy. It takes two arguments: Origin and destination.
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- `touch`
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- `echo` output text to standard output
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- `test` [more commonly known as `[ ... ]`] used to check whether a condition is true or false, result is passed back using exit code
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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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- `su` [switch user] self-explanatory
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- `rm` [remove] remove a file or directoy
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- `mkdir` [make directory] self-explanatory
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- `find` find a file or directoy by specified criteria
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- `grep` search for regular expressions in text
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- `ps`
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- `id`
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- `uname` [UNIX name] get information about the operating system - usually name, architecture, and build information
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- `chmod` [change mode] change file permissions
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- `chown` [change owner] change file ownership
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- `sed` [stream editor] edit text on the fly
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- `vi` unholy abommination of a text editor :)
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- `kill` kill a given process
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- `less` or `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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. ..
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dev null
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dev urandom
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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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- `free` shows information about used/free RAM on Linux systems
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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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- `sl` a great way to infuriate anyone who happens to type too quickly
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- `sudo` become root (or any other user) by authenticating the current user instead of the target user given the current user is permitted to do so - useful when root login is disabled
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