Linux Command Line(I): Beginner

考試月終於暫告一段落，終於有時間回歸Linux 的懷抱。不知怎的，在VMware Workstation 12 上登入Ubuntu後總是blue screen，明明昨天用terminal 也沒有事啊真令人摸不著頭腦@@ 算吧，反正有kali linux 用著吧。早晚VMware 會被換掉的。

哈哈，但是荒廢太久的關係，連賬號密碼也給忘了。被迫爬文：http://www.technig.com/reset-lost-password-of-kali-linux/

昨天剛開始，試了一下command:

pwd: print working directory

cd: change directory，cd ~ 就是回家的意思。

ls: list current directory

rm -rf <--哈哈這個當然沒有，也千萬不要試(remove, forced recursive: remove every OS files!!!)

在mkdir上除了一點狀況。實際上要用 mkdir -p /apple/orange/banana, 但是這個指令會overwrite existing directories. 就是，無論舊的、同名的directory裡面有什麼，都會被移除。還有，用rmdir 的話，如果targetted directory 不是空的，那就不能用這個。you must have to delete the sub-directories beneath it first.

哎，雖然學了普通話拼音都有這些年了，但是用拼音輸入還是很慢。還不如用英語輸入。

But noted you cannot delete the particular directory where you are in.

Also, for beginners, it's very easy to forget the '/' preceeding the name of directory. It is particular problematic when we are trying to use rmdir, cd, etc.

Another common error when using 'cd' is forgetting to place '/ <current_directory>' in front of the targetted directory. It causes an error "such directory does not exist"...

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mkdir  vs mkdir -p

mkdir -p /apple: establish 'apple' directory in parent directory. Therefore, changes invisible in root.

mkdir /root/apple: changes visible in root

Similarly, 'rmdir /apple' would only remove that in parent directory, not that in the root.

Common mistake in using mkdir, rmdir, cd

$ mkdir root/pineapple 　　　　# missing '/' before 'root'

$ mkdir /root/pineapple/banana　　#greedy declaration: pineapple missing directory

$ rmdir /pineapple/banana　　　　# missing '/root': directory DNE

$ rmdir /root/pineapple/banana

$ cd /banana 　　#missing '/root' in changing directory; directory DNE

$ cd /root/banana

$ rm /root/pineapple　　　　#rm: for removing non-empty directory; rmdir: remove empty dir

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pushd vs popd

remark: pushd, pushes current directory into stack and go to the destinated directory.

!! No confusion: pushd does not push the destinated directory into stack!

popd, pop the lastest visited directory

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File I/O

touch new.txt　　#establish a new text file

cat >> new.txt << EOF　　#open txt file for editing 

START RANDOM WRITING IN FILE

Hello World this is my first linux txt document!!

EOF　　#indicate end of file-input; return to parent directory 

cat new.txt　　#view the txt file in terminal 

cat << new.txt << EOF　　#open txt file and move cursor to the end of file

Overwriting in process...

EOF

cat /root/new.txt

START RANDOM WRITING IN FILE

Hello World this is my first linux txt document!!

Overwriting in process...　　#overwrite existing txt failed :(

echo > a.txt　　#empty the txt file 

echo >> Hello! >> /root/hello_world.txt　　#append at EOF

rm hello_world.txt 　　#directly remove a file in terminal 

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How to check the type of shell your Unix/Linux is using

ps -p $$　　#well, usually it's bash shell

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To-slash, or not to slash

~# ls

> Documents　　Music　　Pictures

~# cd Documents　　#access under current directory, need not '/'

c.f.

~# pwd

/root/Documents

~# cd /root/Pictures　　#access directories not at current level, use '/'

** same principle for 'rmdir', e.g. rmdir apple, rmdir hello_world.txt, iff in current directory

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cp: copy file or directory

#this command bit-wise copy from one document to another

~# ls

a.txt

~# cp a.txt b.txt

~# ls

> a.txt　　b.txt

~# mkdir something

~# ls

> a.txt　　b.txt　　something

~# cp -r something another

~#ls

> a.txt　　b.txt　　something　　another

===========================

Forcefully remove non-empty directory

sudo rm -r somthing　　# '-r' usually for directory-level command

c.f.

cd -r somthing anotherThing　　#copy entire directory

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Move a file from one place to another

~# pwd

> /root/temp

~# ls

> a.txt　　b.txt　　another

~# mv a.txt /root　　#move file from one directory to next 

~# mv another /root 　　#move directory from one to another 

~# pushd /root

~# ls

a.txt　　Documents　　Music　　Pictures　　temp

~# cd temp

> b.txt another

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題外話：昨晚寫了一個小工具程序給哥哥，他好像很喜歡呢<3 太好了！原來路徑用程序下載時default的路徑就可以了，根本不用在batch當裡面動態產生。哎，真爬文爬死了。看來要認真的學batch的基礎。還有，原來哥哥都會用虛擬機，顆顆真棒！都說我的哥哥是宇宙最強的哈哈哈哈<3 瞎扯瞎遠了，言歸正傳。

每次開虛擬機linux terminal 都耗時很久，等到它開了已經沒有熱情了，還是用一下網上的好了：https://www.tutorialspoint.com/execute_bash_online.php

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Opening txt file in bash

less a.txt

q　　#quit viewing txt

less vs cat

less a.txt b.txt 　　#error: less for openning one document only one in a time!

cat a.txt b.txt　　#allowed

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結語

這篇可算是第一篇結案的文章，算是有頭有尾吧，哈哈。這裡都是非常基本的bash shell command lines。接下來可能的發展方向是學windows' command prompt，或是繼續發掘:https://www.gnu.org/software/bash/manual/bashref.html

但是無論如何，都會跟大家在新的一篇裡面見面咯:)

～～全文完～～