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• Count of files and directories

ls -l | grep "^-" | wc -l # count of regular files in current directory
ls -lR | grep "^-" | wc -l # count of regular files in current dir including those in sub-folders

ls -l | grep "^d" | wc -l # count of directories in current directory
ls -lR | grep "^d" | wc -l # count of directories in current directory including those in sub-folders

# Explaination
ls -l # means outputing the information of the specified directory as a list
# -R means recursively, showing the files within directories
| # means using the output of the command before it as the input of the one behind it
grep # means Globally search a Regular Expression and Print
# and "^d" represents directories, "^-" files
wc -l # means counting the number of lines of the output
# which in this case equals to the number of files/directories

Reference: Ubuntu查看文件夹下文件的个数

• View files: cat, more and less

Basically cat can print the whole file in the terminal, which is convenient when file size is relatively small. But when file size is large and pagination is needed, more and less are both competent. A very useful argument that shows the line number is -n (for cat) and -N (for less).

cat是一次性显示整个文件的内容，还可以将多个文件连接起来显示，它常与重定向符号配合使用，适用于文件内容少的情况；more和less一般用于显示文件内容超过一屏的内容，并且提供翻页的功能。more比cat强大，提供分页显示的功能，less比more更强大，提供翻页，跳转，查找等命令；而且more和less都支持：用空格显示下一页，按键b显示上一页。下面详细介绍这3个命令。
Reference: linux中cat、more、less命令区别详解

• Use of scp

# copy local file to remote folder with username designated
# password for the username will then be required
scp local_file remote_username@remote_ip:remote_folder 
# copy local file to remote folder with designated filename
scp local_file remote_username@remote_ip:remote_file 
# copy local file to remote folder without username designated
# username and password will then be required
scp local_file remote_ip:remote_folder 
# copy local file to remote folder with designated filename
scp local_file remote_ip:remote_file 

# copy local folder to remote folder
# attention: the copied folder will be under the remote folder
scp -r local_folder remote_username@remote_ip:remote_folder 
# without username designated
scp -r local_folder remote_ip:remote_folder 

# copy remote file or folder to local
scp root@www.runoob.com:/home/root/others/music /home/space/music/1.mp3 
scp -r www.runoob.com:/home/root/others/ /home/space/music/

Reference: Linux scp命令

• What's in /etc?

/etc/profile # contains the environment variables for all users
~/.bashrc # contains the environment variables for current user

Reference: Linux /etc/profile文件详解

/etc/hosts # is the static table lookup for host name
# which contains the host names and corresponding IP addresses
# web applications will access file like this for hostname-IP mapping
# DNS cache > hosts > DNS service

Reference: linux环境下/etc/hosts文件详解

/etc/fstab # is the file system table configuring partition mounting on startup
# <file system> <mount point> <type> <options> <dump> <pass>
proc             /proc         proc        defaults              0       0
/dev/hda1        /             ext3        errors=remount-ro     0       1
/swapfile        /swap         swap        defaults              0       0
/dev/hdc         /media/cdrom0 udf,iso9660 user,noauto           0       0

Reference: /etc/fstab 参数详解及如何设置开机自动挂载

• Use of curl and wget

# Common use for downloading
curl -O http://man.linuxde.net/text.iso # -O should be uppercase
wget http://www.linuxde.net/text.iso # no argument required

# Download and rename
curl -o rename.iso http://man.linuxde.net/text.iso # -o should be lowercase
wget -O rename.zip http://www.linuxde.net/text.iso # -O should be uppercase

Reference: curl和wget的区别和使用

• Other useful commands

fdisk -l                       # lists all mountable disks
df -l                          # lists all mouted disks
top                            # dynamically shows real-time system status
ps -ef | grep python           # lists all python processes with full format
kill -9 processId              # kills a process with its ID
ifconfig                       # checks the IP address
netstat                        # checks port status
free                           # shows memory usage
fg                             # turns a background job into a foreground one
bg                             # runs a paused job at background
jobs                           # lists all background jobs
&                              # added behind a command to run it at background
# ctrl + z                     # puts a foreground job to background and pauses it
nohup                          # keeps a command running when terminal is closed

Reference:
[1] linux如何mount挂载磁盘并设置开机自动mount
[2] Linux（Ubuntu）之top命令
[3] Ubuntu终止进程的方法(kill、pkill、killall)
[4] Linux 任务相关命令介绍