Linux Commands

grep

search for pattern in filename
grep pattern filename
-i: captial insensitive
-r: search all the file in the directory
grep -r admin /etc/

wc

$ wc demo.txt
7459   15915  398400 demo.txt
lines  words  bytes
行数    单词数  字符数    

sed

sed 's/ /-/g' example.txt
replace all the space with -
Hello This is a Test 1 2 3 4 -> Hello-This-is-a-Test-1-2-3-4

sed 's/[0-9]/d/g' example.txt
replace all the numbers with d
Hello This is a Test 1 2 3 4 -> Hello This is a Test d d d d

sort

sort example.txt
example.txt

f
b
c
g
a
e
d

sort example.txt

a
b
c
d
e
f
g

sort example.txt | sort -R randomly
sort example.txt | uniq show unique lines
sort example.txt | uniq -c show unique lines and times

cut

cut -d " " -f2,7,9 example.txt show the 2,7,9 words connected with “ “
red riding hood went to the park to play -> riding park play

echo

echo -ne "Hello\nWorld\n"
“\n” output
Hello
World

fmt

cat example.txt | fmt -w 20: print each line with 20 words

tr

cat example.txt | tr ‘a-z’ ‘A-Z’: change a-z -> A-Z
cat example.txt | tr ' ' '\n': change ‘ ‘ -> ‘\n’

nl

nl -s". " example.txtshow the number of line

 1. Lorem ipsum
 2. dolor sit amet,
 3. consetetur
 4. sadipscing elitr,
 5. sed diam nonumy
 6. eirmod tempor
 7. invidunt ut labore
 8. et dolore magna
 9. aliquyam erat, sed
10. diam voluptua. At
11. vero eos et
12. accusam et justo
13. duo dolores et ea
14. rebum. Stet clita
15. kasd gubergren,
16. no sea takimata
17. sanctus est Lorem
18. ipsum dolor sit
19. amet.

passwd

change password

date

show current date

cal

show the calendar for this month

finger

finger username display the info of the user

uptime

show the running time

uname

uname -a show kernel info

df

show usage of disk

du

du filename show the size of directory and file
-s: only show the total size

ps

ps -u yourusername show the processes

kill

kill PID

killall

killall processname

top

show current process

bg

fg

dig

dig domain dig the info of the domain

wget

wget file download the file

bash

${varname:-word}    # 如果varname存在且不为null，则返回其值; 否则返回word
${varname:=word}    # 如果varname存在且不为null，则返回其值;否则设置它，然后返回其值
${varname:+word}    # 如果varname存在并且不为null，返回word; 否则返回null 
${varname:offset:length}    # 执行子字符串扩展。它返回$ varname的子字符串，从offset开始，最多为length的字符
2.2 String Substitution

string

${variable#pattern}         # if the pattern matches the beginning of the variable's value, delete the shortest part that matches and return the rest
${variable##pattern}        # if the pattern matches the beginning of the variable's value, delete the longest part that matches and return the rest
${variable%pattern}         # if the pattern matches the end of the variable's value, delete the shortest part that matches and return the rest
${variable%%pattern}        # if the pattern matches the end of the variable's value, delete the longest part that matches and return the rest
${variable/pattern/string}  # the longest match to pattern in variable is replaced by string. Only the first match is replaced
${variable//pattern/string} # the longest match to pattern in variable is replaced by string. All matches are replaced
${#varname}     # returns the length of the value of the variable as a character string

expression

statement1 && statement2  # 两边的条件都为true
statement1 || statement2  # 其中一边为true

str1=str2       # str1 匹配 str2
str1!=str2      # str1 不匹配 str2
str1<str2       # str1 是否小于 str2
str1>str2       # str1 是否大于 str2
-n str1         # str1 不为空(长度大于 0)
-z str1         # str1 为空(长度为 0)

-a file         # 文件存在 
-d file         # 文件存在，是一个目录 
-e file         # 文件存在; 相同的-a 
-f file         # 文件存在，是一个常规文件（即不是目录或其他特殊类型的文件） 
-r file         # 你有读权限 
-r file         # 文件存在，不为空 
-w file         # 你有写权限 
-x file         # 你有文件的执行权限

file1 -nt file2     # file1 is newer than file2
file1 -ot file2     # file1 is older than file2

-lt     # 小于 
-le     # 小于或等于 
-eq     # 等于
-ge     # 大于或等于 
-gt     # 大于
-ne     # 不等于

Ref:
https://www.cnblogs.com/savorboard/p/bash-guide.html