Shell Command: awk. For getting substring from origin string

入门参考：  ref: https://www.tohoho-web.com/ex/awk.html

awk: for getting substring from origin string

$ cat --help

Usage: cat [OPTION]... [FILE]...

Concatenate FILE(s) to standard output.

With no FILE, or when FILE is -, read standard input.

  -A, --show-all           equivalent to -vET

  -b, --number-nonblank    number nonempty output lines, overrides -n

  -e                       equivalent to -vE

  -E, --show-ends          display $ at end of each line

  -n, --number             number all output lines

  -s, --squeeze-blank      suppress repeated empty output lines

  -t                       equivalent to -vT

  -T, --show-tabs          display TAB characters as ^I

  -u                       (ignored)

  -v, --show-nonprinting   use ^ and M- notation, except for LFD and TAB

      --help     display this help and exit

      --version  output version information and exit

-F       set delimiter

Examples:

  cat f - g  Output f's contents, then standard input, then g's contents.

  cat        Copy standard input to standard output.

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

Examples for using:

$cat test.tsv

111 222 333 444 555
111 222 333 444 555
111 222 333 444 555

$cat test.tsv | awk '{print $2}'  #print column2

222
222
222

$cat test.tsv | awk '{print $3,$4,$2}'  #print format column3,column4,column2

333 444 222
333 444 222
333 444 222

$ cat test2.csv
111 111 111
222 222 222
333 333 333
444 444 444
555 555 555

$cat test2.csv | awk 'NR==2 {print}'  #print all columns of row2.  NR==2 

222 222 222

$ ls -l

total 11232

drwxrwxr-x  7 usr usr      107 Jun 25  2019 cmd

-rw-rw-r--  1 usr usr      102 Aug 17 11:20 compile.log

-rwxrw-r--  1 usr usr      726 Aug  5 15:53 compile.sh

drwxrwxr-x  3 usr usr       74 Jun 25  2019 doc

-rw-rw-r--  1 usr usr      620 Jan 20  2020 go.mod

-rw-rw-r--  1 usr usr     6550 Jun 25  2019 Gopkg.lock

-rw-rw-r--  1 usr usr      655 Jun 25  2019 Gopkg.toml

...

$ls -l |awk '$5<=100 {print $9}' #print filename on size <=100.

doc

...

$ cat testdata
aa bb test@mail.local 001 002
cc dd test2@test.local 003 004

$ cat testdata | awk -F@ '{print $2}'   # -F: set splitword as @, print col2
mail.local 001 002
test.local 003 004

$ cat testdata | awk -F@ '{print $2}' | awk '{print $1}'   #print col1
mail.local
test.local

$cat var_list.txt

./test.sh: line 1: echo: No such file or directory

echo; var_count:0; var:

./duplicate_error_page.html; var_count:0; var:

./main.html; var_count:3; var:,scre_cd,fileName,ajaxParameter

./main.html; var_count:3; var:,scre_cd,parameter,toPlanForm

./main.html; var_count:1; var:,scre_cd

$cat var_list.txt | awk -F '[;:]' '{print $1,$3,$5}'

echo; var_count:0; var:

./duplicate_error_page.html  0 

./main.html 3 ,scre_cd,fileName,ajaxParameter

./main.html 3 ,scre_cd,parameter,toPlanForm

./main.html 1 ,scre_cd

#get lines between including pattern1 and including pattern2. ex.

$cap tmp.html | awk '/<ul class="list-main"/,/<\ul>/' 

<ul class="list-main"> <li>aaaaa</li> <li>bbbbb</li> </ul>

$ awk 'BEGIN {
   num = 5; if (num >= 0 && num <= 7) printf "%d is in octal format\n", num 
}'

$ awk 'BEGIN {
   ch = "\n"; if (ch == " " || ch == "\t" || ch == "\n") 
   print "Current character is whitespace." 
}'

'BEGIN { name = ""; if (! length(name)) print "name is empty string." }'

ref:https://www.tutorialspoint.com/awk/awk_logical_operators.htm