Shell:. Day09 notes

awk [single programming language interpreter]
1, awk introduce
 full name: Aho Weinberger Kernaighan three men acronym;

 In 1970 for the first time on a Unix machine, then use it in the open field;
 so we use in Linux, renamed GNU awk; so, in fact, Linux is called gawk;

 grep line filter
  -o -i -v -E grep egrep fgrep write anything to match what []
 sed line editor
  -np 1, address delimited 3,5, / pat / e [2], the command pa \ i \ WC \ S / PAT / STR / G |. 1 | 2 .. | &
  $ {var / PAT / STR $} var {// PAT / STR}
  $ {} Test, Test # $ {}, {$ Test: offset : length}, $ {test # * word}, $ {test ## * word}, $ {test% word}, $ {test %% * word}, $ {test / pattern / string}, $ {test / / pattern / String}, {$ Test: -word}
 
 awk report generator
  by pattern matching and language format itself, to obtain, and outputs the required contents of the client;
 [default, not three tools to edit Source File】

 Examples: Get user id above system is equal to 1 less than the user 500 user name and user ID
 for I in $ (Cut -d: - f3 / etc / the passwd); do
  IF [I $ I $ -ge 1 -a - 500 Le]; the then
   echo grep $ I | Cut -d: -f1,3
  Fi
 DONE
 
 awk -F: '{IF ($. 3> = $. 3. 1 && <= 500) {}}. 3 Print $' / etc / the passwd
 # final awk to achieve this function only when you need a word on it!

 Significance formatted output:
 awk -F: -v the OFS = ":" 'the BEGIN {the printf "username UID \ n-======================== \ n "} {if ($ 3> = 1 && $ 3 <= 500) {printf" user name:% - 10s UID:% - 10d \ n ", $ 1, $ 3}} END {printf" ------- -------------------- \ nend \ n "} ' / etc / passwd

2, awk works
 xxxxx (Figure)

3, awk usage
 awk [the Option] ... 'Program' FILE ...
 1, Program must be used! apostrophe!
 2, a number of program statements use braces contain up to be side by side, you can nest

 awk '{print}' /etc/passwd

4, the common Option awk
 -F specified delimiter
 awk -F [/:] '{ print $ 1, $ 3}' a.txt
  which represents any one of a plurality of characters in []
 -v because awk is a compiled language , a variable can define their own, but also has built-in variables (environment variables and the like)
  manually specify variable parameters
 awk -va = "a / b" '{print a}' a.txt
  to assign a print a variable
  1, a A custom variable is the FS = -v ":"
  2, call in awk variable call without adding $ symbol
 awk '{a = "a / b"; print a}' a.txt

 Extended: understand the difference between the cut and awk;

. 5, the syntax awk - Program
 . 1, Print
  default output (on the screen)
  is not stored in the command awk, we can associate another command to save the results of awk;
 awk 'A = { "A / B"; Print } a 'a.txt | TEE a.bak
 
 2, the printf - the formatting output
  printf "% s% d is the best class students learn" variable 1, variable 2 
  [] with only sequential relationship
  specifier
   % string s
   % d% i value
   % e% E scientific calculation value
   % c ACSii code value
   % f float
   % u unsigned integer
   %%% show only their escape character
  modifier
   default is right-aligned
   - left aligned Representative
   % 5.4F  
   5-digit percentage
   four decimal places taken
 awk '/ ^ UUID / {printf " is mounted file:% - 50s mount point:% - 10s file system formats:% - 10s \ n", $ 1, $ 2, $ 3 } '/ etc / fstab
  Note: the address mentioned here delimited
  sed / PAT1 /, / PAT2 /
 
 3, variables (built-in variables, custom variable)
  built-in variables - environmental variables (bash) (env, set -C + C)
   variable awk language supported by default
   FS separator defining input variables
   OFS separator defining output variables
   the number (a variable after the last partition $ NF) NF parameters after the partition definition line
    * time variable references, do not add $, $ 0, $ ... $. 1 n-
  awk -v the FS = ":" '/ \ / the bash $ / {Print $. 1, $ of NF} '/ etc / the passwd
   NR definitions file number of rows, defining a plurality of files, the line number is superimposed
   FNR document count only their row number
  awk' {print NR} '/ etc / fstab / etc / the passwd
  awk '{Print the FNR}' / etc / fstab / etc / the passwd
   name of the file is stored fILENAME
  awk '{print fILENAME}' / etc / passwd // N times to print the file name, the file number of rows N
  awk 'bEGIN {print FILENAME}' / etc / passwd // bEGIN { statement} row beginning of the cycle only performed once;
   the number of stages of the entire command ARGC [Note: do not contain 'program' Itself]
   ARGV array for calling command, a designated section ARGV [2] [Note: the array nor the 'program']
  awk 'Print ARGC the BEGIN {}' / etc / the passwd / etc / fstab / etc / Shadow
  awk 'Print ARGV the BEGIN {[. 3]}' / etc / the passwd / etc / fstab / etc / Shadow 
   the RS specified newline \ n can specify a new line breaks, line breaks itself does not affect the
   ORS output when the specified line breaks, line breaks replace the default specified character
  awk -v the RS = "" 'Print {}' / etc / the passwd
  awk ORS = -v " " '{print}' / etc / passwd // wrap may be used to cancel the
  
  custom variable
   -v variable = value
    when the rear 'program', calls the custom variables can be directly used
   or" variable value = "statement written directly 'program' can;
  awk -VA = "A / B" 'Print {A}' a.txt
  awk 'A = { "A / B";} A Print' a.txt