Linux text file reading

describe

Text viewing and reading of any operating system is the most basic function;
There are mainly seven commands (cat/tac/nl/more/less/head/tail) involved in text reading in linux system

cat command

• Command purpose

  • Read all information from the file
  • Link and print the contents of multiple files to standard output

• Command syntax
  cat [OPTION]... [FILE]...

• Common options

  option	describe
  -A	Displays invisible characters (control characters, line breaks, tabs) in the contents of the file
  -n	All line numbers are displayed while displaying the content, starting from 1
  -b	Display the non blank line number while displaying the content, starting from 1

• Example

  [root@centos-36_2 data]# cat myfile
  first line
          second line 
  
   have space
  last line
  
  [root@centos-36_2 data]# cat -A myfile
  first line$
  ^Isecond line $
  $
   have space$
  last line$
  $
  [root@centos-36_2 data]# cat -n myfile
       1  first line
       2          second line 
       3
       4   have space
       5  last line
       6
  [root@centos-36_2 data]# cat -b myfile
       1  first line
       2          second line 
  
       3   have space
       4  last line
  
  [root@centos-36_2 data]# 
  

tac command

• Command purpose

  • When reading a file, all information is displayed in reverse lines

• Command syntax
  tac [OPTION]... [FILE]...

• Example

  [root@centos-36_2 data]# cat myfile
  first line
          second line 
  
   have space
  last line
  
  [root@centos-36_2 data]# 
  [root@centos-36_2 data]# tac myfile
  
  last line
   have space
  
          second line 
  first line
  [root@centos-36_2 data]# 
  

nl command

• Command purpose

  • Add line number for non empty line when reading file

• Command syntax
  nl [OPTION]... [FILE]...

• Example

  [root@centos-36_2 data]# cat myfile
  first line
          second line 
  
   have space
  last line
  
  [root@centos-36_2 data]# nl myfile
       1  first line
       2          second line 
         
       3   have space
       4  last line
         
  [root@centos-36_2 data]# 
  

more command

• Command purpose

  • Displays one screen at a time when reading a file
  • Text viewing tool based on vi editor, which can flip the screen up and down or find keywords

• Command syntax
  more [OPTION]... [FILE]...

• Common options

  option	describe
  -<num>	Set the number of lines displayed per screen
  +<num>	Sets the display from the specified number of rows

• Example

  [root@centos-36_2 data]# seq 10
  1
  2
  3
  4
  5
  6
  7
  8
  9
  10
  [root@centos-36_2 data]# seq 10 | more +5 -3
  5
  6
  7
  --More--
  

less command

• Command purpose

  • Displays one screen at a time when reading a file
  • It is similar to the more command, but supports flipping back and forth

• Command syntax
  less [OPTION]... [FILE]

• Common options

  option	describe
  -e	Automatically exit after displaying content
  -N	All line numbers are displayed while displaying the content, starting from 1

head command

• Command purpose

  • When reading a file, the first few lines of information are displayed, and the first 10 lines are displayed by default

• Command syntax
  head [OPTION]... [FILE]

• Common options

  option	describe
  -n <num>	Sets the number of rows to display header content
  -c <num>	Sets the number of characters to display the header content
  -v	Display file name information

• Example

  [root@centos-36_2 data]# head -n 3 -v  myfile
  ==> myfile <==
  first line
          second line 
  
  [root@centos-36_2 data]# 
  [root@centos-36_2 data]# head -c 10 myfile
  first line[root@centos-36_2 data]# 
  [root@centos-36_2 data]# 
  [root@centos-36_2 data]# cat myfile
  first line
          second line 
  
   have space
  last line
  
  [root@centos-36_2 data]# head -n 3 -v  myfile
  ==> myfile <==
  first line
          second line 
  
  [root@centos-36_2 data]# head -c 10 myfile
  first line[root@centos-36_2 data]# 
  

tail command

• Command purpose

  • When reading a file, several lines of information at the end are displayed, and the last 10 lines are displayed by default

• Command syntax
  tail [OPTION]... [FILE]

• Common options

  option	describe
  -n <num>	Sets the number of lines to display trailing content
  -c <num>	Sets the number of characters to display the trailing content
  -f	Display the newly added content in the file in real time
  -v	Display file name information

• Example

  [root@centos-36_2 data]# cat myfile
  first line
          second line 
  
   have space
  last line
  
  [root@centos-36_2 data]# tail -n2 -v myfile
  ==> myfile <==
  last line
  
  [root@centos-36_2 data]# tail -c10 -v myfile
  ==> myfile <==
  ast line
  
  [root@centos-36_2 data]# 
  [root@centos-36_2 data]# tail -n3 -vf myfile
  ==> myfile <==
   have space
  last line