Basic Unix Commands - Part3

File Descriptors:

Standard input        <          0

Standard Output     >          1  Append >>

Standard Error       2>         2

cat < file1   To list the contents of the file. 

ls –l > out1 --- Redirecting the output to the file out1.

ls –l >> out1 --- Appending the output to the file out1.

cp file1 file3   2> error1   --- If any error, the error redirects to the file error1.

Pipes and Filters:

Ist command | 2^nd command

First command output is going to input of 2^nd command.

wc  -- word count

ls –l | wc –l

who | wc –l

cut  --- cut the columns or characters :

cat /etc/passwd | cut –d “:” –f1   ----- This command cut the username alone in passwd file.

cat /etc/passwd | cut –d “:” –f1-4

cat /etc/passwd | cut –d “:” –f1,3

cat /etc/passwd | cut –c1-7

cat /etc/passwd | cut –c5-8

cat /etc/passwd | cut –c1,7

To list contents of the file:

ls –l | more

ls –l | less

To list first 10 lines of the file:

cat /etc/passwd | head

To list last 10 lines of the file:

cat /etc/passwd | tail

cat /etc/passwd | head -5

cat /etc/passwd | tail -5

tail –f /var/log/messages     -------- It follow the growth of the file. This –f  option is very helpful to the administrators.

cat /etc/passwd | pg   ----- pagewise

To search the file names in the file system:

find <path><-options><actions>

find /home/senthil/  -name f1 –print   --- to search a file name called f1.

Few options in find command:

-name

-print

-type

-inum

-perm

-user

-mtime

-atime

-mmin

-amin

-depth

-exec

-ok

find /home/senthil –name f1 –type f –print

find . –type f –print ----- to list only files in the current directory.

find . –type l –print  ---- to list the softlink files.

find . –type f –mtime 2 –print

find . –name f1 –type f –exec rm { } \ ;  --- To find and delete.

Read more on man find

To search a pattern in the file contents use grep command:

cat /etc/passwd | grep root  

cat /etc/passwd | grep –w root

cat /etc/passwd | grep ^root    ------------- To start with root

ls –l | grep ^-    ---- to list only files.

cat /etc/passwd | grep nologin$    ---- the word end  with nologin

To sort the output  use sort command:

cat names | sort   --- sorted output of names file.

cat names | sort | uniq  -- suppress the duplicate values.

cat names |sort|uniq |tee snames |more

tr command is used to translate

Senthilkumar Muthusamy

9865831007