Searching inside files with grep

• The “grep” command can be used to search inside files.

                                $ grep word filename

        

         Ex: to find the lines having the word "File" inside a file named foo.txt

• if you need to match the whole word use the -w option with grep

                             $ grep -w 'word' filename

       Ex: in the previous example even though you searched for the word "File" other words which contain the word "File" as a part of it also got selected.by using -w option we can avoid that

      Ex: i have a file named yumlist.txt containing names of packages,and there are thousands of package names in that file.i need to find the package gcj from that file.if i just search using grep other packages which has gcj as a part of its name will also get selected(eg:-libgcj) , but when i use grep with -w option only the gcj packages will get selected

    

• to ignore case use -i option

         Ex:the word "File" in the file foo.txt starts with a capital "F" ,if i just  search as "file" no line will get selected, but when -i option is used, grep will ignore the case and select the lines.
 

          

• to recursively search files in current directory and all its sub directories use the “grep” command with -r option

   Ex : i need to find if there are any servlet source code files present in the current directory or any of its sub directories.

• to display the line numbers of the lines containing the matching word use -n option

• to list only the file names of the files containing the matching word use the -l option with grep

• grep can also be used to search inside outputs generated by other commands using a pipe

                           $ command | grep 'word'

   

                 Ex : I'm in the directory /usr/bin and i need to find all the icewm files in that directory.

                        if i just type ls 3626 files will be displayed

               so i direct the output of ls command to a grep command and search for icewm files

• you can also use regular expressions when searching using the “grep” command

            

                                    $ grep “regular expression” filename

                                                                  or

                                   $ grep -e “regular expression” filename

                                                           or

                                  $ grep --regexp=“regular expression” filename

Ex: website1.sql is a backup file of an sql database having data on customers,items,orders,purchases,etc . i need to find the customer email addresses ending with a .com from that file.

if you want to learn more on using regular expressions using grep click here

cat and more

“cat” command

 

the cat command can be used to display contents of files ,write into files or to concatenate files.

Displaying a file using “cat” command

     

        $ cat filename

   ex:

  

Writing into files using “cat” command

        $ cat > filename

            #things to be

            # written

            #in the file

            #

once you finish press enter to go to a new line and then press ctrl +D to close file.

   ex:

 

Appending in to a file using “cat” command

      similar to writing a file using cat but use >>

           $ cat >> filename

             # new content

            #to be appended

            #to the file

   ex:

Concatenating files using “cat” command

several files can be concatenated to a single file using cat command

           $ cat file1 file2 file3 > newFile

(* this is useful to merge a file that has been split into several parts by the “split” command )

   ex:

“more” command

The “cat” command displays the entire file at once. So it maybe a disadvantage when viewing a large file using cat because we may only see the last part of the file. Therefore viewing large files the “more” command is a better option.

The “more” command displays a file page by page. 

           $ more filename

Basic File Handling ( Copy ,Move,Rename ,Delete )

"cp" command

     used to copy files.

          cp filepath/filename  destination/filename

        or

         cp filepath/filename  destination

   ex:
        $ cp exit.png  mydir/q.png

               copies the file exit.png into mydir as q.png

        $ cp Library.sql  mydir

            copies the file Library.sql inside the directory mydir

  copying multiple files ex:

"mv" command

     used to move files

       ex:
          $ mv out1-1.ogv mydir/out1-1.ogv

          the file out1-1.ogv is moved in to mydir

    moving multiple files with mv

         mv file1 file2 file3 destination 

 ex:

   use * when copying or moving files to copy or move all the files in the   current directory     

   the "mv" command can be also used to rename files

   ex :

"rename" command

         used to rename files

        rename a b c

      a  - part of the file name to be renamed
      b - new part of the filename
      c - the file

   ex: to rename a file named "aa" to "newAA"

      $ rename  aa  newAA  aa

      to rename the "newAA" as "oldAA" only the part "new" has to be renamed to old

      $ rename new old newAA

 using " -v " option with the command rename prints what is going to be done

 ex: to change the extension of all the png files to jpg

    $ rename -v png jpg *.png

"rm" command

      used to delete files

      rm filepath/filename

     ex:  $ rm myfile
 
         deletes the file named myfile in current directory    

    deleting multiple files with rm

   ex:   $ rm file1 file2 file3