Working With Files

Accessing File Contents in UNIX

Accessing file contents is one of the most beneficial features of unix.

Accessing file contents allows us to do significant work.

Opening & Working With Files

To display and work with files in the terminal, the common commands we will be used are the Cat and Tac commands.

The Cat command is a standard command you can use on files such as text files and CSV’s that will allow you to display the contents of the file.

'we will use a simple command prompt to open a file'

cat <file_name.csv>

'we can also read multiple files with the cat command' 

cat file_name1 file_name2 

The cat command goes beyond reading files, we can also Create new files, Modify existing files, and Combine files.

Let’s pick up where we left off in a file system, and make a new file using the cat command.

cd C:/Subdirectory_2/dir_1


                                 |--> file_2.csv
           -----> Subdirectory_1 |
           |                     |---> file.csv
  root ----|    
           |                     
           -----> Subdirectory_2 ---> dir_1
           
cat > file_3.txt



                                 |--> file_2.csv
           -----> Subdirectory_1 |
           |                     |---> file.csv
  root ----|    
           |                     
           -----> Subdirectory_2 ---> dir_1 ---> file_3.txt

Doing this allows us to add contents to a new text file which we can place in our subdirectory dir_1.

We can also Merge Files

cd /Subdirectory_1

cat file file_2 > combined_file

                                
           -----> Subdirectory_1 ---> combined_file 
           |                    
  root ----|    
           |                     
           -----> Subdirectory_2 ---> dir_1 ---> file_3.txt

There are a series of specific modifications we can amke to the cat command to change its function.

Option	Purpose
-n	show line numbers
-b	number non blank lines
file.txt	show full file content
-A	show tabs/chars/ and non printing lines
-s	squeeze repeated blank lines
> combine	join several text files
*.log	Merge matching files
head - n	Select a number of lines to read starting from the top
tail - n	Select a number of lines to read starting from the bottom
> script.sh «‘EOF	write a script

the tac command is another useful tool that builds off of the cat command

While these commands are similar, some functions of the tac command work in Reverse, such as reading files from bottom to top instead of from top to bottom.

we can use the tac command to create files that have reversed contents

 cd /Subdirectory_1
 
                                
           -----> Subdirectory_1 ---> combined_file 
           |                    
  root ----|    
           |                     
           -----> Subdirectory_2 ---> dir_1 ---> file_3.txt
           
           
tac combined_file > reversed_file

                                 
                                 |---> reversed_file
           -----> Subdirectory_1 |
           |                     |---> combined_file 
  root ----|                     
           |                     
           -----> Subdirectory_2 ---> dir_1 ---> file_3.txt

           

Head and Tail commands are other tools in Unix that allow you to view sections of a file with ease

the Head command allows you to view the first 10 Rows of a file. However this can be modified by adding options to our command using the n feature.

head file_3.txt 

'this would automatically read the first 10 lines

head -n 15 file_3.txt'

'this would read the first 15 lines instead'

head -n 5 reversed_file combined_file 

'this would read the first five lines of two files'

Tail

Similar to the tac command, the tail command provides a similar function as the head command except starting at the bottom of the file instead

tail file_3.txt

'this would read the bottom 10 lines of a file.'

With knowledge of both the head and tail commands. We can read sections of a files in the Middle

head -n 10 file_3.txt | tail -n 20 file_3.txt

The open command is another useful tool that allows you to open non text files such as PDF’s.

Finally, we can use commands such as Grep to search for keywords and values in files

Grep -in "keyword" 

We can also tell grep to ignore certain words in files.

Grep -i "keyword"

If we want any words except for some, we can tell grep to find files that do not match to a word.

Grep -v "keyword"

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Summary

The shell can be used for a variety of applications related to file management and working with data. Understanding the file directory is extremely importannt in having strong data organization skills and clean workflows and efficiency when working on large scale projects.

Next Steps

Learning to apply UNIX to script writing import code, and work on other computers are all key when using terminals. Soon, we will be releasing Our BASH fundamentals guide which will go over basic script writing in the terminal!