5 ways to examine the content of files on Linux

Linux provides many commands for examining the contents of files including cat, more, head and tail, but that’s just a start.

For one thing, even the most obvious commands have a lot more options than many Linux users ever get around to using. And there are some less obvious commands that offer some unique features. In this post, we’ll both at commands for viewing the contents of files and options for tailoring those views to better cater to your needs.

cat

The cat command sends the entire contents of text files to your terminal window for viewing. In fact, if you type “cat” followed by the name of a file with thousands of lines, those lines will whiz by your window so fast, you won’t be able to make out much more of it than the last screenful of text. Yet, as familiar as the cat command is to Linux users, even this basic command provides a lot of useful options such as numbering the lines in the output that many of us likely haven’t ever used. To expand on that, not only can you number lines; you have some choices in how you do the numbering.

Numbering every line looks like this:

$ cat -n msg
     1  Hello --
     2
     3  I hope you are having a wonderful day!
     4
     5
     6  That's it for ...       now
     7
     8  bye!
     9
    10  s.

You can also number only lines with content. Note that a line containing only blanks is not considered “empty” for this command and would be numbered.

$ cat -b msg
     1  Hello --

     2  I hope you are having a wonderful day!


     3  That's it for ...       now

     4  bye!

     5  s.

The cat command allows you to ignore repeated blank lines with the -s option, but you have to add another command to suppress blanks lines altogether.

$ cat -s msg
Hello --

I hope you are having a wonderful day!

That's it for ...       now

bye!

s.

To ignore all of the blank lines, just pipe the output from cat to a grep command as follows. The dot (.) matches text including blank characters, so it will display lines that contain only blanks and only look empty.

$ cat msg | grep .
Hello --
I hope you are having a wonderful day!
That's it for ...       now
bye!
s.

The -E optoin provides a visual cue to show whether there are extra blanks at the ends of lines by sticking a $ at the end of every line.

$ cat -E msg
Hello --$
$
I hope you are having a wonderful day!  $
$
$
That's it for ...       now$
$
bye!$
$
s.$

With -A, you get both the $ characters at the end of each line and tabs showing up as ^I rather than empty space.

$ cat -A msg
Hello --$
$
I hope you are having a wonderful day!$
$
$
That’s it for …^Inow$
$
bye!$
$
s.$

Displaying portions of files with head and tail

The head and tail commands show the tops or bottoms of files and default to ten lines. You can specify a different number of lines to view by using strings like -3 (show 3 lines) or -11 (show 11 lines). The tail command works the same way as head but displays the bottoms of files rather than the tops.

$ head -3 msg
Hello --
I hope you are having a wonderful day!
$ tail -3 msg
bye!

s.

You can also combine head and tail commands to view text between the tops of bottoms of files. You just have to pick your starting point and how many lines you want to see. In this example, the command would display the second hundred lines in a file and, with cat‘s help, numbering those lines.

$ cat -b mybigfile | head -200 | tail -100
   101  Invoice #2020-06-07a sent to vendor
   ...

Browsing text a screenful at a time with more or less

The more command is an obvious choice for browsing through a file’s content a screenful at a time while less adds the ability to move up and down in a file by using the up and down keyboard arrows so that you can start scanning and then back up in the file.

Looking at text files two ways at once with od

The od (octal dump) command allows you to view a file both as normal text and as a series of ASCII values (i.e., how that text is actually encoded in the files). As you can see in the example below, the numbered lines show the numeric ASCII values and the alternate lines show the text and non-printable characters.

$ od -bc msg
0000000 110 145 154 154 157 040 055 055 012 012 111 040 150 157 160 145
          H   e   l   l   o       -   -  n  n   I       h   o   p   e
0000020 040 171 157 165 040 141 162 145 040 150 141 166 151 156 147 040
              y   o   u       a   r   e       h   a   v   i   n   g
0000040 141 040 167 157 156 144 145 162 146 165 154 040 144 141 171 041
          a       w   o   n   d   e   r   f   u   l       d   a   y   !
0000060 012 012 012 124 150 141 164 047 163 040 151 164 040 146 157 162
         n  n  n   T   h   a   t   '   s       i   t       f   o   r
0000100 040 056 056 056 011 156 157 167 012 012 142 171 145 041 012 012
              .   .   .  t   n   o   w  n  n   b   y   e   !  n  n
0000120 163 056 012
          s   .  n

Notice how newline characters are shown as n (octal 012) and tabs are shown as t (octal 011).

One of the especially helpful uses of the od command is for looking at non-text files for information that identifies the file types. In this case, we see the JFIF (JPEG File Interchange Format) label that identifies it as a jpg file to commands like file that report on file types. There’s a lot of other useful information in there as well, especially if you’re curious about how these files are formatted.

In this next command, we are looking at the beginning portion of a jpg file.

$ od -bc arrow.jpg | head -12
0000000 377 330 377 340 000 020 112 106 111 106 000 001 001 000 000 001
        377 330 377 340