Cat command, the acronym for Concatenate, is one of the most used commands in *nix systems. The most basic usage of the command is to read files and display them to stdout, meaning to display the content of files on your terminal.
# cat file.txt
Another usage of the cat command is to read or combine multiple files together and send the output to a monitor as illustrated in the below examples.
# cat file1.txt file2.txt file3.txt
The command can also be used to concatenate (join) multiple files into one single file using the “>” Linux redirection operator.
# cat file1.txt file2.txt file3.txt > file-all.txt
By using the append redirector you can add the content of a new file to the bottom of the file-all.txt with the following syntax.
# cat file4.txt >> file-all.txt
The cat command can be used to copy the content of file to a new file. The new file can be renamed arbitrary. For example, copy the file from the current location to /tmp/ directory.
# cat file1.txt > /tmp/file1.txt
Copy the file from the current location to /tmp/ directory and change its name.
# cat file1.txt > /tmp/newfile.cfg
A less usage of the cat command is to create a new file with the below syntax. When finished editing the file hit CTRL+D to save and exit the new file.
# cat > new_file.txt
In order to number all output lines of a file, including empty lines, use the -n switch.
# cat -n file-all.txt
To display only the number of each non-empty line use the -b switch.
# cat -b file-all.txt
Learn How to Use Tac Command in Linux
On the other hand, a lesser known and less used command in *nix systems is tac command. Tac is practically the reverse version of cat command (also spelled backwards) which prints each line of a file starting from the bottom line and finishing on the top line to your machine standard output.
# tac file-all.txt
One of the most important option of the command is represented by the -s switch, which separates the contents of the file based on a string or a keyword from the file.
# tac file-all.txt --separator "two"
Next, most important usage of tac command is, that it can provide a great help in order to debug log files, reversing the chronological order of log contents.
$ tac /var/log/auth.log Or to display the last lines $ tail /var/log/auth.log | tac
Sample Output ----------------------------------------------------------------------------------------------------- rootadminz@rootadminz ~ $ tac /var/log/auth.log pr 6 16:09:01 rootadminz CRON[17714]: pam_unix(cron:session): session closed for user root Apr 6 16:09:01 rootadminz CRON[17714]: pam_unix(cron:session): session opened for user root by (uid=0) Apr 6 16:05:01 rootadminz CRON[17582]: pam_unix(cron:session): session closed for user root Apr 6 16:05:01 rootadminz CRON[17583]: pam_unix(cron:session): session closed for user root Apr 6 16:05:01 rootadminz CRON[17583]: pam_unix(cron:session): session opened for user root by (uid=0) Apr 6 16:05:01 rootadminz CRON[17582]: pam_unix(cron:session): session opened for user root by (uid=0) Apr 6 16:00:01 rootadminz CRON[17434]: pam_unix(cron:session): session closed for user root ....
rootadminz@rootadminz ~ $ tail /var/log/auth.log | tac Apr 6 16:09:01 rootadminz CRON[17714]: pam_unix(cron:session): session closed for user root Apr 6 16:09:01 rootadminz CRON[17714]: pam_unix(cron:session): session opened for user root by (uid=0) Apr 6 16:05:01 rootadminz CRON[17582]: pam_unix(cron:session): session closed for user root Apr 6 16:05:01 rootadminz CRON[17583]: pam_unix(cron:session): session closed for user root Apr 6 16:05:01 rootadminz CRON[17583]: pam_unix(cron:session): session opened for user root by (uid=0) Apr 6 16:05:01 rootadminz CRON[17582]: pam_unix(cron:session): session opened for user root by (uid=0) Apr 6 16:00:01 rootadminz CRON[17434]: pam_unix(cron:session): session closed for user root Apr 6 16:00:01 rootadminz CRON[17434]: pam_unix(cron:session): session opened for user root by (uid=0) Apr 6 15:55:02 rootadminz CRON[17194]: pam_unix(cron:session): session closed for user root Apr 6 15:55:01 rootadminz CRON[17195]: pam_unix(cron:session): session closed for user root ...
