Saturday, 27 October 2012
Linux Cat Command Examples
The cat command displays the content of file on the standard output. If multiple files are specified, the contents of all files will be concatenated and then displayed on the standard output. Likewise, if no file is specified, it will assume standard input (keyboard input) as the input to the command. The Ctrl + d is the shortcut used to save the contents in the appropriate output placeholder specified and exit the cat command.
Print content of file in standard output
samar@samar-Techgaun:~$ cat workers.txt List of workers, designations & salary (in K): Kshitiz Director 30 Bikky Manager 20 Abhis Sweeper 10 Rajesh Guard 12
Print line numbers
samar@samar-Techgaun:~$ cat -n workers.txt
1 List of workers, designations & salary (in K):
2 Kshitiz Director 30
3 Bikky Manager 20
4
5
6 Abhis Sweeper 10
7 Rajesh Guard 12
Print line numbers for non-empty lines only
samar@samar-Techgaun:~$ cat -b workers.txt
1 List of workers, designations & salary (in K):
2 Kshitiz Director 30
3 Bikky Manager 20
4 Abhis Sweeper 10
5 Rajesh Guard 12
Create a new file
samar@samar-Techgaun:~$ cat > newfile.txt We can create text files using cat command once u finish writing, press ctrl+d to save file ^d
Display content of multiple files
samar@samar-Techgaun:~$ cat workers.txt newfile.txt List of workers, designations & salary (in K): Kshitiz Director 30 Bikky Manager 20 Abhis Sweeper 10 Rajesh Guard 12 We can create text files using cat command once u finish writing, press ctrl+d to save file
Combine multiple files to new file
samar@samar-Techgaun:~$ cat workers.txt newfile.txt > concat.txt samar@samar-Techgaun:~$ cat concat.txt List of workers, designations & salary (in K): Kshitiz Director 30 Bikky Manager 20 Abhis Sweeper 10 Rajesh Guard 12 We can create text files using cat command once u finish writing, press ctrl+d to save file
Append data to existing file
samar@samar-Techgaun:~$ cat >> newfile.txt New line added ^d samar@samar-Techgaun:~$ cat newfile.txt We can create text files using cat command once u finish writing, press ctrl+d to save file New line added
Alternatively, you can use the syntax below if you wish to create new file combining the content of already existing file and standard input.
samar@samar-Techgaun:~$ cat newfile.txt - > myfile thanks for everything ^d samar@samar-Techgaun:~$ cat myfile We can create text files using cat command once u finish writing, press ctrl+d to save file New line added thanks for everything
Another possibility is to combine two text files with data from standard input (keyboard) in-between the contents of these two text files.
samar@samar-Techgaun:~$ cat workers.txt - newfile.txt > myfile ---------------------------------- ^d samar@samar-Techgaun:~$ cat myfile List of workers, designations & salary (in K): Kshitiz Director 30 Bikky Manager 20 Abhis Sweeper 10 Rajesh Guard 12 ---------------------------------- We can create text files using cat command once u finish writing, press ctrl+d to save file New line added
Display $ sign at the end of each line
samar@samar-Techgaun:~$ cat -E workers.txt List of workers, designations & salary (in K):$ Kshitiz Director 30$ Bikky Manager 20$ $ $ Abhis Sweeper 10$ Rajesh Guard 12$
Display ^I sign instead of TABs
samar@samar-Techgaun:~$ cat -T workers.txt List of workers, designations & salary (in K): Kshitiz^IDirector^I30 Bikky^IManager^I^I20 Abhis^ISweeper^I^I10 Rajesh^IGuard^I^I12
Display files with non-printing characters
samar@samar-Techgaun:~$ cat -v /bin/nc
In the example above, the non-printing characters are replaced with ^ and M- notation except for line breaks and TABs. This can be used to display the contents of binary files which would otherwise have shown gibberish text all over the console.
Show contents with tabs, line breaks and non-printing characters
samar@samar-Techgaun:~$ cat -A /bin/nc
The tab will be substituted by ^I, line breaks with $ and non-printing characters with ^ and M- notation. Actually, the -A switch is equivalent to -vET switch.
Supress/squeeze repeated empty lines
samar@samar-Techgaun:~$ cat -s workers.txt List of workers, designations & salary (in K): Kshitiz Director 30 Bikky Manager 20 Abhis Sweeper 10 Rajesh Guard 12
Using -s switch, we can squeeze repeatedly occurring blank lines and replace all the adjacent empty lines with a single empty line in the output. This might be useful to reformat a file with several empty lines in-between (eg. cat -s workers.txt > formatted_workers.txt).
Display last line first
samar@samar-Techgaun:~$ tac workers.txt Rajesh Guard 12 Abhis Sweeper 10 Bikky Manager 20 Kshitiz Director 30 List of workers, designations & salary (in K):
It is the tac, not the cat that is doing the magic but just thought that this is the right place to make a note about this little known command.
Edit: Added here-doc examples. Thanks rho dai for pointing me this.
Parameter substitution using here-document strings
samar@samar-Techgaun:~$ cat > test << TEST samar@samar-Techgaun:~$ I am $USER. My home is $HOME samar@samar-Techgaun:~$ I came here from $OLDPWD samar@samar-Techgaun:~$ TEST samar@samar-Techgaun:~$ cat test I am samar. My home is /home/samar I came here from /home/samar/Downloads
Command expansion example
samar@samar-Techgaun:~$ cat > test << TEST samar@samar-Techgaun:~$ $(ls /) samar@samar-Techgaun:~$ TEST samar@samar-Techgaun:~$ cat test bin boot cdrom dev etc home initrd.img initrd.img.old lib lost+found media mnt opt proc root run sbin selinux srv sys tmp usr var vmlinuz vmlinuz.old
Parameter substitution turned off
samar@samar-Techgaun:~$ cat > test << 'TEST' samar@samar-Techgaun:~$ I am $USER. My home is $HOME samar@samar-Techgaun:~$ I came here from $OLDPWD samar@samar-Techgaun:~$ TEST samar@samar-Techgaun:~$ cat test I am $USER. My home is $HOME I came here from $OLDPWD
Note the difference between the last example and previous two examples. Enclosing the limit string TEST with quotes prevents the substitutions and expansions.
I hope these examples are useful. :)
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