CODEWITHSUNDEEP
unix
Bruce
Bruce

Reputation: 35233

Understanding how 'cat' command works

cat concatenates files or stdin and redirects it to stdout. 

$ cat file1 > file5 file2 file3 file4

concatenates file1, file2, file3 and file 4 and writes it to file5.

$ cat file1 > file5 < file2 file3 file4

concatenates file1, file3 and file4 (not file2) and writes to file5

Please explain these outputs

Examples of what happens: 

~/test$ echo "this is file 1"> file1
~/test$ echo "this is file 2"> file2
~/test$ echo "this is file 3"> file3
~/test$ echo "this is file 4">file4
~/test$ cat file1 > file5 file2 file3 file4
~/test$ cat file5
this is file 1
this is file 2
this is file 3
this is file 4
~/test$ cat file1 > file5  < file2 file3 file4
~/test$ cat file5
this is file 1
this is file 3
this is file 4

Upvotes: 0

Views: 3937

Answers (4)

Dave
Dave

Reputation: 11162

This is less about how cat works, and more about shell redirection. The shell processes the command line before it runs the program. It's easier to see if you push all the io redirection to the end of the command. The first becomes:

cat file1 file2 file3 file4 > file5

The shell then changes the output of cat from the terminal to file5. This is completely independent of cat.

The second command is then

cat file1 file3 file4 >file5 <file2

This this changes the standard input from the keyboard to file2 and the output, like before, to file5. In this instance, because there are files specified on the command line, cat ignores the standard input, reading only from the 1, 3, and 4. The - argument tells cat to read from standard input, so

cat file1 - file3 file4 >file5 < file2

Would output the contents of files 1-4 to file 5.

Upvotes: 2

jlliagre
jlliagre

Reputation: 30813

Redirections can be placed anywhere in the command line, although the usual way is at the end.

Your second statement is incorrect. file2 should be is ignored. (was corrected later)

Upvotes: 1

Jan Vorcak
Jan Vorcak

Reputation: 19989

Try to create simple bash file

#!/bin/bash

echo $@

And run

./bash.sh file1 file2 file3 file4 > file5

gives you

file1 file2 file3 file4

./bash.sh file1 file3 file4 >file5 <file2

gives you

file1 file3 file4

in file 5 you see list of arguments, what cat do is that takes list of arguments and write it to the std output, that's why file 2 is ignored, it's not a parameter, it's an input in fact

if you want cat to read from std input assign - after the command

cat file1 > file5 < file2 file3 file4 -

This will write

1111
3333
4444
2222

to the file5

< means redirection of std input, if you write

cat file1 > file5 file3 file4 -

it reads from file1, file3, and file4 and your keyboard to the file5

Upvotes: 1

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 798566

Only one redirection takes place in the shell; the rest are passed as arguments.

The first command is

cat file1 file2 file3 file4 > file5

The second command is

cat file1 file3 file4 > file5 < file2

The second command doesn't include file2 because cat was never told to read from stdin with -.

Upvotes: 1

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