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I was working through a tutorial and saw use of both cat myfile.txt and cat < myfile.txt. Is there a difference between these two sequences of commands? It seems both print the contents of a file to the shell.

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up vote 91 down vote accepted

In the first case, cat opens the file, and in the second case, the shell opens the file, passing it as cat's standard input.

Technically, they could have different effects. For instance, it would be possible to have a shell implementation that was more (or less) privileged than the cat program. For that scenario, one might fail to open the file, while the other could.

That is not the usual scenario, but mentioned to point out that the shell and cat are not the same program.

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71  
Yes, and for example you can do sudo cat myfile.txt. But sudo cat < myfile.txt will not work if you don't have privileges to read the file. – zuazo Jan 31 at 22:07
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Note that ksh93 has cat builtin (not enabled by default unless you have /opt/ast/bin early in your $PATH though). – Stéphane Chazelas Feb 2 at 13:18
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Some programs behave differently depending on whether they get a filename argument or stdin. For instance, wc will print the filename before the counts when given an argument. – Barmar Feb 3 at 19:51

There is no major visible difference in your test case. The most obvious one would be the error message you get if there is no file named myfile.txt in the current directory, or if you are not allowed to read it.

In the former case, cat will complain and in the latter case, your shell will, clearly showing which process is trying to open the file, cat in the former one and the shell in the latter one.

$ cat myfile.txt
cat: myfile.txt: No such file or directory
$ cat < myfile.txt
ksh93: myfile.txt: cannot open [No such file or directory]

In a more general case, a major difference is using redirections cannot be used to print the content of more than one file, which is after all the original purpose of the cat (i.e. catenate) command. Note that the shell will anyway try to open all files passed as redirected input, but only actually pass the last one to cat unless you use zsh and its multios "zshism".

$ echo one > one
$ echo two > two
$ cat one two # cat opens one, shows one, opens two, shows two
one
two
$ cat < one < two # sh opens one then opens two, cat shows stdin (two)
two
$ rm one two
$ echo one > one
$ cat one two # cat opens and shows one, fails to open two
one
cat: two: No such file or directory
$ cat < one < two # the shell opens one then opens two, fails and 
                  # displays an error message, cat gets nothing on stdin
                  # so shows nothing
ksh93: two: cannot open [No such file or directory]

On a standard system, the shell and cat have no difference in file access rights so both will succeed of fail equally. Using sudo to raise cat's privileges will make a big difference in behavior, as Thomas Dickey reply and attached comments already suggested.

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Out of curiosity, do you really use use ksh of your own volition, and if so... why? – cat Feb 1 at 1:51
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@cat - that question is obviously based in ignorance. build it yourself and see. – mikeserv Feb 1 at 1:57
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@mikeserv that was intended to be flippant, not seriously rude, but fair enough, i suppose – cat Feb 1 at 2:01
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@cat - i didnt suppose otherwise. ignorance is not a thing to be ashamed of - it is only a lack of knowledge. if you dont understand why someone might choose to use ksh93, then i can only assume it is because you've never used it. so i recommend you do. its worth trying, to be sure. and believe me when i tell you, that, compared to bash, ksh93 is far and away the better shell. it's the shell, almost. – mikeserv Feb 1 at 2:05
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As @mikeserv pointed out elsewhere, cat < file1 > file2 has a very different effect from cat file1 > file2 in the case where file1 is unreadable or nonexistent. (The latter form truncates file2; the former will not.) – Wildcard Feb 1 at 18:02

cat myfile.txt reads the file myfile.txt then prints it to the standard output.

cat < myfile.txt here cat isn't given any file(s) to open, so -like many Unix commands do- reads the data from the standard input, which is directed there from file.txt by the shell, and prints to standard output.

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@Thomas Dickey's answer is brilliant.

I just want to add some obvious facts about the case of reading several files (loosely related to your question, but still):

  • cat <file1 <file2 <file3 will read only file3, at least in bash. (Actually, it depends on shell, but most shells will dup every specified file to stdin, which causes the last one to effect.)
  • cat file1 file2 file3 will read all the specified files sequentially (actually cat is shortened form of the word concatenate).
  • cat file1 file2 file3 <file4 <file5 <file6 will read only file1, file2, file3 (as cat ignores stdin when filename arguments are passed).
    • cat file1 file2 - file3 <file4 <file5 <file6 will read file1, file2, file6, file3 (as hyphen forces cat not to ignore stdin).

And about errors. In case of inability to open some of files specified as arguments (without <), cat will skip failed files (with outputting relevant message to stderr), but still read other files. In case of inability to open at least one of files specified as redirections (with >), shell won't even start cat (this occurs even for redirections actually not used by cat). In both cases erroneous exit code will be returned.

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Note that in your first example cat will nevertheless open file1 and file2, same with file4 and file5 in your third example. It will only show file3, resp. file6 contents if these previous open instructions succeed. – jlliagre Feb 2 at 19:36
    
@jlliagre, thanks, I didn't know that. Strace obviously proved your correctness. I corrected parenthesised text for cases 1 and 3a. – sasha Feb 3 at 0:40

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