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Both of the following commands try to open a non-existing file foo, but the error messages are a little different. What could be the reason?

$ cat foo
cat: cannot open foo
$ cat < foo
-bash: foo: No such file or directory
Jeff Schaller
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Zain Rafi
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2 Answers2

30
cat foo

This runs the cat command with argument foo. The error printed onscreen depends entirely on what was decided by the programmer of the command cat.

cat < foo

This feeds the contents of the file foo to the cat command by using the Bash stdin redirection. If the file doesn't exist, it is Bash that complains about it, not cat.

dr_
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20

In $ cat foo the shell (here bash) executes the cat command and passes the parameter foo. The cat program chooses to interpret that parameter as a file name - and tries to open the file. The error you see is from the cat program which (naturally) cannot open the file.

The version $ cat < foo is a redirection which is handled by the shell. < is a shell operator which instructs the shell to open a file and redirect it to stdin. The file does not exist so you get a "No such file" and cat is not even run. This time the error comes from the shell (bash) and looks a little different.

This is why you see 2 different errors. The cause is the same - but it is from 2 different programs (cat and bash).

Stéphane Chazelas
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Claus Andersen
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    No. in `cat < foo`, `cat` is not invoked if the redirection fails. That (and having consistent error message) is one reason why it's often better to use redirection where possible. Also consider the `cat < in > out` vs `cat in > out` where the former prevents `out` to be overridden if `in` doesn't exist (the shell cancels the command right after the failing `< in` redirection and doesn't do the next `> out` redirection, let alone invoke `cat`). – Stéphane Chazelas Nov 24 '16 at 11:19
  • Cool! Pure logic. I'll edit my nonsense. – Claus Andersen Nov 24 '16 at 11:34
  • @StéphaneChazelas Hello, why is it better to use the redirection operator wherever possible? – FuRinKaZan_001 Jan 25 '20 at 18:41
  • @FuRinKaZan_001, it is not necessarily always better, but there are several advantages in most cases where it's possible. See for instance [When should I use input redirection?](//unix.stackexchange.com/a/70759) – Stéphane Chazelas Jan 25 '20 at 19:07