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root@raspberrypi:~# cat >test
echo succeed
root@raspberrypi:~# <test
root@raspberrypi:~# `<test`
succeed

i guess the reason is a missing of enter signal.But how to send enter

linux bash

share|improve this question

edited Apr 5 at 18:32

Paradox

478317

asked Apr 5 at 8:26

Jack WalterJack Walter

257

New contributor

Jack Walter is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If <test ran the shell commands in test, then how would one be able to redirect the contents of a file into a command, as in cat <test?
  
  – Kusalananda♦
  Apr 5 at 9:14
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Do you mean that “<filename” like input something "eof".not "enter"?
  
  – Jack Walter
  Apr 6 at 2:42
  

  
  
  
  

add a comment | 

0

root@raspberrypi:~# cat >test
echo succeed
root@raspberrypi:~# <test
root@raspberrypi:~# `<test`
succeed

i guess the reason is a missing of enter signal.But how to send enter

linux bash

share|improve this question

edited Apr 5 at 18:32

Paradox

478317

asked Apr 5 at 8:26

Jack WalterJack Walter

257

New contributor

Jack Walter is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If <test ran the shell commands in test, then how would one be able to redirect the contents of a file into a command, as in cat <test?
  
  – Kusalananda♦
  Apr 5 at 9:14
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Do you mean that “<filename” like input something "eof".not "enter"?
  
  – Jack Walter
  Apr 6 at 2:42
  

  
  
  
  

add a comment | 

root@raspberrypi:~# cat >test
echo succeed
root@raspberrypi:~# <test
root@raspberrypi:~# `<test`
succeed

i guess the reason is a missing of enter signal.But how to send enter

linux bash

share|improve this question

edited Apr 5 at 18:32

Paradox

478317

asked Apr 5 at 8:26

Jack WalterJack Walter

257

New contributor

Jack Walter is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

root@raspberrypi:~# cat >test
echo succeed
root@raspberrypi:~# <test
root@raspberrypi:~# `<test`
succeed

share|improve this question

edited Apr 5 at 18:32

Paradox

478317

asked Apr 5 at 8:26

Jack WalterJack Walter

257

New contributor

Jack Walter is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|improve this question

edited Apr 5 at 18:32

Paradox

478317

asked Apr 5 at 8:26

Jack WalterJack Walter

257

New contributor

Jack Walter is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

edited Apr 5 at 18:32

Paradox

478317

edited Apr 5 at 18:32

Paradox

478317

asked Apr 5 at 8:26

Jack WalterJack Walter

257

New contributor

Jack Walter is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked Apr 5 at 8:26

Jack WalterJack Walter

257

1 Answer
1

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3

In Bash, and some other shells, `<test` is a shorthand for the equivalent of `cat test`:

The command substitution $(cat file) can be replaced by the equivalent but faster $(< file).

(the same is true for `...`-style command substitution).

That means that your command

$ `<test`

is the same as

$ `cat test`

- that is, insert the contents of the test file onto the command line in place of the command substitution expression and then, since it's then at the start of the command, execute the result with the first word as the command name.

---

On the other hand,

$ <test

is just an empty command with its standard input redirected from the test file, in any POSIX-like shell. It is essentially the same as : <test: does nothing and succeeds. That's not very useful, but it will raise an error if test doesn't exist or isn't readable.

The reverse direction, >test, will create or truncate test, and is sometimes useful.

---

The special behaviour of `<test` and $(<test) in Bash, zsh, ksh, etc is inconsistent with the way command substitution generally replicates the ordinary behaviour of the same command. It's a convenience and performance feature for the sake of commands like

printf '%s@%sn' $(<username.txt) $(<host.txt)

that want to read file contents into the command line, and get to save an extra process (by not spawning cat and letting the shell read the file itself).

The difference isn't anything to do with Enter or anything else like that - it's just a special case. I wouldn't recommend using $(<file) to execute a file: use . or source or some similar method instead that says what you're doing (even eval if it really comes to it). I wouldn't even recommend using $(<file) at all unless you're really sure you're in a tight loop where spawning an extra cat process mattered, because it's uncommon enough that people will be confused in exactly the way you were here.

share|improve this answer

answered Apr 5 at 8:48

Michael HomerMichael Homer

50.7k8140177

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  thanks for u reply.i know the correct way.i wannna know the difference between keyboard input and file input. So $<test equals type Enter directly?@Michael Homer
  
  – Jack Walter
  Apr 5 at 9:11
  

  
  
  
  

add a comment | 

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3

In Bash, and some other shells, `<test` is a shorthand for the equivalent of `cat test`:

The command substitution $(cat file) can be replaced by the equivalent but faster $(< file).

(the same is true for `...`-style command substitution).

That means that your command

$ `<test`

is the same as

$ `cat test`

- that is, insert the contents of the test file onto the command line in place of the command substitution expression and then, since it's then at the start of the command, execute the result with the first word as the command name.

---

On the other hand,

$ <test

is just an empty command with its standard input redirected from the test file, in any POSIX-like shell. It is essentially the same as : <test: does nothing and succeeds. That's not very useful, but it will raise an error if test doesn't exist or isn't readable.

The reverse direction, >test, will create or truncate test, and is sometimes useful.

---

The special behaviour of `<test` and $(<test) in Bash, zsh, ksh, etc is inconsistent with the way command substitution generally replicates the ordinary behaviour of the same command. It's a convenience and performance feature for the sake of commands like

printf '%s@%sn' $(<username.txt) $(<host.txt)

that want to read file contents into the command line, and get to save an extra process (by not spawning cat and letting the shell read the file itself).

The difference isn't anything to do with Enter or anything else like that - it's just a special case. I wouldn't recommend using $(<file) to execute a file: use . or source or some similar method instead that says what you're doing (even eval if it really comes to it). I wouldn't even recommend using $(<file) at all unless you're really sure you're in a tight loop where spawning an extra cat process mattered, because it's uncommon enough that people will be confused in exactly the way you were here.

share|improve this answer

answered Apr 5 at 8:48

Michael HomerMichael Homer

50.7k8140177

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  thanks for u reply.i know the correct way.i wannna know the difference between keyboard input and file input. So $<test equals type Enter directly?@Michael Homer
  
  – Jack Walter
  Apr 5 at 9:11
  

  
  
  
  

add a comment | 

3

In Bash, and some other shells, `<test` is a shorthand for the equivalent of `cat test`:

The command substitution $(cat file) can be replaced by the equivalent but faster $(< file).

(the same is true for `...`-style command substitution).

That means that your command

$ `<test`

is the same as

$ `cat test`

- that is, insert the contents of the test file onto the command line in place of the command substitution expression and then, since it's then at the start of the command, execute the result with the first word as the command name.

---

On the other hand,

$ <test

is just an empty command with its standard input redirected from the test file, in any POSIX-like shell. It is essentially the same as : <test: does nothing and succeeds. That's not very useful, but it will raise an error if test doesn't exist or isn't readable.

The reverse direction, >test, will create or truncate test, and is sometimes useful.

---

The special behaviour of `<test` and $(<test) in Bash, zsh, ksh, etc is inconsistent with the way command substitution generally replicates the ordinary behaviour of the same command. It's a convenience and performance feature for the sake of commands like

printf '%s@%sn' $(<username.txt) $(<host.txt)

that want to read file contents into the command line, and get to save an extra process (by not spawning cat and letting the shell read the file itself).

The difference isn't anything to do with Enter or anything else like that - it's just a special case. I wouldn't recommend using $(<file) to execute a file: use . or source or some similar method instead that says what you're doing (even eval if it really comes to it). I wouldn't even recommend using $(<file) at all unless you're really sure you're in a tight loop where spawning an extra cat process mattered, because it's uncommon enough that people will be confused in exactly the way you were here.

share|improve this answer

answered Apr 5 at 8:48

Michael HomerMichael Homer

50.7k8140177

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  thanks for u reply.i know the correct way.i wannna know the difference between keyboard input and file input. So $<test equals type Enter directly?@Michael Homer
  
  – Jack Walter
  Apr 5 at 9:11
  

  
  
  
  

add a comment | 

In Bash, and some other shells, `<test` is a shorthand for the equivalent of `cat test`:

The command substitution $(cat file) can be replaced by the equivalent but faster $(< file).

(the same is true for `...`-style command substitution).

That means that your command

$ `<test`

is the same as

$ `cat test`

- that is, insert the contents of the test file onto the command line in place of the command substitution expression and then, since it's then at the start of the command, execute the result with the first word as the command name.

---

On the other hand,

$ <test

is just an empty command with its standard input redirected from the test file, in any POSIX-like shell. It is essentially the same as : <test: does nothing and succeeds. That's not very useful, but it will raise an error if test doesn't exist or isn't readable.

The reverse direction, >test, will create or truncate test, and is sometimes useful.

---

The special behaviour of `<test` and $(<test) in Bash, zsh, ksh, etc is inconsistent with the way command substitution generally replicates the ordinary behaviour of the same command. It's a convenience and performance feature for the sake of commands like

printf '%s@%sn' $(<username.txt) $(<host.txt)

that want to read file contents into the command line, and get to save an extra process (by not spawning cat and letting the shell read the file itself).

The difference isn't anything to do with Enter or anything else like that - it's just a special case. I wouldn't recommend using $(<file) to execute a file: use . or source or some similar method instead that says what you're doing (even eval if it really comes to it). I wouldn't even recommend using $(<file) at all unless you're really sure you're in a tight loop where spawning an extra cat process mattered, because it's uncommon enough that people will be confused in exactly the way you were here.

share|improve this answer

answered Apr 5 at 8:48

Michael HomerMichael Homer

50.7k8140177

$ `<test`

$ `cat test`

$ <test

printf '%s@%sn' $(<username.txt) $(<host.txt)

share|improve this answer

answered Apr 5 at 8:48

Michael HomerMichael Homer

50.7k8140177

answered Apr 5 at 8:48

Michael HomerMichael Homer

50.7k8140177

answered Apr 5 at 8:48

Michael HomerMichael Homer

50.7k8140177