echo "output of echo" > f1.txt what is this

As a result, f1.txt was created(it did not exist earlier), and this is the content of f1.txt

cat f1.txt
output of echo what is this

It seems that ">" can take two arguments, the first argument is the file name, and the second is the input for the file.

but when I changed echo to cat, it doesn't work

cat < a.txt > f1.txt ohlala
cat: ohlala: No such file or directory```





Please explain how this line - "echo "output of echo" > f1.txt what is this" works exactly. I can't find the > syntax for this kind. 

CodePudding user response：

The output redirection doesn't have to be the last pair of words of the command; they can occur any time after the command name (and in the case of a simple command, even before the command name.

What the shell does is first scan the entire command looking for > <word>, then processes that as an output redirection before running the command with the remaining words as the arguments. All of the following are equivalent:

• echo "output of echo" > f1.txt what is this
• echo "output of echo" what > f1.txt is this
• echo "output of echo" what is > f1.txt this
• echo "output of echo" what is this > f1.txt
• echo > f1.txt "output of echo" what is this
• > f1.txt echo "output of echo" what is this

because > f1.txt is removed, leaving behind

echo "output of echo" what is this

as the command to actually execute, namely echo with 4 arguments.

> takes one "argument"; the shell uses it to open a file for writing and passes that file descriptor to the command (when it finally runs) to use as standard output. In other words, no command is passed to >, but rather whatever > produces is passed to the process that executes the command.

Your cat command, after removing the input and output redirections, is cat ohlala, and there is no file named ohlala for cat to open. (It's not a shell error, but a cat error.)