测试命令和重定向bas​​h

Question

Why does test > file exit with 1 while test arg1 > arg2 exit with 0 ?

I thought that in test arg1 > arg2 , test arg1 is applied first then the output(it has none) written to file arg2 and since the last command(redirection) was successful the exit is 0. But so should the first case. I am a bit confused.

Answer 1

Redirection is not a command.

| creates a new command, because not only the is output of the previous command "piped" somewhere else, but also another command is executed on that output. For example,

ls | grep 'abc' ls | grep 'abc' will exit 1 if nothing matches that pattern, regardless of whether ls completed successfully or not). The exit status you see with $? is the status of the last pipe.

Redirection is not the same, because it doesn't give the output to the next command in line, just to a file. Note that redirection can make you return 1 when your command would otherwise have worked because you don't have permissions to write to the file. In that case, your command won't be executed.

TL;DR: A command requires a valid redirection to run. If it can run, the exit status will be the exit status of the command. Exit status of redirection is the OR of the command and the redirection (0 OR 1, 1 OR 1, 1 OR 0 will all return 1; only a successful redirection and successful command returns 0)