Ubuntu – Why doesn’t the command “ls | file” work

command linefile-commandlspipe

I've been studying about the command line and learned that | (pipeline) is meant to redirect the output from a command to the input of another one.
So why does the command ls | file doesn't work?

file input is one of more filenames, like file filename1 filename2

ls output is a list of directories and files on a folder, so I thought ls | file was supposed to show the file type of every file on a folder.

When I use it however, the output is:

    Usage: file [-bcEhikLlNnprsvz0] [--apple] [--mime-encoding] [--mime-type]
        [-e testname] [-F separator] [-f namefile] [-m magicfiles] file ...
    file -C [-m magicfiles]
    file [--help]

As there was some error with the usage of the file command

Best Answer

The fundamental issue is that file expects file names as command-line arguments, not on stdin. When you write ls | file the output of ls is being passed as input to file. Not as arguments, as input.

What's the difference?

Command-line arguments are when you write flags and file names after a command, as in cmd arg1 arg2 arg3. In shell scripts these arguments are available as the variables $1, $2, $3, etc. In C you'd access them via the char **argv and int argc arguments to main().
Standard input, stdin, is a stream of data. Some programs like cat or wc read from stdin when they're not given any command-line arguments. In a shell script you can use read to get a single line of input. In C you can use scanf() or getchar(), among various options.

file does not normally read from stdin. It expects at least one file name to be passed as an argument. That's why it prints out usage when you write ls | file, because you didn't pass an argument.

You could use xargs to convert stdin into arguments, as in ls | xargs file. Still, as terdon mentions, parsing ls is a bad idea. The most direct way to do this is simply:

file *

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Ubuntu – How to output the linux command and the returned result in the same text file

The redirection operator command > file will redirect the STDOUT of a command, or STDERR of a command if used like command 2> file or both if used like &> in bash or in any shell by portable command > file 2>&1 way. I am not aware of any direct method using redirection that can achieve what you need.

Although it is possible using the script program. It will basically save everything printed on the terminal in that script session.

From man script:

script makes a typescript of everything printed on your terminal. 
It is useful for students who need a hardcopy record of an 
interactive session as proof of an assignment, as the typescript file 
can be printed out later with lpr(1).

You can start a script session by just typing script in the terminal, all the subsequent commands and their outputs will all be saved in a file named typescript in the current directory. You can save the result to a different file too by just starting script like:

script output.txt

To logout of the screen session (stop saving the contents), just type exit.

Here is an example:

$ script output.txt
Script started, file is output.txt

$ ls
output.txt  testfile.txt  foo.txt

$ exit
exit
Script done, file is output.txt

Now if i read the file:

$ cat output.txt

Script started on Mon 20 Apr 2015 08:00:14 AM BDT
$ ls
output.txt  testfile.txt  foo.txt
$ exit
exit

Script done on Mon 20 Apr 2015 08:00:21 AM BDT

script also has many options e.g. running quietly -q (--quiet) without showing/saving program messages, it can also run a specific command -c (--command) rather than a session, it also has many other options. Check man script to get more idea.

Best Answer

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