Reading an argument list with a glob from a file

Question

I am trying to read arguments of ll command from file. I have file args.txt with content some?. I have files with names some1, some2 and some3 and some other files. when I execute cat args.txt | ll I get the same result as when I execute ll.

Can someone explain me why is that and how can I achieve the desired result. Thanks in advance.

Answer 1

^{Note: I'm assuming that ll is an alias for ls -l or some variation.
As Charles Duffy points out, (by default) only interactive shells know about aliases, and scripts as well as tools such as xargs are unaware of them.}

What you send through a pipeline to a command is received via its stdin stream, which is distinct from a command's arguments (options such as -l and operands such as some?).

Therefore, an explicit transformation is needed to convert the contents of file args.txt to an argument you can pass to ls -l:

ls -l $(cat args.txt)   # In Bash you can simplify to: ls -l $(< args.txt)

Note that the command substitution ($(...)) is deliberately unquoted so as to ensure that globbing (filename expansion) is applied to the contents of args.txt, as desired.

Yours is a rare case where this technique - which is brittle - is actually needed.

To illustrate the brittleness: If your globbing pattern were "some file"?, for instance (you'd need the double quotes for the pattern to still be recognized as a single argument), the command wouldn't work anymore, because the " characters lose their syntactic function when they're the result of a command substitution (or variable expansion).

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The standard utility for transforming stdin or file content into arguments is xargs. However, in your particular case it is not an option, because your file contains a glob (filename pattern) that must be expanded by the shell, but xargs only invokes external utilities, without involving a shell:

$ xargs ls -l < args.txt # !! Does NOT work as intended here.
ls: file?: No such file or directory

Filename file? was passed literally to ls (and no file actually named file? exists) - no globbing happened, because no shell was involved.