Piping the output of a program that doesn't write to stdout

Question

I'm looking for the best way to use pipes when the input program doesn't write to stdout. Specifically I want to pipe objcopy into hexdump like this

objcopy -I ifmt -O binary ifile - | hexdump -C

but objcopy doesn't accept '-' as a file meaning 'write to stdout' as some programs do.

At the moment I am doing

objcopy -I ifmt -O binary ifile tmpfile; hexdump -C tmpfile; rm tmpfile

but was wondering if there was a better way.

I am using bash 4.1.10 on cygwin.

Answer 1

I wrote a comment prescribing a process substitution, but it can't work for objcopy since objcopy will try to open a seekable file (as it may need to move back and forth in the file).

In short: objcopy can't write to a stream as stdout, that's why its output must be a file that can be seeked. Your solution is very likely the only reasonable possibility.

---

To answer your question

I'm looking for the best way to use pipes when the input program doesn't write to stdout

in a more general fashion (but that's not applicable with objcopy or any command that requires seeking the file), in Bash you can use process substitution: if mycommand takes a parameter that is an output file, and doesn't accept - for standard output and doesn't write to standard output by default, you can use it as:

mycommand >(cat)

or if you want to pipe it through, e.g., hexdump -C:

mycommand >(hexdump -C)

With this, mycommand will see an argument of the form /dev/fd/42 (where 42 may differ), and will be able to open it for writing, as if it where a regular file (but not seekable), and hexdump will get on its standard input the written data.

You can experiment process substitution like so: call the following script mycommand:

#!/bin/bash

if [[ $1 ]]; then
    echo "Hi, this is mycommand, and I was called with first argument: \`$1'"
    echo "I'm outputting this to the file given as argument" > "$1"
else
    echo >&2 "Please provide an argument (file to write to)"
    exit 1
fi

This script makes sure that you give a non-empty argument (otherwise shows an error message), outputs this argument to standard output, and a little line in the file the name of which is given as argument.

Then chmod +x mycommand and play with it:

$ ./mycommand
Please provide an argument (file to write to)
$ ./mycommand -
Hi, this is mycommand, and I was called with first argument: `-'
$ ls
- mycommand
$ rm ./-
$ ./mycommand >(cat)
Hi, this is mycommand, and I was called with first argument: `/dev/fd/63'
I'm outputting this to the file given as argument
$ ./mycommand >(tr -d e)
Hi, this is mycommand, and I was called with first argument: `/dev/fd/63'
I'm outputting this to th fil givn as argumnt
$ ./mycommand >(hexdump -C)
Hi, this is mycommand, and I was called with first argument: `/dev/fd/63'
00000000  49 27 6d 20 6f 75 74 70  75 74 74 69 6e 67 20 74  |I'm outputting t|
00000010  68 69 73 20 74 6f 20 74  68 65 20 66 69 6c 65 20  |his to the file |
00000020  67 69 76 65 6e 20 61 73  20 61 72 67 75 6d 65 6e  |given as argumen|
00000030  74 0a                                             |t.|
00000032
$ ./mycommand >(cat) > /dev/null
I'm outputting this to the file given as argument