Shell Process Substitution as Temp File

A Brief Intro

This is all started when i’m trying to make a shell alias using fzf and git add -p command. And i was surprised that the interactive selection from git add -p immediately terminated before i can even press any key. Let’s get started, shall we?

First Attempt

My first attempt to make that shell alias was something like this:

alias gap='git status -s | awk "{print \$2}" | fzf | xargs -r git add -p'

Let me briefly explain one by one the command that i use in my shell alias:

git status -s -> The short format of git status, without unnecessary info (at least for me).
awk "{print \$2}" -> Only use the second column. The backslash is to prevent shell to expand $2 into a second argument instead of a second column in awk.
fzf -> Fuzzy finder by Junegunn¹.
xargs -r -> Do not run the command if there’s no standard input.
git add -p -> To add a chunk of changes, only works if the file already tracked by git.

This alias is where the problem arise. After i select the file name using fzf, the interactive interface of git add -p only appear for a few seconds and then terminated.

According to stackoverflow answer²:

Without further arguments xargs does not work with interactive (command line) applications.

The reason for that is, by default xargs gets its input from stdin but interactive applications also expect input from stdin.

To prevent the applications from grabbing input that is intended for xargs, xargs redirects stdin from /dev/null for the applications it runs.

This leads to the application just receiving an EOF³.

With that in mind, we need to use --arg-file=<file> or -a <file> flag, which means that xargs will read from a <file> instead of stdin ( standard input) so that the stdin remains unchanged. Alright, let’s go to the second attempt.

Second Attempt

Following the stackoverflow answer², my second attempt was something like this:

alias gap='xargs -a <(git status -s | awk "{print \$2}" | fzf) git add -p'

The expression <(git status -s | awk "{print \$2}" | fzf) is what we called process substitution (or to be precise, shell process substitution).

Instead of using a file, we use a command to act like a temporary file.

Now, the problem is, in my experiment i can’t use fzf with the shell process substitution because shell process substitution produces a special file that can only be opened and read, but not written or seeked⁴.

Commands that treat their arguments as pure streams will works with shell process substitution, but the commands that seek a file they are given (or write to a file) won’t work⁴.

And that is why we can’t use interactive command such as fzf with shell process substitution.

Conclusion

With all the information from my experiment before, i decided to remove fzf from the alias, which result with this alias:

alias gap='xargs -a <(git status -s | awk "{print \$2}") git add -p'

Is it fulfilling what i need? Not really, i might replace that with shell function instead of shell alias soon. But, i learn something new about shell process substitution.

Extra Note

For more info about the xargs and git flags, check the manpage. For example: man xargs or man git-add.

If we use echo with shell process substitution like this:

echo <(git status -s)

That will show us where the temporary file created.