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Questions tagged [process-substitution]

Process substitution a form of inter-process communication that allows the input or output of a command to appear as a file (such as: `<(cmd)` or `>(cmd)`).

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cat in process substitution hangs: what is it waiting for? [duplicate]

I have this piece of code using Process Substitution. When I run it, it works fine, but it does not exit - just hangs there waiting for input. Why and how to overcome? prepro() { in=$(<...
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2answers
1k views

Why do I seem to lose data using this bash pipe construction?

I am trying to combine a few programs like so (please ignore any extra includes, this is heavy work-in-progress): pv -q -l -L 1 < input.csv | ./repeat <(nc "host" 1234) Where the source of ...
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3answers
115 views

Can I duplicate output of a pipe?

Well, the task is simple: a part of my script has to compute both md5 and sha1 hashes. The input is a file - big file - and hashes have to be put into MD and SH variables for later output composition. ...
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1answer
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Process substitution to script in order to monitor iotop

I am actively trying to get the output of the iotop for monitoring. As you know, the iotop is interactive, meaning that once started, it'll provide live updates on disk usage on the system. I'm ...
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2answers
36 views

Implement file read option from command line - “anonymous” file [closed]

I have a command line program like so: my_cmd -f <file> which would normally be used like: my_cmd -f some-file.json my question is - is there a way to read an inline file, something like ...
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45 views

Write stdout to a temp file using command line

Say for example we wanted to write the env from the host to Docker container, one way would be to use a temp file: env > env_file && docker run --env-file env_file image_name to avoid a ...
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1answer
44 views

How to send stdin to while loop using redirection operator

Not sure if I used the right words in the title, I am trying to do this: while read line; do echo "$line" exit; done; <( echo "foo\n"; exit ) but it hangs, I don't think the while loop ...
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46 views

How to pass to ssh the key stored in a shell environment variable?

Presumably ssh tries to access the fd several times, but the fd is destroyed after it has been read the very first time: # ssh -i <(echo $KEY) user@example.com Warning: Identity file /dev/fd/11 ...
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1answer
140 views

Process substitution and redirection using tee

Suppose that you redirect, in bash, the standard output of a command cmd to a file named f.out, and the standard error to f.err, using tee to preserve console printing: cmd 1> >(tee f.out) 2>...
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1answer
75 views

concurrent processing in bash using process expansion, and redirection

Apart from possible races that have to be controlled by using proper synchronisation, it's possible in bash to feed a data source concurrently into multiple pipelines and collect all their outputs ...
3
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1answer
427 views

Process substitution inside a subshell to set a variable

I'm trying to run a script remotely and use its standard output to populate a variable. I'm doing this to avoid temporary files. Here's the pattern I'm trying: var=$(bash <(curl -fsSkL http://...
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1answer
46 views

How to use bash process substitution for a sftp private key file?

With my team, we need to share a few private keys and we store them in a pass repository. So instead of extracting the key and storing it in cleartext on the filesystem, I am looking for a way to ...
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1answer
142 views

Why is the stdout of a `>()` subshell different if it's part of a redirection (e.g. `> >()`)?

This is a followup question to: Why does `... | sed 's/^/stdout: /'` print on empty stdin when `... > >(sed 's/^/stdout: /')` doesn't? To be more concrete, why is this a ...
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4answers
206 views

Why does `… | sed 's/^/stdout: /'` print on empty stdin when `… > >(sed 's/^/stdout: /')` doesn't?

I'm trying to understand what causes the difference in these 2 constructs that I thought were functionally equivalent: $ ( echo foo >&2 ) 2> >(sed 's/^/stderr: /') | sed 's/^/stdout: /' ...
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1answer
63 views

Confused why script does not exit

I have this script verbatim: #!/usr/bin/env bash handle_json(){ while read line; do cat <<EOF {"@json-stdio":true,"value":{"mark":"$1","v":"$line"}} EOF done; } ( echo; echo; echo '...
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1answer
62 views

Is this bash argument substitution?

While studying bash process substitution, I found this: counter=0 while IFS= read -rN1 _; do ((counter++)) done < <(find /etc -printf ' ') echo "$counter files" If I understand correctly,...
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Does process substitution need to be double quoted?

Double quoting command substitution is a good practice. Is it the same for process substitution (<() and >())? It seems double quotes allow command substitution, but disallow process ...
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Using a here-doc with trailing backslashes inside a process substitution in bash

This: cat <<EOF one two three EOF Prints: one two three And this: cat <<EOF one \\ two \\ three EOF Prints: one \ two \ three But while this: cat <( cat <<EOF one two ...
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1answer
71 views

Specifying the file extension produced by zsh process substitution

I want to use zsh's process substitution to create a temporary file that can then be read by another program. However, the file it generates has no file extension, so the program reading it refuses to ...
3
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3answers
80 views

Pass input to multiple commands and compare their outputs

I am trying to pass standard input into multiple commands and compare their outputs. My current attempt seems close, but doesn't quite work - plus it relies on temporary files which I feel would not ...
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0answers
45 views

Command requires file, avoid creating temporary file [duplicate]

<() doesn't seem to do what I expect. This command requires a text file (it's JLinkExe, the text file contains commands for JLinkExe to run. But that shouldn't matter). JLinkExe -CommanderScript &...
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35 views

Process substitution of commands on separate lines of a string

I have a variable, COMMANDS, that contains a list of newline-separated commands (I know for example in this case that there will never be newlines, but they could be separated by NUL). For example, ...
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4answers
360 views

simple cat echo process substitution hangs

Simple process substitution with cat seems to hang: cat >( echo hello; ) never finishes. Also tried: cat >( echo hello; exit; ) Manually closing the standard out file descript from the ...
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1answer
348 views

multiple pipes in loop, saving pipeline-result to array

I am trying to do the following (using bash): Search for files that always have the same name and extract data from these files. I want to store the extracted data in new arrays I am almost there, I ...
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2answers
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How to create sh compatible bash function

I have this bash function: zmx(){ "$@" \ 2> >( while read line; do echo -e "r2g: $line"; done ) \ 1> >( while read line; do echo -e "r2g: $line"; done ) } this function is ...
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2answers
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Which signal >(process) receives after main shell exits?

This is a Zshell question, although Bash, if it has >(command) syntax (i.e. process substitution of such kind), can hint a solution too. This really basic code explains all: % fun() { setopt ...
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1answer
187 views

Using process substitution, only send stderr to process

I have this: exec > >( while read line; do echo " stdout: $line"; done ) exec 2> >( while read line; do echo " stderr: $line"; done ) echo "rolo" >&2 echo "cholo" if you run that ...
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1answer
346 views

Why process substitution is faster than pipe in this case? [duplicate]

I am wondering why ls -1 | while read file; do echo $file; tail -n 100 $file > >(sleep 1 && cat > $file) done is faster than ls -1 | while read file; do echo $...
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1answer
283 views

Process substitution vs FIFO (created by `makefifo`)

As far as I know, process substitution in bash and FIFO (created by mkfifo) are both named pipes. They both seem viable solutions for communication between processes. I was wondering how to decide ...
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1answer
51 views

When does a command in a process substitution or FIFO start and finish running?

When does a command in a process substitution in bash start and finish running? E.g. when does command1 in command2 >(command1) start and finish running respectively? when does command3 in ...
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GPG error: “do_plaintext: wrote X bytes but expected Y bytes”

I'm having trouble using gpg when I use the <() syntax for process substitution. Here is a minimal-working example of my problem: $ gpg2 -c -o out.gpg <(head -c 73729 /dev/zero) gpg: ...
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3answers
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Conditional process substitution

I have a command with a number of arguments: my_command --arg1 <arg1> \ --arg2 <arg2> \ --arg3 <arg3> I've wrapped the invocation of my_command in a function that sets up ...
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1answer
37 views

Misunderstanding the purpose of a process substitution

I guess I'm missing some understanding on the use cases of a process substitution. My intuition was that a process substitution, of the form <(COMMANDS) would execute COMMANDS and then feed the ...
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1answer
82 views

non-deterministic output from independent processes run concurrently by process substitution

On bash v4.1.2(2), the following simple statement, chosen merely as a minimal example demonstrating the problem, gives seemingly random output: $ for n in {0..1000}; do echo "$n"; done | tee >(...
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103 views

Bash Process Substitution Hangs When Listing Bindings

TLDR: cat <(INPUTRC=/dev/null bash -c 'bind -pm emacs') # freezes # I can't use this because I need to pipe the output of the bind cat <(INPUTRC=/dev/null bash -c 'bind -pm emacs' &) # ...
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1answer
339 views

Iteratively write to the same process substitution file on MacOs

The following bash-fu code works fine on Linux but breaks on MacOS: files="foo bar" echo PROG 1 for file in $files do echo $file | tee -a tempfile.txt done sort -u tempfile.txt echo PROG 2 ...
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2answers
310 views

`tee` and `bash` process substitution order

I'd have supposed this code would have printed "oof" first: echo foo | tee >(rev) | ( sleep 1 ; cat ; ) Output: foo oof Increasing the sleep time doesn't change the order. Why doesn't that ...
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1answer
437 views

Bash - process substitution in chroot without /dev and /proc

I have a chroot jail without /dev and /proc mounted. When I try to do process substitution here, this happens: bash-4.4# cat <(ls) cat: open /dev/fd/63: No such file or directory I tried to ...
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2answers
843 views

The process substitution output is out of the order

The echo one; echo two > >(cat); echo three; command gives unexpected output. I read this: How process substitution is implemented in bash? and many other articles about process ...
4
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1answer
730 views

Difference between subshells and process substitution

In bash, I want to assign my current working directory to a variable. Using a subshell, I can do this. var=$(pwd) echo $var /home/user.name If I use process substitution like so: var=<(pwd) ...
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1answer
1k views

bash wait for process in process substitution even if command is invalid [duplicate]

I'm trying to write a utility script errpipe with a simple api that runs stderr through a filter. At first I tried to implement it using bash's process substitution feature. #!/bin/bash com="$1" ...
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1answer
337 views

Difference between 'cmd1 | cmd2' and 'cmd2 <(cmd1)' and 'mkfifo foo; cmd1>foo; cmd2 foo'?

Or: When and why to not prefer STDIN when cmd2 can use both, and is a single command? I come across questions on this site, where 3 of their answers are one of each of the above, like this one. This ...
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2answers
296 views

bash timeout command kills randomly before duration

I'm trying to show thumbnails from youtube for specific timeout to user and then show next one I made this command: while read line; do\ wget -O- -q "$line" | \ timeout 10s display ;\ ...
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1answer
56 views

Equivalence to the usage of process substitution?

Is the usage of process substitution command1 <(command2) equivalent to command2 > ~/myfile command1 ~myfile ? Can the two above be replaced with each other? Does command1 and command2 ...
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2answers
218 views

Sensitive data in process substitution

Let's say I do: #!/bin/bash #content=$(cat -) content="foo" pass=$1 echo $content | ccrypt -f -k <(echo -n $pass) Can the permission set on the process substition /dev/fd/* files be trusted to ...
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2answers
268 views

How to execute an output string without bash's command substitution?

How to execute an output string without bash's command substitution? For example, if I type in terminal tshark, and the program isn't installed yet, I will recieve an error message like: _some text_ ...
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2answers
402 views

How to detect an error using process substitution

This question is similar to the following link, but focused on using the command line (bash shell). Using a simple example, when doing the following command: $ cat <(date); echo $? Fri Jul 7 21:...
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4answers
3k views

Why is there a “/dev/fd/63” in the output of “echo 123 >(cat)”?

$ echo 123 | cat 123 is doing what I expected, both commands run inside the same shell. But when I connect them with the >( ... ) expression, which connects the output of one command in the ...
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1answer
654 views

Subshell and process substitution

Apologies if this is a basic question - I'm stuck trying to solve a larger problem, and it's come down to how a shell script is invoked - directly (shellScript.sh) or using sh shellScript.sh. Here's ...
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1answer
1k views

Process substitution in GNU Makefiles

On a bash prompt, one can execute diff using pseudo files: diff <(echo test) <(echo test) Adding this as is into a Makefile fails: all: diff <(echo test) <(echo test) The error ...