Can I avoid using a FIFO file to join the end of a Bash pipeline to be stored in a variable in the current shell? - linux

Question

I have the following functions:
execIn ()
{
local STORE_INvar="${1}" ; shift
printf -v "${STORE_INvar}" '%s' "$( eval "$#" ; printf %s x ; )"
printf -v "${STORE_INvar}" '%s' "${!STORE_INvar%x}"
}
and
getFifo ()
{
local FIFOfile
FIFOfile="/tmp/diamondLang-FIFO-$$-${RANDOM}"
while [ -e "${FIFOfile}" ]
do
FIFOfile="/tmp/diamondLang-FIFO-$$-${RANDOM}"
done
mkfifo "${FIFOfile}"
echo "${FIFOfile}"
}
I want to store the output of the end of a pipeline into a variable as given to a function at the end of the pipeline, however, the only way I have found to do this that will work in early versions of Bash is to use mkfifo to make a temp fifo file. I was hoping to use file descriptors to avoid having to create temporary files. So, This works, but is not ideal:
Set Up: (before I can do this I need to have assigned a FIFO file to a var that can be used by the rest of the process)
$ FIFOfile="$( getFifo )"
The Pipeline I want to persist:
$ printf '\n\n123\n456\n524\n789\n\n\n' | grep 2 # for e.g.
The action: (I can now add) >${FIFOfile} &
$ printf '\n\n123\n456\n524\n789\n\n\n' | grep 2 >${FIFOfile} &
N.B. The need to background it with & - Problem 1: I get [1] <PID_NO> output to the screen.
The actual persist:
$ execIn SOME_VAR cat - <${FIFOfile}
Problem 2: I get more noise to the screen
[1]+ Done printf '\n\n123\n456\n524\n789\n\n\n' | grep 2 > ${FIFOfile}
Problem 3: I loose the blanks at the start of the stream rather than at the end as I have experienced before.
So, am I doing this the right way? I am sure that there must be a way to avoid the need of a FIFO file that needs cleanup afterwards using file descriptors, but I cannot seem to do this as I cannot assign either side of the problem to a file descriptor that is not attached to a file or a FIFO file.
I can try and resolve the problems with what I have, although to make this work properly I guess I need to pre-establish a pool of FIFO files that can be pulled in to use or else I have a pre-req of establishing this file before the command. So, for many reasons this is far from ideal. If anyone can advise me of a better way you would make my day/week/month/life :)
Thanks in advance...

Answer 1

Process substitution was available in bash from the ancient days. You absolutely do not have a version so ancient as to be unable to use it. Thus, there's no need to use a FIFO at all:
readToVar() { IFS= read -r -d '' "$1"; }
readToVar targetVar < <(printf '\n\n123\n456\n524\n789\n\n\n')
You'll observe that:
printf '%q\n' "$targetVar"
...correctly preserves the leading newlines as well as the trailing ones.
By contrast, in a use case where you can't afford to lose stdin:
readToVar() { IFS= read -r -d '' "$1" <"$2"; }
readToVar targetVar <(printf '\n\n123\n456\n524\n789\n\n\n')
If you really want to pipe to this command, are willing to require a very modern bash, and don't mind being incompatible with job control:
set +m # disable job control
shopt -s lastpipe # in a pipeline, parent shell becomes right-hand side
readToVar() { IFS= read -r -d '' "$1"; }
printf '\n\n123\n456\n524\n789\n\n\n' | grep 2 | readToVar targetVar

Answer 2

The issues you claim to run into with using a FIFO do not actually exist. Put this in a script, and run it:
#!/bin/bash
trap 'rm -rf "$tempdir"' 0 # cleanup on exit
tempdir=$(mktemp -d -t fifodir.XXXXXX)
mkfifo "$tempdir/fifo"
printf '\n\n123\n456\n524\n789\n\n\n' >"$tempdir/fifo" &
IFS= read -r -d '' content <"$tempdir/fifo"
printf '%q\n' "$content" # print content to console
You'll notice that, when run in a script, there is no "noise" printed to the screen, because all that status is explicitly tied to job control, which is disabled by default in scripts.
You'll also notice that both leading and tailing newlines are correctly represented.

Answer 3

One idea, tell me I am crazy, might be to use the !! notation to grab the line just executed, e.g. if there is a command that can terminate a pipeline and stop it actually executing, whilst still as far as the shell is concerned, consider it as a successful execution, I am thinking something like the true command, I could then use !! to grab that line and call my existing function to execute it with process substitution or something. I could then wrap this into an alias, something like: alias streamTo=' | true ; LAST_EXEC="!!" ; myNewCommandVariation <<<' which I think could be used something like: $ cmd1 | cmd2 | myNewCommandVariation THE_VAR_NAME_TO_SET and the <<< from the alias would pass the var name to the command as an arg or stdin, either way, the command would be not at the end of a pipeline. How mad is this idea?

Answer 4

Not a full answer but rather a first point: is there some good reason not using mktemp for creating a new file with a random name? As far as I can see, your function called getFifo() doesn't perform much more.
mktemp -u
will give to you a free new name without creating anything; then you can use mkfifo with this name.