CODEWITHSUNDEEP
linuxbashpipe
jliu83
jliu83

Reputation: 1723

Pipe output to bash function

I have as simple function in a bash script and I would like to pipe stdout to it as an input.

jc_hms(){
  printf "$1"
}

I'd like to use it in this manner.

var=`echo "teststring" | jc_hms`

Of course I used redundant functions echo and printf to simplify the question, but you get the idea. Right now I get a "not found" error, which I assume means my parameter delimiting is wrong (the "$1" part). Any suggestions?

Originally the jc_hms function was used like this:

echo `jc_hms "teststring"` > //dev/tts/0

but I'd like to store the results in a variable for further processing first, before sending it to the serial port.

EDIT: So to clarify, I am NOT trying to print stuff to the serial port, I'd like to interface to my bash functions should the "|" pipe character, and I am wondering if this is possible.

EDIT: Alright, here's the full function.

jc_hms(){
  hr=$(($1 / 3600))
  min=$(($1 / 60))
  sec=$(($1 % 60))

  printf "$hs:%02d:%02d" $min $sec
}

I'm using the function to form a string which come this line of code

songplaytime=`echo $songtime | awk '{print S1 }'`
printstring="`jc_hms $songplaytime`"  #store resulting string in printstring

Where $songtime is a string expressed as "playtime totaltime" delimited by a space.

I wish I can just do this in one line, and pipe it after the awk

printstring=`echo $songtime | awk '{print S1 }' | jc_hms`

like so.

Upvotes: 112

Views: 108171

Answers (10)

MegaV0lt
MegaV0lt

Reputation: 26

I have modified answer from 'Der Schley' https://stackoverflow.com/a/36432966/21633953:

f_myfunc() {
  local data=("${@:-$(</dev/stdin)}")
  for parm in "${data[@]}" ; do
    # do what needs to be done on each input value
  done
}

Upvotes: 0

George Vasiliou
George Vasiliou

Reputation: 6335

I know that this is an old post, but it was the first result brought to me by search engines when i was looking the correct way to do it.

I prefer this way since it can cover all cases:
(a) accept data by argument like $1 inside a function
(b) accept data by pipe
(c) avoid hanging if /dev/stdin is not open - no data in pipe or no argument is given

function somefunc() {
if test -n "$1"; then
   dt="$1"
   #Read from positional argument $1;
elif test ! -t 0; then
   dt="$(</dev/stdin)"
   #Read from stdin if file descriptor /dev/stdin is open
else
   echo "No standard input."
fi
}

source: https://www.baeldung.com/linux/pipe-output-to-function

Upvotes: 1

zinking
zinking

Reputation: 5685

seems nothing works, but there are work arounds

mentioned work around xargs ref function

$ FUNCS=$(functions hi); seq 3 | xargs -I{} zsh -c "eval $FUNCS; hi {}"

then this doesn't work either because your function could reference another function. so I ended up writing some function that accepts pipe inputs, like this:

somefunc() {
    while read -r data; do
        printf "%s" "$data"
    done
}

Upvotes: 1

chepner
chepner

Reputation: 530823

To answer your actual question, when a shell function is on the receiving end of a pipe, standard input is inherited by all commands in the function, but only commands that actually read form their standard input consume any data. For commands that run one after the other, later commands can only see what isn't consumed by previous commands. When two commands run in parallel, which commands see which data depends on how the OS schedules the commands.

Since printf is the first and only command in your function, standard input is effectively ignored. There are several ways around that, including using the read built-in to read standard input into a variable which can be passed to printf:

jc_hms () {
    read foo
    hr=$(($foo / 3600))
    min=$(($foo / 60))
    sec=$(($foo % 60))
    printf "%d:%02d:%02d" "$hr" "$min" "$sec"
}

However, since your need for a pipeline seems to depend on your perceived need to use awk, let me suggest the following alternative:

printstring=$( jc_hms $songtime )

Since songtime consists of a space-separated pair of numbers, the shell performs word-splitting on the value of songtime, and jc_hms sees two separate parameters. This requires no change in the definition of jc_hms, and no need to pipe anything into it via standard input.

If you still have a different reason for jc_hms to read standard input, please let us know.

Upvotes: 97

moopet
moopet

Reputation: 6175

You can't pipe stuff directly to a bash function like that, however you can use read to pull it in instead:

jc_hms() {
  while read -r data; do
      printf "%s" "$data"
  done
}

should be what you want

Upvotes: 79

Farway
Farway

Reputation: 811

The proposed solutions require content on stdin or read to be only conditionally called. Otherwise the function will wait for content from the console and require an Enter or Ctrl+D before continuing.

A workaround is to use read with a timeout. e.g. read -t <seconds>

function test ()
{
  # ...
  # process any parameters
  # ...
  read -t 0.001 piped
  if [[ "${piped:-}" ]]; then
    echo $piped
  fi
}

Note, -t 0 did not work for me.
You might have to use a different value for the time-out. Too small a value might result in bugs and a too large time-out delays the script.

Upvotes: 1

Der Schley
Der Schley

Reputation: 476

I like user.friendly's answer using the Bash built-in conditional unset substitution syntax. Here's a slight tweak to make his answer more generic, such as for cases with an indeterminate parameter count:

function myfunc() {
    declare MY_INPUT=${*:-$(</dev/stdin)}
    for PARAM in $MY_INPUT; do
        # do what needs to be done on each input value
    done
}

Upvotes: 23

user.friendly
user.friendly

Reputation: 2258

1) I know this is a pretty old post

2) I like most of the answers here

However, I found this post because I needed to something similar. While everyone agrees stdin is what needs to be used, what the answers here are missing is the actual usage of the /dev/stdin file.

Using the read builtin forces this function to be used with piped input, so it can no longer be used in a typical way. I think utilizing /dev/stdin is a superior way of solving this problem, so I wanted to add my 2 cents for completeness.

My solution:

jc_hms() { 
  declare -i i=${1:-$(</dev/stdin)};
  declare hr=$(($i/3600)) min=$(($i/60%60)) sec=$(($i%60));
  printf "%02d:%02d:%02d\n" $hr $min $sec;
}

In action:

user@hostname:pwd$ jc_hms 7800
02:10:00
user@hostname:pwd$ echo 7800 | jc_hms 
02:10:00

I hope this may help someone.

Happy hacking!

Upvotes: 79

Medievalist
Medievalist

Reputation: 743

Hmmmm....

songplaytime=`echo $songtime | awk '{print S1 }'`
printstring="`jc_hms $songplaytime`"  #store resulting string in printstring

if you're calling awk anyway, why not use it?

printstring=`TZ=UTC gawk -vT=$songplaytime 'BEGIN{print strftime("%T",T)}'`

I'm assuming you're using Gnu's Awk, which is the best one and also free; this will work in common linux distros which aren't necessarily using the most recent gawk. The most recent versions of gawk will let you specify UTC as a third parameter to the strftime() function.

Upvotes: 2

Jo So
Jo So

Reputation: 26501

Or, you can also do it in a simple way.

jc_hms() {
    cat
}

Though all answers so far have disregarded the fact that this was not what OP wanted (he stated the function is simplified)

Upvotes: 32

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