Read values into a shell variable from a pipe

Question

I am trying to get bash to process data from stdin that gets piped into  but no luck  What I mean is none of the following work   echo  hello world    test     lt   dev stdin    echo test  test test   echo  hello world    read test  echo test  test test   echo  hello world    test  cat   echo test  test test    where I want the output to be test hello world  I ve tried putting    quotes around   test  that doesn t work either

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In my eyes the best way to read from stdin in bash is the following one  which also lets you work on the lines before the input ends   while read LINE  do     echo  LINE done  lt   dev stdin

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read won t read from a pipe  or possibly the result is lost because the pipe creates a subshell   You can  however  use a here string in Bash     read a b c  lt  lt  lt    echo 1 2 3    echo  a  b  c 1 2 3   But see  chepner s answer for information about lastpipe

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Piping something into an expression involving an assignment doesn t behave like that   Instead  try   test   echo  hello world    echo test  test

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The following code   echo  hello world      test     lt   dev stdin    echo test  test     will work too  but it will open another new sub-shell after the pipe  where  echo  hello world      test     lt   dev stdin    echo test  test      won t     I had to disable job control to make use of chepnars  method  I was running this command from terminal    set  m shopt -s lastpipe echo  hello world    read test  echo test  test echo  hello world    test     lt  dev stdin    echo test  test   Bash Manual says      lastpipe      If set  and job control is not active  the shell runs the last command   of a pipeline not executed in the background in the current shell   environment    Note  job control is turned off by default in a non-interactive shell and thus you don t need the set  m inside a script

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I wanted something similar - a function that parses a string that can be passed as a parameter or piped  I came up with a solution as below  works as    bin sh and as    bin bash     bin sh  set -eu  my func         local content  quot  quot             if the first param is an empty string or is not set     if   -z   1 x     then             read content from a pipe if passed or from a user input if not passed         while read line  do content  quot   content  line quot   done  lt   dev stdin        first param was set  it may be an empty string      else         content  quot  1 quot      fi      echo  quot Content    content  quot        printf  quot 0    my func  quot  quot   n quot  printf  quot 1    my func  quot one quot   n quot  printf  quot 2    echo  quot two quot    my func  n quot  printf  quot 3    my func  n quot  printf  quot End n quot   Outputs  0  Content     1  Content   one  2  Content   two  typed text 3  Content   typed text  End  For the last case  3   you need to type  hit enter and CTRL D to end the input

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This is another option    read test  lt   lt  echo hello world     echo  test hello world

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How about this   echo  hello world    echo test   cat

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bash 4 2 introduces the lastpipe option  which allows your code to work as written  by executing the last command in a pipeline in the current shell  rather than a subshell   shopt -s lastpipe echo  hello world    read test  echo test  test

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I m no expert in Bash  but I wonder why this hasn t been proposed   stdin   cat   echo   stdin    One-liner proof that it works for me     fortune   eval  stdin   cat   echo   stdin

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Because I fall for it  I would like to drop a note  I found this thread  because I have to rewrite an old sh script to be POSIX compatible  This basically means to circumvent the pipe subshell problem introduced by POSIX by rewriting code like this   some command   read a b c   into   read a b c  lt  lt  EOF   some command  EOF   And code like this   some command   while read a b c  do       something done   into   while read a b c  do       something done  lt  lt  EOF   some command  EOF   But the latter does not behave the same on empty input  With the old notation the while loop is not entered on empty input  but in POSIX notation it is  I think it s due to the newline before EOF  which cannot be ommitted  The POSIX code which behaves more like the old notation looks like this   while read a b c  do     case  a in      break  esac       something done  lt  lt  EOF   some command  EOF   In most cases this should be good enough  But unfortunately this still behaves not exactly like the old notation if some command prints an empty line  In the old notation the while body is executed and in POSIX notation we break in front of the body   An approach to fix this might look like this   while read a b c  do     case  a in   something guaranteed not to be printed by some command   break  esac       something done  lt  lt  EOF   some command  echo  something guaranteed not to be printed by some command  EOF

User · Answer

I think you were trying to write a shell script which could take input from stdin  but while you are trying it to do it inline  you got lost trying to create that test  variable  I think it does not make much sense to do it inline  and that s why it does not work the way you expect   I was trying to reduce           head -n  X   tail -n 1     to get a specific line from various input  so I could type      cat program file c   line 34   so I need a small shell program able to read from stdin  like you do   22 14     cat   bin line     bin sh  if      -ne 1    then echo enter a line number to display  exit  fi cat   head -n  1   tail -n 1 22 16        there you go

User · Answer

if you want to read in lots of data and work on each line separately you could use something like this   cat myFile   while read x   do echo  x   done   if you want to split the lines up into multiple words you can use multiple variables in place of x like this   cat myFile   while read x y   do echo  y  x   done   alternatively   while read x y   do echo  y  x   done  lt  myFile   But as soon as you start to want to do anything really clever with this sort of thing you re better going for some scripting language like perl where you could try something like this   perl -ane  print   F 0  n    lt  myFile   There s a fairly steep learning curve with perl  or I guess any of these languages  but you ll find it a lot easier in the long run if you want to do anything but the simplest of scripts  I d recommend the Perl Cookbook and  of course  The Perl Programming Language by Larry Wall et al

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A smart script that can both read data from PIPE and command line arguments     bin bash if    -p  proc self fd 0        then     PIPE   cat -      echo  quot PIPE  PIPE quot  fi echo  quot ARGS    quot   Output    bash test arg1 arg2 ARGS arg1 arg2    echo pipe data1   bash test arg1 arg2 PIPE pipe data1 ARGS arg1 arg2  Explanation  When a script receives any data via pipe  then the stdin  proc self fd 0 will be a symlink to a pipe   proc self fd 0 - gt  pipe  155938   If not  it will point to the current terminal   proc self fd 0 - gt   dev pts 5  The bash    -p option can check it it is a pipe or not  cat - reads the from stdin  If we use cat - when there is no stdin  it will wait forever  that is why we put it inside the if condition

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Use  IFS  read var  lt  lt  EOF   foo  EOF   You can trick read into accepting from a pipe like this   echo  hello world      read test  echo test  test      or even write a function like this   read from pipe     read       lt  amp 0      But there s no point - your variable assignments may not last  A pipeline may spawn a subshell  where the environment is inherited by value  not by reference  This is why read doesn t bother with input from a pipe - it s undefined   FYI  http   www etalabs net sh tricks html is a nifty collection of the cruft necessary to fight the oddities and incompatibilities of bourne shells  sh

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The syntax for an implicit pipe from a shell command into a bash variable is  var   command    or  var  command    In your examples  you are piping data to an assignment statement  which does not expect any input

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The first attempt was pretty close  This variation should work   echo  hello world      test    lt   dev stdin   echo  test  test        and the output is      test hello world   You need braces after the pipe to enclose the assignment to test and the echo    Without the braces  the assignment to test  after the pipe  is in one shell  and the echo  test  test  is in a separate shell which doesn t know about that assignment  That s why you were getting  test   in the output instead of  test hello world

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