How to read from a file or STDIN in Bash

Question

The following Perl script  my pl  can read from either the file on the command line args or from STDIN  while   lt  gt        print         perl my pl will read from STDIN  while perl my pl a txt will read from a txt  This is very handy  Wondering is there an equivalent in Bash

User · Answer

I don t find any of these answers acceptable  In particular  the accepted answer only handles the first command line parameter and ignores the rest  The Perl program that it is trying to emulate handles all the command line parameters  So the accepted answer doesn t even answer the question  Other answers use bash extensions  add unnecessary  cat  commands  only work for the simple case of echoing input to output  or are just unnecessarily complicated   However  I have to give them some credit because they gave me some ideas  Here is the complete answer      bin sh  if        0   then         DEFAULT INPUT FILE  dev stdin else         DEFAULT INPUT FILE  fi    Iterates over all parameters or  dev stdin for FILE in       DEFAULT INPUT FILE do         while IFS  read -r LINE         do                   Do whatever you want with LINE here                  echo  LINE         done  lt    FILE  done

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The following solution reads from a file if the script is called with a file name as the first parameter  1 otherwise from standard input       while read line do   echo   line  done  lt     1 - dev stdin     The substitution   1 -     takes  1 if defined otherwise the file name of the standard input of the own process is used

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The echo solution adds new lines whenever IFS breaks the input stream   fgm s answer can be modified a bit   cat    1 - dev stdin    gt     2 - dev stdout

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Perl s behavior  with the code given in the OP can take none or several arguments  and if an argument is a single hyphen - this is understood as stdin  Moreover  it s always possible to have the filename with  ARGV  None of the answers given so far really mimic Perl s behavior in these respects  Here s a pure Bash possibility  The trick is to use exec appropriately      bin bash            set -- - while         do          1   -       exec  lt    1      amp  amp     while read -r  do       printf   s n    REPLY     done    shift done   Filename s available in  1   If no arguments are given  we artificially set - as the first positional parameter  We then loop on the parameters  If a parameter is not -  we redirect standard input from filename with exec  If this redirection succeeds we loop with a while loop  I m using the standard REPLY variable  and in this case you don t need to reset IFS  If you want another name  you must reset IFS like so  unless  of course  you don t want that and know what you re doing    while IFS  read -r line  do     printf   s n    line  done

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Here is the simplest way      bin sh cat -   Usage     echo test   sh my script sh test   To assign stdin to the variable  you may use  STDIN   cat -  or just simply STDIN   cat  as operator is not necessary  as per  mklement0 comment      To parse each line from the standard input  try the following script      bin bash while IFS  read -r line  do   printf   s n    line  done     To read from the file or stdin  if argument is not present   you can extend it to      bin bash file   1--    POSIX-compliant    1 --  can be used either  while IFS  read -r line  do   printf   s n    line    Or  env POSIXLY CORRECT 1 echo   line  done  lt   lt  cat --   file        Notes       - read -r - Do not treat a backslash character in any special way  Consider each backslash to be part of the input line       - Without setting IFS  by default the sequences of Space and Tab at the beginning and end of the lines are ignored  trimmed        - Use printf instead of echo to avoid printing empty lines when the line consists of a single -e  -n or -E  However there is a workaround by using env POSIXLY CORRECT 1 echo   line  which executes your external GNU echo which supports it  See  How do I echo  -e     See  How to read stdin when no arguments are passed  at stackoverflow SE

User · Answer

Please try the following code   while IFS  read -r line  do     echo   line  done  lt  file

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I think this is the straight-forward way     cat reader sh    bin bash while read line  do   echo  reading    line   done  lt   dev stdin   --    cat writer sh    bin bash for i in  0  5   do   echo  line   i   done   --      writer sh     reader sh reading  line 0 reading  line 1 reading  line 2 reading  line 3 reading  line 4 reading  line 5

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More accurately     while IFS  read -r line   do     printf   s n    line  done  lt  file

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I combined all of the above answers and created a shell function that would suit my needs  This is from a cygwin terminal of my 2 Windows10 machines where I had a shared folder between them  I need to be able to handle the following    cat file cpp   tx tx  lt  file cpp tx file cpp   Where a specific filename is specified  I need to used the same filename during copy  Where input data stream has been piped thru  then i need to generate a temporary filename having the hour minute and seconds  The shared mainfolder has subfolders of the days of the week  This is for organizational purposes    Behold  the ultimate script for my needs   tx        if      -eq 0    then     local TMP  tmp tx   date    H M S       while IFS  read -r line  do         echo   line      done  lt   dev stdin  gt   TMP     cp  TMP    OTHER stargate   date    a        rm -f  TMP   else       -r  1    amp  amp  cp  1    OTHER stargate   date    a       echo  cannot read file    fi     If there is any way that you can see to further optimize this  I would like to know

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Perhaps the simplest solution is to redirect stdin with a merging redirect operator      bin bash less  lt  amp 0   Stdin is file descriptor zero  The above sends the input piped to your bash script into less s stdin   Read more about file descriptor redirection

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Code   1 - dev stdin  will just understand first argument  so  how about this   ARGS      if   -z         then   ARGS  -  fi eval  cat --  ARGS    while read line do    echo   line  done

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The following works with standard sh  Tested with dash on Debian  and is quite readable  but that s a matter of taste   if   -n   1     then     cat   1  else     cat fi   commands and transformations   Details  If the first parameter is non-empty then cat that file  else cat standard input  Then the output of the whole if statement is processed by the commands and transformations

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How about  for line in  cat   do     something  line   done

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as a workaround you can use the stdin device in  dev directory        for item in  cat  dev stdin    do echo  item  done

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The Perl loop in the question reads from all the file name arguments on the command line  or from standard input if no files are specified  The answers I see all seem to process a single file or standard input if there is no file specified   Although often derided accurately as UUOC  Useless Use of cat   there are times when cat is the best tool for the job  and it is arguable that this is one of them   cat        while read -r line do     echo   line  done   The only downside to this is that it creates a pipeline running in a sub-shell  so things like variable assignments in the while loop are not accessible outside the pipeline  The bash way around that is Process Substitution   while read -r line do     echo   line  done  lt   lt  cat         This leaves the while loop running in the main shell  so variables set in the loop are accessible outside the loop

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usr bin bash  if   -p  dev stdin    then      for FILE in  quot    quot   dev stdin     for FILE in  dev stdin     do         while IFS  read -r LINE         do             echo  quot    quot   quot  LINE quot     print line argument and stdin         done  lt   quot  FILE quot      done else     printf  quot   -p  dev stdin   is false n quot       dosomething fi  running  echo var var2   bash std sh   result  var var2  running  bash std sh  lt   lt  cat  etc passwd   result  root x 0 0   root  usr bin bash                                                                                                                                                                                         bin x 1 1     usr bin nologin                                                                                                                                                                                           daemon x 2 2     usr bin nologin                                                                                                                                                                                        mail x 8 12   var spool mail  usr bin nologin

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This one is easy to use on the terminal     echo  1 n2 n3 n    while read -r  do echo  REPLY  done 1 2 3

[bash] How to read from a file or STDIN in Bash?

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