Bash Scripting: Pipe input to script vs. $1

jhwilliams · 07-20-2007, 10:55 AM

I have a script - let's callit MyCoolShellScript, which takes $1 as a list of files. It would be more convenient in this instance, and in the future if instead of saying

Code:

(method to create filelist) > .filelist
MyCoolShellScript .filelist

I could just say

Code:

(method to create filelist) | MycoolShellScript

However, it would also be convenient to maintain the $1 functionality.
How do you handle piped stdin within a shell script?

unSpawn · 07-20-2007, 11:06 AM

Code:

#!/bin/sh
case "$#" in
0) while read line; do
        line=(${line}); echo -en "Input contains ${#line[@]} elements"
        [ ${#line[@]} -ge 3 ] && echo ", the 3rd element is ${line[2]}." \
        || echo .
   done
   ;;
-*|--*) echo help; exit 1
   ;;
*) args=($@); echo -en "Input comprises of ${#args[@]} arguments"
        [ $# -ge 3 ] && echo ", the 3rd element is ${args[2]}." \
        || echo .
   ;;
esac

exit 0

Code:

]$ echo 0 1 nonexistent 3  | sh example.sh
Input contains 4 elements, the 3rd element is nonexistent.

]$ sh example.sh 0 1 garbage 3
Input comprises of 4 arguments, the 3rd element is garbage.

Not foolproof but it'll do as an example I hope.

SeRGeiSarov · 07-20-2007, 11:11 AM

May be

Code:

(method to create filelist) | xargs MycoolShellScript

carlanderson · 12-21-2007, 10:54 PM

On my system (probably on yours too) you can test for pipe input by looking into /proc/$$/fd/0 - which is the standard input (stdin) file descriptor symlink.
Here's an example script and three examples of it in use:

Code:

#!/bin/sh
# file: example.sh
readlink /proc/$$/fd/0

Code:

bash$ ./example.sh
/dev/pts/0

bash$ ls | ./example.sh
pipe:[27112838]

bash$ ./example.sh < input.txt
input.txt

Also, if you want to read the input that was piped into the script, you can simply use cat to read it from the pipe.

Code:

#!/bin/sh

if readlink /proc/$$/fd/0 | grep -q "^pipe:"; then
  echo "Pipe input:"
elif file $( readlink /proc/$$/fd/0 ) | grep -q "character special"; then
  echo "Standard input:"
else
  echo "File input:"
fi

cat # read from stdin and write to stdout