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Old 07-20-2007, 11:55 AM   #1
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Bash Scripting: Pipe input to script vs. $1

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

(method to create filelist) > .filelist
MyCoolShellScript .filelist
I could just say

(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?

Last edited by jhwilliams; 07-20-2007 at 11:56 AM.
Old 07-20-2007, 12:06 PM   #2
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case "$#" in
0) while read line; do
        line=(${line}); echo -en "Input contains ${#line[@]} elements"
        [ ${#line[@]} -ge 3 ] && echo ", the 3rd element is ${line[2]}." \
        || echo .
-*|--*) echo help; exit 1
*) args=($@); echo -en "Input comprises of ${#args[@]} arguments"
        [ $# -ge 3 ] && echo ", the 3rd element is ${args[2]}." \
        || echo .

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

]$ 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.

Last edited by unSpawn; 07-20-2007 at 12:08 PM.
Old 07-20-2007, 12:11 PM   #3
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May be
(method to create filelist) | xargs MycoolShellScript
Old 12-21-2007, 11:54 PM   #4
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detecting pipe input from a script

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

bash$ ls | ./

bash$ ./ < 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.

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:"
  echo "File input:"

cat # read from stdin and write to stdout


bash, pipe, stdin

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