Passing Parameter To Bash Shell Script Through Input Redirection (<)

Question

So, I need to write a shell script that accepts a file path as an argument and uses it within the shell script. I know that I could do something like:

$ ./script filename

With this solution, this would make it so that I could access this filename string using $1 in the shell script.

However, I'm wondering if it is possible to use input redirection instead so I could pass the filename using it:

$ ./script < filename

If so, how would I access this filename in the shell script?
I'm fairly new to shell script coding so I'm not sure if this is even possible.

Raul Andres · Accepted Answer · 2014-02-25 09:22:11Z

5

You can extract the name of file passed with < by listing /proc/<pid>/fd directory, as follows:

ls -ltr /proc/$$/fd

Then

$ cat myscript.sh
ls -ltr /proc/$$/fd
$ ./myscript.sh < hello
total 0
lr-x------ 1 u g 64 Feb 25 08:42 255 -> /tmp/myscript.sh
lrwx------ 1 u g 64 Feb 25 08:42 2 -> /dev/pts/0
lrwx------ 1 u g 64 Feb 25 08:42 1 -> /dev/pts/0
lr-x------ 1 u g 64 Feb 25 08:42 0 -> /tmp/hello

I can't guess whether this is useful

And doesn't work when input is passed through a pipe

$ cat hello| ./myscript.sh 
total 0
lr-x------ 1 u p 64 Feb 25 08:50 255 -> /tmp/myscript.sh
lrwx------ 1 u p 64 Feb 25 08:50 2 -> /dev/pts/0
lrwx------ 1 u p 64 Feb 25 08:50 1 -> /dev/pts/0
lr-x------ 1 u p 64 Feb 25 08:50 0 -> pipe:[82796]

Alternatively, you can use lsof and a little line handling to extract value

 filename=$(/usr/sbin/lsof -p $$| grep " 0u"| cut -c 60-)

edited Feb 25, 2014 at 9:22

answered Feb 25, 2014 at 8:44

Raul Andres

3,81617 silver badges24 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

isedev Over a year ago

nice one, hadn't thought of that... not sure if that's portable, but neat trick all the same... come to think of it, the same could be achieve parsing the output of lsof -p $$.

Raul Andres Over a year ago

@isedev Thank you, lsof -p $$ seems more portable. It's possible too to use lsof to follow pipes, but it seems as useless as the basic case

Bryan Muscedere Over a year ago

Is it possible to translate /proc/$$/fd/0 to the actual file name passed?

Raul Andres Over a year ago

filename=/usr/sbin/lsof -p $$| grep " 0u" cut -c 60- works for me

isedev · Accepted Answer · 2014-02-25 08:38:57Z

2

You cannot access the filename in the case of ./script < filename but only its contents. The reason is that the shell opens filename and set the corresponding file descriptor to be the standard input of the ./script.

answered Feb 25, 2014 at 8:38

isedev

19.7k3 gold badges65 silver badges60 bronze badges

Comments

SzG · Accepted Answer · 2014-02-25 09:08:03Z

1

With your second solution you don't access the filename, but your script's standard input is coming from the file.

With the first solution printing the file would look like this:

cat "$1"

With the second:

while read line; do echo "$line"; done

edited Feb 25, 2014 at 9:08

answered Feb 25, 2014 at 8:38

SzG

12.7k4 gold badges31 silver badges43 bronze badges

1 Comment

Olivier Dulac Over a year ago

should be while IFS= read -r line;, and you're also missing quotes around $line (echo "$line") .

Collectives™ on Stack Overflow

Passing Parameter To Bash Shell Script Through Input Redirection (<)

3 Answers 3

4 Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related