What does the -b option do in SFTP

Question

I'm looking at some old shell scripts and there is a line that I don't quite understand:

~]$ sftp -b /dev/fd/3 [email protected]

I can see from the man file that -b is for batch, and that the argument is supposed to be a batchfile.

In this case, it looks like the batch file is supposed to be on a /dev/fd/3 - a floppy drive ? I can't seem to get to it.

Any ideas what this is supposed to do ?

Answer 1

The "/dev/fd*" files are special devices. These aren't really taking up that much space on your system. They allow a process to access file descriptors by number; 0,1,2 are standard input, standard output and standard error, and other open files start with 3

In your case sftp using -b to read command from /dev/fd/3

Example:

[root@04 fd]# exec 3< /etc/resolv.conf
[root@04 fd]# cat /dev/fd/3
search example.com 

nameserver 10.10.10.10
nameserver 20.20.20.20

You can read data using read command

[root@04 fd]# read -u 3 a b
[root@04 fd]# echo $a $b
nameserver 10.10.10.10

output of /dev/fd directoy

[root@04 fd]# ls -l /dev/fd/
total 0
lrwx------ 1 root root 64 Feb 20 14:34 0 -> /dev/pts/0
lrwx------ 1 root root 64 Feb 20 14:34 1 -> /dev/pts/0
lrwx------ 1 root root 64 Feb 20 14:34 2 -> /dev/pts/0
lr-x------ 1 root root 64 Feb 20 14:34 3 -> /etc/resolv.conf

Notes: In your case that input file could be different