<<Anchor(faq89)>>
== I'm reading a file line by line and running ssh or ffmpeg, only the first line gets processed! ==
When [[BashFAQ/001|reading a file line by line]], if a command inside the loop also reads stdin, it can exhaust the input file.  For example:

{{{#!highlight bash
# Non-working example
while IFS= read -r file; do
  ffmpeg -i "$file" -c:v libx264 -c:a aac "${file%.avi}".mkv
done < <(find . -name '*.avi')
}}}

{{{#!highlight bash
# Non-working example
while read host; do
  ssh "$host" some command
done < hostslist
}}}

What's happening here?  Let's take the first example.  `read` reads a line from standard input (FD 0), puts it in the file parameter, and then `ffmpeg` is executed.  Like any program you execute from BASH, `ffmpeg` inherits standard input.  However, `ffmpeg` additionally uses standard input to detect quit commands indicated by user input of `q`, thus sucking up all the input from the `find` command and starving the loop.

Use the `-nostdin` global option in `ffmpeg` to disable interaction on standard input:
{{{#!highlight bash
while IFS= read -r file; do
  ffmpeg -nostdin -i "$file" -c:v libx264 -c:a aac "${file%.avi}".mkv
done < <(find . -name '*.avi')
}}}
Alternatively you could use [[BashGuide/InputAndOutput#Redirection|redirection]] at the end of the ffmpeg line: `</dev/null`. The ssh example can be fixed the same way, or with the `-n` switch (at least with [[http://www.openssh.org/|OpenSSH]]).

Sometimes with large loops it might be difficult to work out what's reading from stdin, or a program might change its behaviour when you add `</dev/null` to it.  In this case you can make read use a different FileDescriptor that a random program is less likely to read from:
{{{#!highlight bash
while IFS= read -r line <&3; do
  ...
done 3< file
}}}

In bash, the `read` builtin can also be told to read directly from an fd (`-u fd`) without redirection, and since bash 4.1, an available fd can be assigned (`{var}<file`) instead of hard coding a file descriptor.

{{{#!highlight bash
# bash 4.1+
while IFS= read -r -u "$fd" line; do
  ...
done {fd}< file
exec {fd}<&-
}}}