Peek at the contents of stdin

strace ?

Example below. Start a cat process which is reading stdin and writing to /tmp/foofile. Find the pid, strace it. And in the original cat window, enter some text, hey presto.

# cat >/tmp/foofile


# ps -ef|grep cat
steve     2134  1801  0 22:25 pts/2    00:00:00 cat
# strace -fp 2134
Process 2134 attached
read(0, "test\n", 65536)                = 5
write(1, "test\n", 5)                   = 5
read(0,

To just pluck out the reads from file descriptor 0:

strace -fp 2134 -e trace=read -o "|grep read.0,"