Messing around with stdin, stdout, and stderr

Well, if we tinker a little bit with the file descriptors and special characters we can have some nice, really nice, outcomes; let's see what we can do.

• x < filename: This opens a file in read mode and the descriptor named a, whose value falls between 3 and 9. We can choose any name by the means of which we can easily access the file content through the stdin.
• 1 > filename: This redirects the standard output to filename. If it does exist, it gets created; if it exists, the pre-existing data is overwritten.
• 1 >> filename: This redirects the standard output to filename. If it does exist, it is created; otherwise, the contents get appended to the pre-existing data.
• 2 > filename: This redirects the standard error to filename. If it does not exist, it gets created; if it exists, the pre-existing data is overwritten.
• 2 >> filename: This redirects the standard error to filename. If it does not exist, it is created; otherwise, the contents get appended to the pre-existing data.
• &> filename: This redirects both the stdout and the stderr to filename. This redirects the standard error to filename. If it does not exist, it gets created; if it exists, the pre-existing data is overwritten.
• 2>&1: This redirects the stderr to the stdout. If you use this with a program, its error messages will be redirected to the stdout, that is, usually, the monitor.
• y>&x: This redirects the file descriptor for y to x so that the output from the file pointed by descriptor y will be redirected to the file pointed by descriptor x.
• >&x: This redirects the file descriptor 1 that is associated with the stdout to the file pointed by the descriptor x, so whatever hits the standard output will be written in the file pointed by x.
• x<> filename: This opens a file in read/write mode and assigns the descriptor x to it. If the file does not exist, it is created, and if the descriptor is omitted, it defaults to 0, the stdin.
• x<&-: This closes the file opened in read mode and associated with the descriptor x.
• 0<&- or <&-: This closes the file opened in read mode and associated with the descriptor 0, the stdin , which is then closed.
• x>&-: This closes the file opened in write mode and associated with the descriptor x.
• 1>&- or >&-: This closes the file opened in write mode and associated with the descriptor 1, the stdout, which is then closed.

If you want to see which file descriptors are associated with a process, you can explore the /proc directory and point to the following:

/proc/pid/fd

Under that path, change PID with the ID of the process you want to explore; you will find all the file descriptors associated with it, as in the following example:

gzarrelli:~$ ls -lah /proc/15820/fd
total 0
dr-x------ 2 postgres postgres  0 Jan 20 17:59 .
dr-xr-xr-x 9 postgres postgres  0 Jan 20 09:59 ..
lr-x------ 1 postgres postgres 64 Jan 20 17:59 0 -> /dev/null 
(deleted)
l-wx------ 1 postgres postgres 64 Jan 20 17:59 1 -> /var/log/postgresql/postgresql-9.4-main.log
lrwx------ 1 postgres postgres 64 Jan 20 17:59 10 -> /var/lib/postgresql/9.4/main/base/16385/16587
lrwx------ 1 postgres postgres 64 Jan 20 17:59 11 -> socket:[13135]
lrwx------ 1 postgres postgres 64 Jan 20 17:59 12 -> socket:[1502010]
lrwx------ 1 postgres postgres 64 Jan 20 17:59 13 -> /var/lib/postgresql/9.4/main/base/16385/16591
lrwx------ 1 postgres postgres 64 Jan 20 17:59 14 -> /var/lib/postgresql/9.4/main/base/16385/16593
lrwx------ 1 postgres postgres 64 Jan 20 17:59 15 -> /var/lib/postgresql/9.4/main/base/16385/16634
lrwx------ 1 postgres postgres 64 Jan 20 17:59 16 -> /var/lib/postgresql/9.4/main/base/16385/16399
lrwx------ 1 postgres postgres 64 Jan 20 17:59 17 -> /var/lib/postgresql/9.4/main/base/16385/16406
lrwx------ 1 postgres postgres 64 Jan 20 17:59 18 -> /var/lib/postgresql/9.4/main/base/16385/16408
l-wx------ 1 postgres postgres 64 Jan 20 17:59 2 -> /var/log/postgresql/postgresql-9.4-main.log
lr-x------ 1 postgres postgres 64 Jan 20 17:59 3 -> /dev/urandom
l-wx------ 1 postgres postgres 64 Jan 20 17:59 4 -> /dev/null 
(deleted)
l-wx------ 1 postgres postgres 64 Jan 20 17:59 5 -> /dev/null 
(deleted)
lr-x------ 1 postgres postgres 64 Jan 20 17:59 6 -> pipe:[1502013]
l-wx------ 1 postgres postgres 64 Jan 20 17:59 7 -> pipe:[1502013]
lrwx------ 1 postgres postgres 64 Jan 20 17:59 8 -> /var/lib/postgresql/9.4/main/base/16385/11943
lr-x------ 1 postgres postgres 64 Jan 20 17:59 9 -> pipe:[13125]

Nice, isn't it? So, let us do something that is absolute fun:

First, let's open a socket in read/write mode to the web server of a virtual machine created for this book and assign the descriptor 9:

gzarrelli:~$ exec 9<> /dev/tcp/172.16.210.128/80 || exit 1

Then, let us write something to it; nothing complex:

gzarrelli:~$ printf 'GET /index2.html HTTP/1.1\nHost: 172.16.210.128\nConnection: close\n\n' >&9

We just requested a simple HTML file created for this example.

And now let us read the file descriptor 9:

gzarrelli:~$ cat <&9
HTTP/1.1 200 OK
Date: Sat, 21 Jan 2017 17:57:33 GMT
Server: Apache/2.4.10 (Debian)
Last-Modified: Sat, 21 Jan 2017 17:57:12 GMT
ETag: "f3-5469e7ef9e35f"
Accept-Ranges: bytes
Content-Length: 243
Vary: Accept-Encoding
Connection: close
Content-Type: text/html
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
   "http://www.w3.org/TR/html4/strict.dtd">
<HTML>
   <HEAD>
      <TITLE>This is a test file</TITLE>
   </HEAD>
   <BODY>
      <P>And we grabbed it through our descriptor!
   </BODY>
</HTML>

That's it! We connected the descriptor to a server through a socket, we could write to it and read the response, redirecting the streams over the network.

For dealing just with the command line, we have done a so far, but if we want to go further, we have to see how to script all these commands and make the most out of them. It is time for our first script!