Notes

File Descriptions & Redirections

2 min read

A file descriptor (FD) in Unix/Linux OS is an indicator of connection maintained by the kernel to perform I/O operations. In windows, its called a file-handle. By default there are 3

  • Data stream for input STDIN - 0
  • Data stream for output STDOUT - 1
  • Data stream for output that relates to an error STDERR - 2

STDIN and STDOUT The command cat. When running cat, we give the running program our standard input in STDIN - FD 0 and upon confirming with enter, it is returned to the terminal as standard output i.e. STDOUT - 1.

The same idea works even for standard output and its standard error. Lets take the example with the find command. We can redirect the resulting errors to the null devices, which discards all data.

We can redirect errors and output with greater-than (>) character. This also works for lower-than sign (<). However, it serves as standard input. So, you can use the STDIN from the previous stdout.txt file. Likewise, you can append to a file using (>>). The single greater than overwrites the info file. And a double lower-than is called the End of File function and defines the input’s end.

0xCrvngr@htb[/htb]$ find /etc/ -name shadow
find: '/etc/ssl/private': Permission Denied <- (STDERR - 2)
/etc/shadow <- (STDOUT - 1)
find: '/etc/dovecot/private': Permission Denied <- (STDERR - 2)
 
 
0xCrvngr@htb[/htb]$ find /etc/ -name shadow 2>/dev/null
/etc/shadow <- (STDOUT - 1)
 
0xCrvngr@htb[/htb]$ find /etc/ -name shadow 2> stderr.txt 1>stdout.txt
0xCrvngr@htb[/htb]$ cat stderr.txt
find: '/etc/ssl/private': Permission Denied <- (STDERR - 2)
find: '/etc/dovecot/private': Permission Denied <- (STDERR - 2)
 
0xCrvngr@htb[/htb]$ cat stdout.txt
/etc/shadow
 
0xCrvngr@htb[/htb]$ cat < stdout.txt
/etc/shadow
 
0xCrvngr@htb[/htb]$ find /etc/ -name passwd >> stdout.txt 2>/dev/null
/etc/pam.d/passwd
/etc/cron.daily/passwd
/etc/passwd
 
0xCrvngr@htb[/htb]$ cat << EOF > stream.txt
> HackTheBox
> EOF

Pipes

Can also use pipes (|) to redirect STDOUT. Commonly used ones are grep. Grep is used to filter STDOUT according to the pattern we define. Another one is wc which counts the total number of obtained results from the given input.

0xCrvngr@htb[/htb]$ find /etc/ -name *.conf 2>/dev/null | grep systemd
/etc/systemd/system.conf
/etc/systemd/login.conf
/etc/systemd/resolved.conf
 
0xCrvngr@htb[/htb]$ find /etc/ -name *.conf 2>/dev/null | grep systemd | wc -l
3

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