on input and output file descriptors

Let’s start with some basics. The basics works as well on Unix, Linux and Windows. Later techniques only work on linux/unix

$ ls -l hosts          
-rw-r--r--. 1 root root 211 Oct  5  2015 hosts
$ ls -l xxx  
ls: cannot access xxx: No such file or directory
$ read x
foo
$ 

Outpout and error are displayed on screen and input is read from your keyboard

The output is kwown as file-descriptor-1 or stdout. Sometimes, depending on your OS, it may be exposed as /dev/fd/1 or /dev/stdout. But not all *nix have this.
The error is kwown as file-descriptor-2 or stderr.
The input is known as file-descriptor-0 or stdin.

Instead of keyboard and screen, it could be a file or any other devices, e.g. /dev/null or just a simple file.

$ ls -l hosts 1>file1
$ ls -l xxx 2>file2          
$ read x 0<file3

0 and 1 are optional here.

If is also possible to redirect stdout and vice versa

$ ls -l hosts 1>&2         
-rw-r--r--. 1 root root 211 Oct  5  2015 hosts
$ ls -l xxx 2>&1 
ls: cannot access xxx: No such file or directory

It is possible to close the file descriptor.

sleep 1 1>&- 2>&- 0<&-

Well, sleep has no output and no input and no error, so the effect is not impressive.

If you write to a closed file descriptor, you get an error. Ok, if you close both stdout and stderr, the error will be silent. But there will still be an error.

$ (echo foo) 1>&-     
bash: echo: write error: Bad file descriptor
$ echo $?        
1
$ (echo bar 1>&2) 2>&-      
$ echo $?             
1

if you want to redirect stdin to stdout and stdout to stdin, you better use a new file descriptor

$ (ls -l hosts xxx 1>&2 2>&3) 3>&1            
xxx not found
-rw-rw-r-- 1 root system 2133 Jun 22 2017 hosts

An old trick is to use additional file descriptor to find a return code of command before the pipe.

$ ((((ls hosts; echo $? >&3) | tr '[a-z]' '[A-Z]' >&4) 3>&1) | (read rc; exit $rc)) 4>&1       
HOSTS
$ echo $?
0
$ ((((ls xxx; echo $? >&3) | tr '[a-z]' '[A-Z]' >&4) 3>&1) | (read rc; exit $rc)) 4>&1        
ls: cannot access xxx: No such file or directory
$ echo $?
2

As I know the trick for so long that I could not credit the author, only found some 21st century posts

If you want to redirect all your outputs to a logfile, you can use exec

#!/bin/ksh
exec 1>>/tmp/mylog
exec 2>>/tmp/mylog
cd /etc 
ls -l hosts          
ls -l xxx
exit

If you want to be able to still use your stdout / stderr, again, open new descriptors

#!/bin/ksh
exec 3>&1
exec 4>&2
exec 1>>/tmp/mylog
exec 2>>/tmp/mylog
cd /etc 
ls -l hosts          
ls -l xxx
echo INFO >&3
echo ERROR >&4
exec 3>&-
exec 4>&-
exit

Bash has also one shortcut

ls xxx host &>log

& redirect both 1 and 2 in one step. Doesn't work on ksh.

Published by Laurent Schneider

Oracle Certified Master

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