File Descriptors and Redirection
File descriptors provide a uniform way for processes to read and write streams.
Overview
0 is stdin, 1 is stdout, and 2 is stderr- Redirection rewires those streams to files or
/dev/null
- Pipes connect the output of one command to the input of another
Reference
Standard file descriptors
| FD |
Name |
Purpose |
0 |
STDIN |
Standard input |
1 |
STDOUT |
Standard output |
2 |
STDERR |
Standard error |
Redirection
| Operator |
Meaning |
> |
Overwrite a file with stdout |
>> |
Append stdout to a file |
< |
Use a file as stdin |
2> |
Redirect stderr |
1> |
Redirect stdout explicitly |
find /etc/ -name networks 2>/dev/null
find /etc/ -name networks > results.txt
find /etc/ -name networks 2>/dev/null 1>stdout.txt
cat < stdout.txt
Here documents
cat << EOF >> stream.txt
My
Input
Stream
EOF
Pipes
find /etc/ -name "*.conf" 2>/dev/null | grep systemd | wc -l
Mental Model
- Each process reads from FD
0 and writes to FD 1 or 2
- Redirection changes where those streams go
- Pipes build dataflow pipelines between commands
/dev/null is a sink for discarded output