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Redirection

Every command has three standard connections to the outside world:

Connection Number Associated What it does
stdin 0 Standard input (usually your keyboard)
stdout 1 Standard output (usually your screen)
stderr 2 Standard error (error messages)

You've been using stdin, stdout, and stderr already. You just didn't know the names.

  • When you type echo "Hello" - you see "Hello" on screen (that's stdout)
  • When you type ls fakefile.txt - you see an error on screen (that's stderr)
  • When you type a command - your keyboard sends input (that's stdin)

Redirection lets you change where these connections go. For example, to files instead of your screen.

> - Save output to a file (stdout)

Instead of printing to your screen, send the output to a file.

ls > files.txt

What happens:

  • ls runs normally
  • Output goes to files.txt instead of your screen
  • If files.txt exists, it gets overwritten
echo "Hello" > hello.txt   # creates hello.txt with "Hello"
cat hello.txt
# Hello

cat file1.txt file2.txt > merged.txt # merge two files into one new file

> default is 1> - meaning stdout, so no need to write the number 1.

>> - Append output to a file (add, don't overwrite)

Same as >, but adds to the end instead of overwriting.

echo "First line" > file.txt
echo "Second line" >> file.txt
cat file.txt
# First line
# Second line

Use >> when you want to keep what's already there.

2> - Save error messages (stderr)

By default, error messages go to your screen too. 2> captures only errors.

ls fakefile.txt 2> errors.txt
cat errors.txt
# ls: cannot access 'fakefile.txt': No such file or directory

Combining stdout and stderr

Syntax What it does
command > output.txt 2>&1 Send both stdout and stderr to same file
command &> output.txt Same as above (shorter) 
ls legit.txt fake.txt &> all.txt   # both output and error go to all.txt

| - Pipe: send output from one command to another

The pipe | takes the stdout of the left command and passes it as stdin to the right command.

ls -la | head -n 5
# Shows only the first 5 lines of ls output

You can chain pipes - [command1] | [command2] | [command3]

Symbol What it does Example
> Save stdout to file (overwrite) ls > files.txt
>> Append stdout to file echo "Second line" >> file.txt
2> Save stderr to file ls fakefile.txt 2> errors.txt
2>&1 Send stderr to same place as stdout ls > all.txt 2>&1
| Pipe stdout to another command ls -la | head -n 5

Check Your Understanding

Test yourself with these real terminal scenarios.

1. Save a directory listing

You want to save the output of ls -la to a file called directory.txt.

What command do you type?

Reveal Answer 
ls -la > directory.txt

2. Add to a log file

You want to add the current date to events.log without erasing what's already there.

What command do you type?

Reveal Answer 
date >> events.log

3. Capture errors only

You run ls fakefile.txt realfile.txt and only want to save the error message to errors.txt.

What command do you type?

Reveal Answer 
ls fakefile.txt realfile.txt 2> errors.txt