Shell, explained
What does << EOF mean? Here-documents explained
cat << EOF > config.txt
<< starts a here-document: the lines that follow, up to a line containing only the word you chose (EOF by habit), are given to the command as its input. cat << EOF > config.txt therefore writes those lines into config.txt. It is the standard way to put several lines of text into a script.
The lines up to EOF become the input
cat reads the two lines as if they came from a file and prints them; > sends that into config.txt. The closing EOF is not part of the text.
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output → config.txt
Variables are filled in
With name=ana set, the shell replaces $name before cat sees the text, just as it would inside double quotes.
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Quote the word to switch that off
With 'EOF' in quotes, the text is passed on exactly as written. Use this when the text is itself code, full of $ signs.
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Part by part
cat- Prints whatever it reads on its input.
<<- Says: my input is not a file, it is the lines that follow.
EOF- The end marker. Any word works; EOF (“end of file”) is the convention.
> config.txt- An ordinary redirection: what cat prints goes into the file.
EOF- Alone on a line, with no spaces around it, it ends the text.
Common mistakes
Indenting the closing EOF
The end marker must start in the first column, with nothing after it. Indented by even one space it is treated as text, and the shell keeps reading to the end of the script.
Expecting <<- to strip spaces
<<- removes leading tabs from every line, so the text can be indented inside an if or a function. It removes tabs only: with spaces, the closing marker is not found.
Try it
$ cat << EOF
Hello $USER
EOFPrints Hello and your user name.
$ cat << 'EOF'
Hello $USER
EOFPrints Hello $USER, untouched.
$ wc -l << EOF
a
b
c
EOFAny command that reads input works: prints 3.
Questions
Does the word have to be EOF?
No. Any word works, as long as the same one closes the text: END, SQL, HTML. Pick one that does not appear on a line of its own inside the text.
What is the difference between << and <<<?
<< takes several lines up to a marker. <<< is a here-string: it takes the one string that follows, as in grep o <<< "hello". <<< is a bash and zsh feature, not plain sh.
How do I write a file that needs sudo?
sudo cat << EOF > /etc/file fails, because the > is done by your own shell. Let tee write the file instead: sudo tee /etc/file << EOF.
Can I use a heredoc with ssh?
Yes: ssh server bash << 'EOF' runs the lines on the remote machine. Quote the marker so variables are expanded there and not on your machine first.
Terminaster
Stop looking up the same command twice.
Ask about any command you run into and see what it actually does. Terminaster then brings it back just before you would forget it, until you can type it without searching.
The ideas behind it
Redirection
Redirection is shell syntax that opens, closes, or duplicates file descriptors for a command before that command begins executing.
Standard input, output, and error
A command-line process normally starts with three conventional open streams: standard input for incoming data, standard output for primary results, and standard error for diagnostics.
Quoting and escaping
Shell quoting is syntax the shell consumes while building command arguments. Single quotes preserve every enclosed character literally; double quotes still allow parameter, command, and arithmetic expansion while suppressing word splitting and pathname expansion; an unquoted backslash protects the next character, except that backslash-newline continues the line.