Heredocs#

Bash Here Documents (short heredocs) are a great way to pass multi-line input to a command.

I mostly use them to initialize files when writing documentation or Dockerfiles, e.g.

some_env_var=foo

cat <<EOF > /some/file.yaml
nested:
  value: $some_env_var
EOF

with variable substitution#

foo=42
cat <<EOF
The answer is $foo.
EOF

returns

The answer is 42.

foo=42
cat <<'EOF'
The answer is $foo.
EOF

returns

The answer is $foo.

cat <<MY_CUSTOM_DELIMITER
foo
bar
baz
MY_CUSTOM_DELIMITER

Using cat is great to print to STDOUT or redirect to a file, but heredocs are universal, e.g.

psql <<EOF
SELECT * FROM users;
EOF

python <<'EOF'
import sys
print(sys.version)
EOF