redirections

posted on May 15th, 2025 by Lee Cattarin

baseline

cat tempest.md

You do look, my son, in a moved sort,
As if you were dismay'd: be cheerful, sir.
Our revels now are ended. These our actors,
As I foretold you, were all spirits and
Are melted into air, into thin air:
And, like the baseless fabric of this vision,
The cloud-capp'd towers, the gorgeous palaces,
The solemn temples, the great globe itself,
Ye all which it inherit, shall dissolve
And, like this insubstantial pageant faded,
Leave not a rack behind. We are such stuff
As dreams are made on, and our little life
Is rounded with a sleep.

direct

grep takes a file or files as an argument.

grep "'d" tempest.md

As if you were dismay'd: be cheerful, sir.
The cloud-capp'd towers, the gorgeous palaces,

redirect

If there’s no files, or if - is passed as an argument, grep reads from standard input. < reads a file into standard input (this is shorthand for 0<, 0 being the file descriptor of stdin).

grep "'d" < tempest.md
# or
grep "'d" - < tempest.md

As if you were dismay'd: be cheerful, sir.
The cloud-capp'd towers, the gorgeous palaces,

re-redirect

<(command) creates a file descriptor for the output of command.

grep "'d" - < <(cat tempest.md)

As if you were dismay'd: be cheerful, sir.
The cloud-capp'd towers, the gorgeous palaces,

(In this case, you could substitute cat tempest.md | grep "'d" - but <(command) is handy for cases where commands don’t take stdin, or need to take in multiple files (such as comm).

re-re-redirect

cat can also take stdin when given - or no arguments.

grep "'d" < <(cat < tempest.md)
# or
grep "'d" < <(cat - < tempest.md)

As if you were dismay'd: be cheerful, sir.
The cloud-capp'd towers, the gorgeous palaces,

conclusion

it’s <s all the way down.

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postscript: heredoc

grep "'d" << tempest.md
heredoc> oh are we interactive now
heredoc> what am i 'doing' anyway
heredoc> tempest.md

what am i 'doing' anyway

(<< starts an ephemeral document or heredoc. The first argument given is the delimiter, and is more commonly EOF (end of file).)

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category: reference

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