¬ just serendipity 🍀 (not just serendipity)

Problem statement: given a command run with a ton of output, how can you take a glance at both (i) its first few lines and (ii) its last few lines?

Let’s hypothetically call it lorem.

This is what we want to achieve:

lorem | head

lorem | tail

…ideally though we would like to run lorem only once.

How about this?

lorem | tee out.log
head out.log
tail out.log
rm out.log

It works, but can we do it without creating an intermediate out.log file?

TIL via Alex Chan:

How to truncate the middle of long command output

Use a command group { head -n 3; echo ‘[…]’; tail -n 5; } to snip print the first few and last few lines.

If I’m running a command with lots of output, I can use head(1) to get the first few lines, or tail(1) to get the last few lines. What if I want to get some lines from the beginning and from the end, but truncate the middle?

lorem | { head; echo '[...]'; tail; }

Docs: bash command grouping:

{ list; }

Placing a list of commands between curly braces causes the list to be executed in the current shell environment. No subshell is created. The semicolon (or newline) following list is required.

[…] The braces are reserved words, so they must be separated from the list by blanks or other shell metacharacters. […]

The exit status […] is the exit status of list.