Brace expansion and why you no longer need seq

I’m here to tell you the naked truth. Not only bash knows how to do everything seq does, but it also does it better. There, I said it. I know, your older brother or your mentor taught you to write for i in $(seq 5); do ... but using pure bash is so much better. Curious how? Read on!

History

Back in the 80s, when the shells were featureless, there was a strong case for seq. Coders needed a way to iterate through a sequence of numbers in their shell scripts. It was so early in the computers era that Perl didn’t even exist as a fallback alternative. But today, and even before one of the most stable and longstanding versions which was 2.05b, bash can do what seq does in a straightforward way.

Generating sequences is part of a feature called Brace Expansion. That’s because the special characters are the braces, along with the double dot and the comma. And I only say sequences and not sequences of numbers because bash knows how to deal with single letters as well.

My real life usage: renaming files

In real life, I have to admit that I don’t use brace expansion for sequences. That’s because I rarely need to use them. I use brace expansion when I want to rename a file quickly or when I want to shorten the line in a script. Especially when I’m dealing with long full paths.

For example, if I want to rename a file by adding the .orig extension. I do that very easy with:

mv /path/to/file{,.orig}

I feel the need to clarify that the above expansion is done by bash before it executes mv. The actual execution string is:

mv /path/to/file /path/to/file.orig

Another example is when I want to rename a file with a completely different name:

mv /path/to/{file,newname}

That expands to

mv /path/to/file /path/to/newname

So the simplest brace expansion

{a,b}

expands to

a b

and

{a,b,c}

expands to

a b c

and these are the ones I use the most in real life. Whether I’m writing scripts or command line.

Sequences

But back to sequences. Here’s how they work. The syntax is: {FIRST..LAST[..INCR]} By the way, when a brace expansion is not valid, it’s left unchanged by bash. But in the syntax above, FIRST and LAST are mandatory. The ..INCR is the increment/step and it’s optional. FIRST and LAST can be numbers or single letters and it doesn’t matter if FIRST is less or greater than LAST. If FIRST or LAST is a number and is prefixed with zero, the expanded numbers are zero padded. Let’s see all of these in action.

$ echo {1..5}
1 2 3 4 5

Descending order can be achieved with:

$ echo {7..3}
7 6 5 4 3

Or, printing numbers from 15 to 5 with an increment (in this case it’s really a decrement) of 3:

$ echo {15..5..3}
15 12 9 6

But the same above:

$ echo {15..05..3}
15 12 09 06

because 05 are 2 characters, so it’s padded with a single zero. However, check on the next one.

$ echo {015..5..3}
015 012 009 006

You did not expect this one, did you?! 🙂

Letter sequences are just as useful

With letters, sequences are expanded in the same way, except there’s no zero padding.

$ echo {a..f}
a b c d e f

while descending order goes like:

$ echo {f..b}
f e d c b

and last one, with increment:

$ echo {a..z..5}
a f k p u z

Preventing brace expansion

As usual, if you want to temporarily prevent bash from expanding a string that is a brace expansion expression, then you can quote it:

echo \{1..3}

echo "{1..3}"

will both print the literal {1..3}

That’s it. No need for seq now, isn’t it? You see, when you use seq, you have to spawn it in a subshell, which is a lot more expensive than using bash builtin functionalities.