Brace yourself, I’m in an expansive mood
September 7, 2019 at 1:08 PM by Dr. Drang
A longstanding truth of this blog is that whenever I write a post about shell t̸r̸i̸c̸k̸s̸ features, I get a note from Aristotle Pagaltzis letting me know of a shorter, faster, or better way to do it. Normally, I add a short update to the post with Aristotle’s improvements, or I explain whay his faster way wouldn’t be faster for me (because some things just won’t stick in my head). But his response to my jot and seq post got me exploring an area of the shell that I’ve seen but never used before, and I thought it deserved a post of its own. I even learned something useful without his help.
Here’s Aristotle’s tweet:
@drdrang Bash/zsh brace expansion can replace 99% of jot/seq uses (though bash < 4.x doesn’t support padding or step size 🙁). Your last example becomes much simpler:
printf '%s\n' 'Apt. '{2..5}{A..D}
You even get your preferred argument order:
printf '%s\n' {10..40..5}
Aristotle Sep 2, 2019 – 2:48 PM
Brace expansion in bash and zsh doesn’t seem like a very important feature because it takes up so little space in either manual. The brief exposure I’ve had to it has been in articles that talked about using it to run an operation on several files at once. For example, if I have a script called file.py that generates text, CSV, PDF, and PNG output files, all named file but with different extensions, I might want to delete all the output files while leaving the script intact. I can’t do
rm file.*
because that would delete the script file. What works is
rm file.{txt,csv,pdf,png}
The shell expands this into
rm file.txt file.csv file.pdf file.png
and then runs the command.
This is cute, but I never thought it worth committing to memory because tab completion and command-line editing through the Readline library makes it very easy to generate the file names interactively.
What I didn’t realize until Aristotle’s tweet sent me to the manuals was that the expansion could also be specified as a numeric or alphabetic sequence using the two-dot syntax. Thus,
mkdir folder{A..T}
creates 20 folders in one short step, which is the sort of thing that can be really useful.
And you can use two sets of braces to do what is effectively a nested loop. With apologies to Aristotle, here’s how I would do the apartment number generation from my earlier post:
printf "Apt. %s\n" {2..5}{A..D}
This gives output of
Apt. 2A
Apt. 2B
Apt. 2C
Apt. 2D
Apt. 3A
Apt. 3B
Apt. 3C
Apt. 3D
Apt. 4A
Apt. 4B
Apt. 4C
Apt. 4D
Apt. 5A
Apt. 5B
Apt. 5C
Apt. 5D
just like my more complicated jot/seq command.
The main limitation to brace expansion when compared to jot and seq is that you can’t generate sequences with fractional steps. If you want numbers from 0 through 100 with a step size of 0.5,
seq 0 .5 100
is the way to go.
And if you’re using the stock version of bash that comes with macOS (bash version 3.2.57), you’ll run into other limitations.
First, you won’t be able to left-pad the generated numbers with zeros. In zsh and more recent versions of bash, you can say
echo {005..12}
and get1
005 006 007 008 009 010 011 012
where the prefixed zeros (which can be put in front of either number or both) tell the expansion to zero-pad the results to the same length. If you run that same command in the stock bash, you just get
5 6 7 8 9 10 11 12
Similarly, the old bash that comes with macOS doesn’t understand the three-parameter type of brace sequence expansion (mentioned by Aristotle), in which the third parameter is the (integer) step size:
echo {5..100..5}
which gives
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
in zsh and newer versions of bash. Old bash doesn’t understand the three-parameter at all and just outputs the input string:
{5..100..5}
We’ve been told that Catalina will ship with zsh as the default shell, which means we shouldn’t have to worry about these deficiencies for long. Because I don’t want to learn a new system of configuration files, I’m sticking with bash, but I have switched to version 5.0.11 that’s installed by Homebrew. My default shell is now /usr/local/bin/bash.2
One more thing. I said last time that seq needs a weirdly long formatting code to get zero-padded numbers embedded in another string. The example was
seq -f "file%02.0f.txt" 5
to get
file01.txt
file02.txt
file03.txt
file04.txt
file05.txt
What I didn’t understand was how the %g specifier works. Based on my skimming of the printf man page, I thought it just chose the shorter output of the equivalent %f and %e specifiers. But it turns out to do further shortening, eliminating all trailing zeros and the decimal point if there’s no fractional part to the number. Therefore, we can use the simpler
seq -f "file%02g.txt" 5
to get the output above. Because printf-style formatting is used in lots of places, this is good to know outside the context of seq.
Of course, now that I understand brace expansion, I wouldn’t use seq at all. I’d go with something like
echo file{01..5}.txt
