Bash – backticks inside brace expansion

bashbrace-expansioncommand-substitution

I'm trying to list the files of httpd processes, and I tried this:

ls -l /proc/{`pidof httpd | tr ' ' ','`}/
ls: cannot access /proc/{{28493,28492,28491,28490,28489,28488,28487,28486,28483}}/: No such file or directory

But if I put the PID inside the brackets myself, it works:

ls -l /proc/{28493,28492,28491,28490,28489,28488,28487,28486,28483}/ | head -6
/proc/28483/:
total 0
dr-xr-xr-x 2 root root 0 Jan 30 11:18 attr
-rw-r--r-- 1 root root 0 Jan 30 11:18 autogroup
-r-------- 1 root root 0 Jan 30 11:18 auxv

I believe in the first case, the shell is considering the backtick output as a single argument, and is not expanding. So I'm wondering, is there a way of making it see as a list to be expanded? I know I could do this with a for loop, or in many other ways. I'm particularly interested in learning more about backticks and brace expansions, assuming it is possible to do it.

Best Answer

The order of evaluation in bash is such that brace expansion is done first, and command substitutions are not handled until later.

Here's the order in which things are done by bash:

brace expansion,
tilde expansion,
parameter and variable expansion,
command substitution,
arithmetic expansion,
word splitting,
pathname expansion.

Alternative solution:

for pid in $( pidof httpd ); do ls -l /proc/"$pid"; done

Related Solutions

Bash – How to perform command substitution before brace expansion

You simply need to use the eval shell builtin:

$ eval ls {$(seq -s , 13 20)}.pdf

Where eval takes the arguments passed to it:

ls {$(seq -s , 13 20)}.pdf

and concatenates them together into a single command:

ls {13,14,15,16,17,18,19,20}.pdf

which is then read and executed by the shell.

$ eval ls {$(seq -s , 13 20)}.pdf
13.pdf  14.pdf  15.pdf  16.pdf  17.pdf  18.pdf  19.pdf  20.pdf

Bash – brace expansion and * expansion

What happens is that bash first expands *.djvu{,.bk} into *.djvu *.djvu.bk, and then does glob-expansion on those. This would explain what you observe: in your case, *.djvu, matches an existing file, say foo.djvu and expands into that, but *.djvu.bk matches no file, and thus expands as itself, *.djvu.bk.

The order of expansion is specified in the bash documentation:

The order of expansions is: brace expansion, tilde  expansion,  parame‐
ter,  variable  and arithmetic expansion and command substitution (done
in a left-to-right fashion), word splitting, and pathname expansion.

I would suggest rewriting your copy command as:

for f in *.djvu; do cp -- "$f" "$f".bk; done

Or perhaps, to avoid the syntactic overhead of an explicit for loop:

parallel -j1 cp -- {} {}.bk ::: *.djvu

(On second thoughts... that's not really much shorter.)

To answer your sub-question "how could it be expanded", one could use a sub-command (example in a directory containing just foo.djvu and bar.djvu):

$ echo $(echo *.djvu){,.bk}
bar.djvu foo.djvu bar.djvu foo.djvu.bk

But that isn't as safe a solution as the for loop or parallel call above; it will break down on file names containing white space.

Best Answer

Related Solutions

Bash – How to perform command substitution before brace expansion

Bash – brace expansion and * expansion

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