Perl backticks using bash

Question

In Perl, the default shell to execute backticks is sh. I'd like to switch to use bash for its richer syntax. So far I found that the suggested solution is

`bash -c \"echo a b\"`

The apparent drawback is that the escaped double quotes, which means I will have difficulty to use double quotes in my bash args. For example, if I wanted to run commands requiring double quotes in bash

echo "a'b"

The above method will be very awkward.

Perl's system() call has a solution for this problem: to use ARRAY args,

system("bash", "-c", qq(echo "a'b"));

This keeps my original bash command unmodified, and almost always.

I'd like to use ARRAY args in backticks too. Is it possible?

Answer 1

For one, one can submit a list to qx; it gets interpolated into a string and then passed to either execvp or a shell (see qx, and the second part of this post and comments). And if you need a shell then presumably that string contains shell metacharacters so it goes via shell.

my @cmd = qw(ls -l "dir with spaces");
#my @cmd = qw(ls -l "dir with spaces" > outfile);
my @out = qx(@cmd);
print for @out;

I make a "dir with spaces" directory with a file in it to test. (For a command with quotes in it a shell does get used.)

Next, I would in principle recommend a module to compose those shell commands, instead of going through a nail-biter to correctly escape and pass it all, like String::ShellQuote

use String::ShellQuote qw(shell_quote); 

my @cmd = ('ls', '-l', q(dir with spaces)); 

my $quoted = shell_quote(@cmd);; 
my @out = qx($quoted); 
#my @out = qx($quoted > outfile); 
print for @out;

I use the q(...) operator form of single quotes to demonstrate another way (also useful for including single quotes); it is not necessary for this simple example. One still need be careful with details; that's in the nature of using complex external commands and cannot be fully avoided by any approach or tool.

As for running bash, note that normally sh delegates to a default-of-sorts shell on your system, and on many systems it is bash that is used. But if it isn't on yours, one way to use bash -c in the command would be to first prepare the command and then add that to the qx string

my @cmd = ('ls', '-l', q(dir with spaces)); 
my $quoted = shell_quote(@cmd); 
my @out = qx(bash -c "$quoted"); 
#my @out = qx(bash -c "$quoted" > outfile); 
print for @out;

A couple more notes I'd like to offer:

• That qx is an ancient demon. How about using modern tools/modules for running external commands? There may be a little more to do in order to prepare your involved bash strings but then everything else will be better. Options abound. For example

  • IPC::System::Simple with its few utility functions

  • Use Capture::Tiny to wrap a system call with syntax you prefer

  • The IPC::Run can do any and all of this and then way way more

• Why use external commands, with Perl's (far) superior richness? It's a whole, very complete, programming language, vs. the command-interpreter with some programming capabilities. If you need shell's capabilities why not run just those things via the shell and do all else in Perl?