CODEWITHSUNDEEP
bashshbuilt-in
eloyesp
eloyesp

Reputation: 3285

How do I prevent bash to use a builtin command?

I'm trying to fix a script that use echo, that is using the builtin command instead of the command, how can I prevent that?

I know I can do /bin/echo to force the usage of that, but I wouldn't like to hardcode the path (for portability).

I thought using something as:

$ECHO=`which echo`
$ECHO -e "text\nhere"

but which echo returns: "echo: shell built-in command".

I've ended up defining an echo function that uses env as @Kenster recommends. This way I don't need to modify the calls to echo in the script.

echo() {
  env echo $*
}

# the function is called before the built-in command.
echo -en "text\nhere"

Upvotes: 22

Views: 7657

Answers (2)

that other guy
that other guy

Reputation: 123460

You can disable the builtin echo:

enable -n echo

Now simply doing echo anything will run the external version. It only affects the current script process, so you can safely do it in your scripts.

Upvotes: 18

Kenster
Kenster

Reputation: 25390

Use the env program. Env is a command which launches another program with a possibly modified environment. Because env is a program, it doesn't have access to shell builtins, aliases, and whatnot.

This command will run the echo program, searching for it in your command path:

$ env echo foo

You can verify this by using strace to monitor system calls while running echo vs env echo:

$ strace -f -e trace=process bash -c 'echo foo'
execve("/bin/bash", ["bash", "-c", "echo foo"], [/* 16 vars */]) = 0
arch_prctl(ARCH_SET_FS, 0x7f153fa14700) = 0
foo
exit_group(0)                           = ?

$ strace -f -e trace=process bash -c 'env echo foo'
execve("/bin/bash", ["bash", "-c", "env echo foo"], [/* 16 vars */]) = 0
arch_prctl(ARCH_SET_FS, 0x7f474eb2e700) = 0
execve("/usr/bin/env", ["env", "echo", "foo"], [/* 16 vars */]) = 0
arch_prctl(ARCH_SET_FS, 0x7f60cad15700) = 0
execve("/usr/local/sbin/echo", ["echo", "foo"], [/* 16 vars */]) = -1 ENOENT (No such file or directory)
execve("/usr/local/bin/echo", ["echo", "foo"], [/* 16 vars */]) = -1 ENOENT (No such file or directory)
execve("/usr/sbin/echo", ["echo", "foo"], [/* 16 vars */]) = -1 ENOENT (No such file or directory)
execve("/usr/bin/echo", ["echo", "foo"], [/* 16 vars */]) = -1 ENOENT (No such file or directory)
execve("/sbin/echo", ["echo", "foo"], [/* 16 vars */]) = -1 ENOENT (No such file or directory)
execve("/bin/echo", ["echo", "foo"], [/* 16 vars */]) = 0
arch_prctl(ARCH_SET_FS, 0x7f0146906700) = 0
foo
exit_group(0)                           = ?

Upvotes: 21

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