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0 pointschasil4y ago0 comments

> that's why we start bash scripts with #!/bin/bash

That will also fail spectacularly, as bash does not behave the same when called as /bin/bash as it does when it is /bin/sh.

I have principally noticed that aliases are not expanded in scripts unless a shopt is issued, which violates POSIX.

Forcing POSIXLY_CORRECT might also help.

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gh02t4y ago

I would assume if you put /bin/bash as your shebang that you're expecting to get bash-isms. I think the problem you're complaining about (which is a real one) is people putting /bin/sh and expecting bashisms. Debuntu being problematic here is more a side effect of bad practice.

chasilOP4y ago

Bash has a POSIX mode.

Knowing when to switch into and out of this mode, and what impact it has, is a more advanced subject that should not burden those learning the Borne family.

It is better to start with Almquist, or another pure POSIX implementation, with documentation specific to standard adherence.

More advanced shell features should wait.

gh02t4y ago

I have mixed feelings. On paper I agree with you, people should start with POSIX shell. In practice I'm not sure how relevant that is anymore. I'm not really convinced bash should be the default thing people learn, but I think there's a decent argument that people should just start off with the shell they're going to actually use. You should, however, be aware that there is a distinction, and if you're learning/writing bash and not POSIX shell you should specify /bin/bash not /bin/sh. But you don't necessarily need to know all the nuances of how bash differs unless you have a need to write POSIX compliant shell.

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ericbarrett4y ago

Not sure what you are saying, bash behaves as bash when invoked as /bin/bash, and Bourne-shell-ish when invoked as /bin/sh. Lots more detail in the man page.

I've never seen use of aliases in a bash script...? They are generally for CLI convenience.

chasilOP4y ago

Alias expansion within scripts is mandated by POSIX.

When bash is not in POSIX mode, it violates the standard.

  $ ll /bin/sh
  lrwxrwxrwx. 1 root root 4 Nov 24 08:40 /bin/sh -> bash

  $ cat s1
  #!/bin/sh
  alias p=printf
  p hello\\n

  $ cat s2
  #!/bin/bash
  alias p=printf
  p world\\n

  $ ./s1
  hello

  $ ./s2
  ./s2: line 3: p: command not found

jjnoakes4y ago

Sure but someone putting #!/bin/bash at the top of their script, written for non-POSIX bash, won't have that issue...

ericbarrett4y ago

That's very nice, it's POSIX-compliant when invoked as #!/bin/sh and sane when invoked as #!/bin/bash — exactly what I'd want.

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gh02t4y ago

chasilOP4y ago

Bash has a POSIX mode.

Knowing when to switch into and out of this mode, and what impact it has, is a more advanced subject that should not burden those learning the Borne family.

It is better to start with Almquist, or another pure POSIX implementation, with documentation specific to standard adherence.

More advanced shell features should wait.

gh02t4y ago

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ericbarrett4y ago

Not sure what you are saying, bash behaves as bash when invoked as /bin/bash, and Bourne-shell-ish when invoked as /bin/sh. Lots more detail in the man page.

I've never seen use of aliases in a bash script...? They are generally for CLI convenience.

chasilOP4y ago

Alias expansion within scripts is mandated by POSIX.

When bash is not in POSIX mode, it violates the standard.

  $ ll /bin/sh
  lrwxrwxrwx. 1 root root 4 Nov 24 08:40 /bin/sh -> bash

  $ cat s1
  #!/bin/sh
  alias p=printf
  p hello\\n

  $ cat s2
  #!/bin/bash
  alias p=printf
  p world\\n

  $ ./s1
  hello

  $ ./s2
  ./s2: line 3: p: command not found

jjnoakes4y ago

Sure but someone putting #!/bin/bash at the top of their script, written for non-POSIX bash, won't have that issue...

ericbarrett4y ago

That's very nice, it's POSIX-compliant when invoked as #!/bin/sh and sane when invoked as #!/bin/bash — exactly what I'd want.

1 more reply

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