Bash – ny sh code that is not syntactically valid bash code

bashposixshell-script

Is there any sh code that is not syntactically valid bash code (won't barf on syntax)?

I am thinking of overwriting sh with bash for certain commands.

Best Answer

Here is some code that does something different in POSIX sh and Bash:

hello &> world

Whether that is "invalid" for you I don't know.

In Bash, it redirects both standard output and standard error from hello into the file world. In POSIX sh, it runs hello in the background and then makes an empty redirection into world, truncating it (i.e. it's treated as & >).

There are plenty of other cases where Bash extensions will do their thing when run under bash, and would have different effects in a pure POSIX sh. For example, brace expansion is another, and it too operates the same under Bash's POSIX mode and not.

As far as static syntax errors go, Bash has both reserved words (like [[ and time) not specified by POSIX, such that [[ x is valid POSIX shell code but a Bash syntax error, and a history of various POSIX incompatibility bugs that may result in syntax errors, such as the one from this question:

x=$(cat <<'EOF'
`
EOF
)
bash: line 2: unexpected EOF while looking for matching ``'
bash: line 5: syntax error: unexpected end of file

Syntax-errors-only is a pretty dangerous definition of "invalid" for any circumstance where it matters, but there it is.

Related Solutions

Bash – Is checking for exit status other than 0 after a `command || return` unreachable

Yes you are correct. The return procedure will exit from the function to the originating caller in the script with an exit status of 1.

Therefore the mount command will never get processed.

To resolve this, just strip the if .. then statement from the mount command:

[ -d ${MKAPP_BUILDDIR}/tmp ] || mkdir ${MKAPP_BUILDDIR}/tmp || return 1

mount -t tmpfs none ${MKAPP_BUILDDIR}/tmp || return 1

Therefore if the directory cannot be created: exit with status of 1.
If the creation is successful, then run the mount command.

Bash – way to have a function in the bash script auto run on any command error

You can use bash trap ERR to make your script quit if any command returns status greater than zero and execute your function on exiting.

Something like:

myfunc() {
  echo 'Error raised. Exiting!'
}

trap 'myfunc' ERR

# file does not exist, causing error
ls asd
echo 123

Note that bash trap ERR does implicit set -o errexit or set -e and is not POSIX.

And the ERR trap is not executed if the failed command is part of the command list immediately following until or while keyword, part of the test following the if or elif reserved words, part of a command executed in && or || list, or if the command’s return status is being inverted using !.

Best Answer

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Bash – Is checking for exit status other than 0 after a `command || return` unreachable

Bash – way to have a function in the bash script auto run on any command error

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