Bash – Why Does dash’s echo Expand \\ Differently?

bashdashechoquotingshell-script

I have a little open source project that for various reasons I've tried to write in reasonably portable shell script. Its automated integration tests check that hostile characters in path expressions are treated properly, among other things.

Users with /bin/sh provided by bash are seeing a failure in a test that I've simplified down to the following:

echo "A bug\\'s life"
echo "A bug\\\\'s life"

On bash, it produces this expected result:

A bug\'s life
A bug\\'s life

With dash, which I've developed against, it does this:

A bug\'s life
A bug\'s life

I'd like to think that I haven't found a bug in dash, that I might be missing something instead. Is there a rational explanation for this?

Best Answer

echo "A bug\\'s life"

Because those are double quotes, and \ is special inside double quotes, the first \ is understood by the shell as escaping/quoting the second \. So a A bug\'s life argument is being passed to echo.

echo "A bug\'s life"

Would have achieved exactly the same. ' being not special inside double quotes, the \ is not removed so it's the exact same argument that is passed to echo.

As explained at Why is printf better than echo?, there's a lot of variation between echo implementations.

In Unix-conformant implementations like dash's, \ is used to introduce escape sequences: \n for newline, \b for backspace, \0123 for octal sequences... and \\ for backslash itself.

Some (non-POSIX) ones require a -e option for that, or do it only when in conformance mode (like bash's when built with the right options like for the sh of OS/X or when called with SHELLOPTS=xpg_echo in the environment).

So in standard (Unix standard only; POSIX leaves the behaviour unspecified) echos,

echo '\\'

same as:

echo "\\\\"

outputs one backslash, while in bash when not in conformance mode:

echo '\\'

will output two backslashes.

Best it to avoid echo and use printf instead:

$ printf '%s\n' "A bug\'s life"
A bug\'s life

Which works the same in this instance in all printf implementations.

Related Solutions

Shell – Avoid backslash expansion with echo in dash

Actually the shell forks and executes the command. But each time a command (which is not a built-in) is executed, the shell forks. That's how shells work, and that's unavoidable.

Note: the shell doesn't translate anything. The echo command may. But you can use printf instead:

your_shell -c 'bar=$(cat foo); printf %s "$bar"'

Bash vs Dash – Why ‘cat ${1:-/dev/stdin} | … &>/dev/null’ Works in Bash Only

[this answer is about asynchronous pipelines in scripts; for the deprecated &> bash operator and why you should always use >output 2>&1 instead, refer to obsolete and deprecated syntax]

#! /bin/sh
cat "${1:-/dev/stdin}" | ... &

Here you have a pipeline running asynchronously (because terminated by &), started from a script, ie is from a shell with the job control disabled.

According to the standard:

command1 & [command2 & ... ]

The standard input for an asynchronous list, before any explicit redirections are performed, shall be considered to be assigned to a file that has the same properties as /dev/null.

The problem is that dash, ksh, mksh, yash, etc intepret "asynchronous list" as any command, including a pipeline, and will redirect the stdin of the first command from /dev/null:

$ echo foo | dash -c 'cat | tr fo FO & echo DONE'
DONE
$ echo | dash -c 'readlink /proc/self/fd/0 | cat & echo DONE'
DONE
/dev/null

But bash will only interpret it as "simple command" and will only redirect its stdin from /dev/null when it's not part of a pipeline:

$ echo foo | bash -c 'cat | tr fo FO & echo DONE'
DONE
FOO
$ echo | bash -c 'readlink /proc/self/fd/0 | cat & echo DONE'
DONE
pipe:[69872]
$ echo | bash -c 'readlink /proc/self/fd/0 & echo DONE'
DONE
/dev/null
$ bash -c 'cat | tr a A & echo DONE'
DONE
cat: -: Input/output error

zsh will only redirect it from /dev/null when the original stdin is a tty, not when it's other kind of file:

$ zsh -c 'readlink /proc/self/fd/0 &' </dev/tty
/dev/null
$ zsh -c 'readlink /proc/self/fd/0 &' </dev/zero
/dev/zero

A workaround which works in all shells is to duplicate the stdin into another file descriptor, and redirect the stdin of the first command from it:

#! /bin/sh
exec 3<"${1:-/dev/stdin}"
cat <&3 | timeout 30 xclip -i -selection clipboard -verbose -r >/dev/null 2>&1 &

Best Answer

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Shell – Avoid backslash expansion with echo in dash

Bash vs Dash – Why ‘cat ${1:-/dev/stdin} | … &>/dev/null’ Works in Bash Only

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