Bash – readlink literal vs variable

bashquotingshelltildevariable substitution

Using BASH: the answer is probably obvious but, not for me.

> echo $PWD
/root/fcm
> readlink -f ~
/root
> # but then with a variable or literal
> a='~'
> readlink -f $a
/root/fcm/~
> readlink -f "~"
/root/fcm/~

I'm expecting to receive just '/root/';
Who is doing the substitution bash or readlink?

Best Answer

The shell does the tilde expansion. readlink doesn't. Bash will not expand tilde within quotes.

readlink -f $a does not do what you want as tilde expansion happens before variable expansion, i.e. the variable is expanded to ~, but that tilde won't be expanded further.

The order in which Bash do things is: brace expansion, tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, word splitting, and pathname expansion.

Using $HOME may be preferable to using tilde under some circumstances, since it behaves as any other variable.

Also, please don't work logged in as root...

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You can access the just-executed command line with the history built-in. (I have no idea why history 1 prints the just-executed command line but fc -nl -1 prints the previous commmand, as does fc -nl 0.)

PROMPT_COMMAND='echo -en "\033[38;5;2m"; history 1; echo -en "\033[0m\n"'

This prints a number before the command text. Here's a version that removes the number. (It may be incorrect if you go beyond 99999 history lines, I don't know how bash formats the number then.)

prompt_function () {
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Bash – How to print “$” in here-document

If you want to write a here-doc and you don't want ANY of the doc to be expanded or any special characters interpreted, you can quote the label with single quotes, like this:

$ cat >file <<'EOF'
echo "$ABC=home_dir"
EOF

However, your situation as described in your example is much more complex, because you're really sending this content through ssh, to a remote system, to be run by sudo which is also invoking a shell (and so that shell will expand the content as well). You're going to need more levels of quoting to get this right, but even with that it still won't work because sudo requires a terminal (so it can ask for a password) and you've redirected from stdin. Even using ssh -t won't help here.

Also I agree with Johan. It's not clear this is really what you want; note that it's not legal to assign a value to a shell variable reference, so if this file you're trying to create is supposed to be a shell script, it won't work as you've described it. Maybe if you back up a bit and describe what you really want to do, we can help more.

Best Answer

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