Bash – Expand Subshell Before Executing It

bashcommand historyshellsubshell

I often run commands using subshells, and sometimes would like to have the subshells expanded before I run something.. This way I could verify what I'm doing, and possibly edit what's about to happen as well.

For example, how can I get the following command line to be expanded before I run it, so I can edit the results of the subshell?

e.g.

$ find -name "test.txt" 
/tmp/test.txt

$ mv $(!!) /tmp/new.txt

I'd like to see the subshell expanded before I run the command, like so:

$ mv /tmp/test.txt /tmp/new.txt

Is there some way to do this?

Best Answer

shell-expand-line (\e\C-e) expands command substitutions in bash.

$ bind -p|grep shell-ex
"\e\C-e": shell-expand-line

$(!!)\e\C-e would run the previous command again and insert the output:

"\eo": "$(!!)\e\C-e"

It also expands other command substitutions, but there is no command like shell-expand-word.

In bash 4.0 or later you could also enable globstar, type **/file.txt, and use glob-complete-word (\eg) or glob-expand-word (\C-x*).

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Bash Shell – How to Make Explicit Operator Precedence Without Creating a Subshell

from man bash:

   { list; }
          list  is  simply executed in the current shell environment.  list must be terminated with a newline or semicolon.
          This is known as a group command.  The return status is the exit status of list.  Note that unlike the  metachar‐
          acters  (  and  ), { and } are reserved words and must occur where a reserved word is permitted to be recognized.
          Since they do not cause a word break, they must be separated from list by whitespace or another shell metacharac‐
          ter.

Bash – Use read Builtin Without a While Loop

It's because the part where you use the vars is a new set of commands. Use this instead:

head somefile | { read A B C D E FOO; echo $A $B $C $D $E $FOO; }

Note that, in this syntax, there must be a space after the { and a ; (semicolon) before the }. Also -n1 is not necessary; read only reads the first line.

For better understanding, this may help you; it does the same as above:

read A B C D E FOO < <(head somefile); echo $A $B $C $D $E $FOO

Edit:

It's often said that the next two statements do the same:

head somefile | read A B C D E FOO
read A B C D E FOO < <(head somefile)

Well, not exactly. The first one is a pipe from head to bash's read builtin. One process's stdout to another process's stdin.

The second statement is redirection and process substitution. It is handled by bash itself. It creates a FIFO (named pipe, <(...)) that head's output is connected to, and redirects (<) it to the read process.

So far these seem equivalent. But when working with variables it can matter. In the first one the variables are not set after executing. In the second one they are available in the current environment.

Every shell has another behavior in this situation. See that link for which they are. In bash you can work around that behavior with command grouping {}, process substitution (< <()) or Here strings (<<<).

Best Answer

Related Solutions

Bash Shell – How to Make Explicit Operator Precedence Without Creating a Subshell

Bash – Use read Builtin Without a While Loop

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