Bash Command Substitution – Behavior with Command from String in Variable

bashcommand linecommand-substitutionvariable substitution

$ echo $(echo x; echo y)
x y
$ a='echo x; echo y'
$ echo $($a)  # expect 'x y'
x; echo y

Why does command substitution behave in such way?
How to perform the command substitution for list of commands stored in a variable without using eval and bash -c?

echo $(eval $a) and echo $(bash -c "$a") actually does what I want, but I heard that using eval often is a wrong way to solve problems, so I want know how to manage without these commands (using bash -c is in fact just the same thing)

Best Answer

Word splitting happens quite late in the evaluation of a command. Most crucially for you, it happens after variable expansion and command substitution.

This means that the second line in

s="echo a; echo b"
echo $($s)

will first have $s expanded to echo a; echo b and then have that command executed without splitting it into a compound command consisting of two echos.

(details: $s gets split into four words, echo, a;, echo and b. The encapsulating $(...) executes this as one command, echo with three arguments, a;, echo and b.)

What is given to the outmost echo is therefore the string a; echo b (actually three words as $(...) is not in quotes) since this was what was outputted by the innermost echo.

Compare that to

s1="echo a"
s2="echo b"
echo $($s1;$s2)

which results in what you'd expect.

Yes, "eval is evil", most of the time, and sh -c is clunky and as fragile. But if you have a bit of shell code that you trust in a string in a shell script, then these tools are the only (?) way to get that code to execute properly since this often requires having to explicitly evaluate the text in the string as shell code (with all phases of the evaluation from start to finish). Especially if it's a compound command.

I think it's only due to the Unix shell's intimate relation to text that you're lucky enough to have echo $($s) execute something at all.

Just think about the steps you'd have to take to get a C program to execute a piece of C code that it gets given as a string...

Related Solutions

Bash – Command Substitution Splitting on Newline but Not Space

Portably:

set -f              # turn off globbing
IFS='
'                   # split at newlines only
cmd $(cat <file)
unset IFS
set +f

Or using a subshell to make the IFS and option changes local:

( set -f; IFS='
'; exec cmd $(cat <file) )

The shell performs field splitting and filename generation on the result of a variable or command substitution that is not in double quotes. So you need to turn off filename generation with set -f, and configure field splitting with IFS to make only newlines separate fields.

There's not much to be gained with bash or ksh constructs. You can make IFS local to a function, but not set -f.

In bash or ksh93, you can store the fields in an array, if you need to pass them to multiple commands. You need to control expansion at the time you build the array. Then "${a[@]}" expands to the elements of the array, one per word.

set -f; IFS=$'\n'
a=($(cat <file))
set +f; unset IFS
cmd "${a[@]}"

Bash – How to list the info for an array of /dev/disks using bash expansion or substitution

The brace expansion you're asking about will only expand for files/directories that match on disk to the pattern you use. The other issue you'll run into is diskutil may not be able to handle more than 1 argument at a time. To expand these you'd need to do a while or for loop, and pass the results to diskutil as you iterate through the loop.

Example

$ for i in /dev/*; do diskutil "$i";done

As to your 2nd question there isn't really any method where brace expansion can help you in situations where there are no corresponding files/directories on disk.

Parsing diskutil

Given the output from this command you'll have to resort to using a tool such as awk, sed, or grep to "parse" this output so that you can get meaningful information about the disks, for further queries calling diskutil a 2nd time.

Example

Here's a rough cut of parsing the output from diskutil using grep and sed. There are more efficient ways to do this but this shows you the general approach.

$ diskutil list | grep -E "[ ]+[0-9]+:.*disk[0-9]+" | sed 's/.*\(disk.*\)/\1/'
disk0
disk0s1
disk0s2
disk0s3

With this approach you can then modify the for loop above like so:

$ for i in $(diskutil list | grep -E "[ ]+[0-9]+:.*disk[0-9]+" | \
    sed 's/.*\(disk.*\)/\1/'); do diskutil info $i; done

Best Answer

Related Solutions

Bash – Command Substitution Splitting on Newline but Not Space

Bash – How to list the info for an array of /dev/disks using bash expansion or substitution

Example

Parsing diskutil

Example

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