Bash – Why Can’t Variables Be Used as Prefix to a Command to Set Environment Variables?

bashenvironment-variablesvariable

Normally, it is possible to set an environment variable for a command by prefixing it like so:

hello=hi bash -c 'echo $hello'

I also know that we can use a variable to substitute any part of a command invocation like the following:

$ cmd=bash
$ $cmd -c "echo hi" # equivalent to bash -c "echo hi"

I was very surprised to find out that you cannot use a variable to prefix a command to set an environment variable. Test case:

$ prefix=hello=hi
$ echo $prefix # prints hello=hi
$ $prefix bash -c 'echo $hello'
hello=hi: command not found

Why can I not set the environment variable using a variable? Is the prefix part a special part? I was able to get it working by using eval in front, but I still do not understand why. I am using bash 4.4.

Best Answer

I suspect this is the part of the sequence that's catching you:

The words that are not variable assignments or redirections are expanded (see Shell Expansions). If any words remain after expansion, the first word is taken to be the name of the command and the remaining words are the arguments

That's from the Bash reference manual in the section on Simple Command Expansion.

In the cmd=bash example, no environment variables are set, and bash processes the command line up through parameter expansion, leaving bash -c "echo hi".

In the prefix=hello=hi example, there are again no variable assignments in the first pass, so processing continues to parameter expansion, resulting in a first word of hello=hi.

Once the variable assignments have been processed, they are not re-processed during command execution.

See the processing and its results under set -x:

$ prefix=hello=hi
+ prefix=hello=hi
$ $prefix bash -c 'echo $hello'
+ hello=hi bash -c 'echo $hello'
-bash: hello=hi: command not found
$ hello=42 bash -c 'echo $hello'
+ hello=42
+ bash -c 'echo $hello'
42

For a safer variation of "variable expansion" -as- "environment variables" than eval, consider wjandrea's suggestion of env:

prefix=hello=hi
env "$prefix" bash -c 'echo "$hello"'
hi

It's not strictly a command-line variable assignment, since we're using the env utility's main function of assigning environment variables to a command, but it accomplishes the same goal. The $prefix variable is expanded during the processing of the command-line, providing the name=value to env, who passes it along to bash.

Related Solutions

Bash – Why does export -p exclude $_ variable

$_ does not seem to be an environmental variable in bash, bash only appears to export it into a child process' environment. Inside bash itself it seems to be a normal shell variable. Note however this is not the case when the first command is executed:

$ bash -c 'export -p | grep _='
declare -x _="/bin/bash"

Afterwards however it shows up as a normal variable:

$ bash -c ':; declare -p | grep _='
declare -- BASH_EXECUTION_STRING=":; declare -p | grep _="
declare -- _=":

Not this is not the case in dash:

$ dash -c 'export -p | grep _='
export _='/bin/dash'
$ dash -c ':; export -p | grep _='
export _='/bin/dash'

Although here it only seems to take on its proper role in interactive mode:

$ dash
$ :
$ export -p | grep _=
export _=':'

Bash – Running a constructed command from bash script

When you leave a variable expansion unquoted, it undergoes word splitting and filename expansion (i.e. globbing). It isn't parsed as a shell command. In general, when you dynamically construct a shell snippet to execute, the right way to execute it is eval "$a" where a contains the string to parse as shell code.

In your first snippet, the value of a is the string my_cmd --verbose. Word splitting breaks it into two words my_cmd and --verbose. Globbing does nothing since there are no wildcards in any of the words. The command resulting from the expansion of $a thus consists of two words my_cmd and --verbose, so the command my_cmd (which could be an alias, a function, a builtin or an executable in the PATH) is executed with the single argument --verbose.

In the second snippet, things are similar, with three words resulting from the expansion: URL=myurl, my_cmd and --verbose. This results in an attempt to execute the command URL=myurl with two arguments.

The shell command URL=myurl my_cmd --verbose is parsed differently: the first word is parsed as an assignment to the variable URL, and since there is a command name after it the assignment sets the environment variable for the duration of the command only. This is part of parsing, not something that's done after expansion, so it requires the equal sign to be part of the shell source code, the equal sign can't be the result of some expansion.

Don't store a command with parameters into a string. Use an array. For a complex command, such as one where you set variables or perform redirection in addition to running the command, use a function if at all possible, and if not use eval (taking great care of proper quoting).

Best Answer

Related Solutions

Bash – Why does export -p exclude $_ variable

Bash – Running a constructed command from bash script

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