Shell Variable Names – Can Variable Names Include a Hyphen or Dash?

environment-variablesshell

I am not able to use - in variables in shell. Is there a way to be able to use it, because I have one script which depends on such named variables:

$export a-b=c
-bash: export: `a-b=c': not a valid identifier

$export a_b=c

First throws the given error and second works fine.

Best Answer

I've never met a Bourne-style shell that allowed - in a variable name. Only ASCII letters (of either case), _ and digits are supported, and the first character must not be a digit.

If you have a program that requires an environment variable that doesn't match the shell restrictions, launch it with the env program.

env 'strange-name=some value' myprogram

Note that some shells (e.g. modern dash, mksh, zsh) remove variables whose name they don't like from the environment. (Shellshock has caused people to be more cautious about environment variable names, so restrictions are likely to become tighter over time, not more permissive.) So if you need to pass a variable whose name contains special character to a program, pass it directly, without a shell in between (env 'strange-name=some value' sh -c'…; myprogram' may or may not work).

Related Solutions

Environment Variables – What Scopes Can Shell Variables Have?

The processes are organized as a tree: every process has a unique parent, apart from init which PID is always 1 and has no parent.

The creation of a new process goes generally through a pair of fork/execv system calls, where the environment of the child process is a copy of the parent process.

To put a variable in the environment from the shell you have to export that variable, so that it is visible recursively to all children. But be aware that if a child change the value of a variable, the changed value is only visible to it and all processes created after that change (being a copy, as previously said).

Take also into account that a child process could change its environment, for example could reset it to default values, as is probably done from login for example.

How to store / load exported environment variables to / from a file

POSIXly, you can do:

# save
export -p > saved-env

...

# restore
blacklisted () {
  case $1 in
    PWD|OLDPWD|SHELL|STORAGE|-*) return 0 ;;
    *) return 1 ;;
  esac
}

eval '
  export() {
    blacklisted "${1%%=*}" || unset -v "${1%%=*}"
  }
  '"$(export -p)"
export() {
  blacklisted "${1%%=*}" || command export "$@"
}
. saved-env
unset -f export

Note that for bash not invoked as sh, you'd need to issue a set -o posix for that to work properly. Also with bash versions prior to 4.4, sourcing the output of export -p is potentially unsafe:

$ env -i 'a;reboot;=1' /bin/bash -o posix -c 'export -p'
export OLDPWD
export PWD="/"
export SHLVL="1"
export a;reboot;

ksh93 has a similar problem. yash doesn't have that particular one, but still has problems with variable names starting with -:

$ env -i -- '-p=' yash -c 'export -p'
export '-p'=''
export OLDPWD
export PWD='/'

Also beware of potential problems if you're not in the same locale when saving and restoring the variables.

bash-4.3$ locale charmap
ISO-8859-15
bash-4.3$ export Stéphane=1
bash-4.3$ export -p > a
bash-4.3$ LC_ALL=en_GB.UTF-8 bash -c '. ./a'
./a: line 5: export: `Stéphane=1': not a valid identifier

Best Answer

Related Solutions

Environment Variables – What Scopes Can Shell Variables Have?

How to store / load exported environment variables to / from a file

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