envsubst: command not found on Mac OS X 10.8

Question 1

Edit: @cobberboy 's anwer is more correct. upvote him.

brew install gettext
brew link --force gettext

Following is my old answer:

envsubst is included in gettext package.

Therefore you may compile it by your own, using standard build tools such as make or using homebrew.

However, it seems to have little issue when installing gettext in MacOS. See following url for details: How to install gettext on MacOS X

Question 2

brew install gettext
brew link --force gettext

This will enable envsubst on OS X, and force it to link properly. It requires homebrew to be installed.

Question 3

To clear up potential confusion:

envsubst is an external executable and thus not part of Bash; external executables are platform-dependent, both in terms of which ones are available as well as their specific behavior and the specific options they support (though, hopefully, there is a common subset based on the POSIX specifications)
Commands directly built into bash are called builtins, and only they can be relied upon to be present on all platforms.

To test whether a given command is a builtin, use

type <cmdName>

In the case at hand, running type envsubst on macOS 10.13 returns -bash: type: envsubst: not found, from which you can infer:

envsubst is NOT a builtin
envsubst is not in your system's $PATH (and thus likely not present on your system)

(By contrast, running the same on command on, e.g., a Ubuntu 12.04 system returns envsubst is hashed (/usr/bin/envsubst), which tells you that the utility is present and where it is located.)

A makeshift alternative to envsubst is to use eval, although the usual caveat applies: use eval only on strings whose content you control or trust:

Assume a sample.txt file containing text with unexpanded variable references; e.g.:

cat > sample.txt <<'EOF'
Honey, I'm $USER
and I'm $HOME.
EOF

The equivalent of:

envsubst < sample.txt

is:

eval "echo \"$(sed 's/"/\\"/g' sample.txt)\""

There is a crucial difference, however:

envsubst expands only environment variable references
whereas eval will expand shell variable references too - as well as embedded command substitutions, which is what makes use of eval a security concern.

Question 4

I'm using this now in my bash script that requires envsubst:

if ! which envsubst > /dev/null 2>&1; then
    envsubst() {
        while read line; do
            line=$( echo $line | sed 's/"/\\"/g' )
            eval echo $line
        done
    }
fi

you can use it as the envsubst command - of course it's not feature complete or something else:

envsubst <<<'Honey, I am $HOME.'
envsubst < input > output 2> corrupt

Question 5

If you don't want to bother installing homebrew and gettext, a one line perl executable will do:

#!/usr/bin/perl -p
$_ =~ s/\Q${$1||$2}/$ENV{$1?$2:$4}/ while $_ =~ /(\$\{([^}]+)})|(\$(\w+))/g;

Question 6

If you don't want to bother installing homebrew and gettext, and can't make heads or tails of perl, a simple python script also does the trick:

envsubst() {
  python -c 'import os,sys;[sys.stdout.write(os.path.expandvars(l)) for l in sys.stdin]'
}

The advantage of this over eval solutions is that it only handles variable replacement and won't execute arbitrary scripts like eval does.

$ cat > sample.txt <<'EOF'
Honey, I'm $USER
and I'm $HOME.
Danger: $( echo 'eval can do evil here, but python expandvars rocks' )
EOF

$ envsubst < sample.txt

Honey, I'm mattmc3
and I'm /Users/mattmc3.
Danger: $( echo 'eval can do evil here, but python expandvars rocks' )