add Homebrew on Linux (#589)

* Add homebrew installation and validation scripts

* Update `/etc/environment` in `updatepath.sh` script

Keep this code for sake of compatibility with the existing installation logic. It will be changed in its own PR

* Remove env. variables from homebrew validation task

* Move reboot to the dedicated script and task

Co-authored-by: Sergey Dolin <v-sedoli@micorosoft.com>

images/linux/scripts/helpers/document.sh

@@ -9,7 +9,7 @@ function WriteItem {
         echo "METADATA_FILE environment variable must be set to output to Metadata Document!"
         return 1;
     else
-        echo -e "$1" >> "$METADATA_FILE"
+        echo -e "$1" | sudo tee -a "$METADATA_FILE"
     fi
 }
 

images/linux/scripts/helpers/etc-environment.sh

@@ -0,0 +1,65 @@
+#!/bin/bash
+################################################################################
+##  File:  etc-environment.sh
+##  Desc:  Helper functions for source and modify /etc/environment
+################################################################################
+
+# NB: sed expression use '%' as a delimiter in order to simplify handling
+#     values containg slashes (i.e. directory path)
+#     The values containing '%' will break the functions
+
+function getEtcEnvironmentVar {
+    var_name="$1"
+    # remove `var_name=` and possible quotes from the line
+    grep "^${var_name}=" /etc/environment |sed -E "s%^${var_name}=\"?([^\"]+)\"?.*$%\1%"
+}
+
+function addEtcEnvironmentVar {
+    var_name="$1"
+    var_value="$2"
+
+    echo "$var_name=\"$var_value\"" | sudo tee -a /etc/environment
+}
+
+function replaceEtcEnvironmentVar {
+    var_name="$1"
+    var_value="$2"
+
+    # modify /etc/environemnt in place by replacing a string that begins with var_name
+    sudo sed -ie "s%^${var_name}=.*$%${var_name}=\"${var_value}\"%" /etc/environment
+}
+
+function setEtcEnvironmentVar {
+    var_name="$1"
+    var_value="$2"
+
+    if grep "$var_name" /etc/environment > /dev/null; then
+        replaceEtcEnvironmentVar $var_name $var_value
+    else
+        addEtcEnvironmentVar $var_name $var_value
+    fi
+}
+
+function addEtcEnvironmentPathElement {
+    element="$1"
+    etc_path=$(getEtcEnvironmentVar PATH)
+    setEtcEnvironmentVar PATH "${element}:${etc_path}"
+}
+
+# Process /etc/environment as if it were shell script with `export VAR=...` expressions
+#    The PATH variable is handled specially in order to do not override the existing PATH
+#    variable. The value of PATH variable read from /etc/environment is added to the end
+#    of value of the exiting PATH variable exactly as it would happen with real PAM app read
+#    /etc/environment
+#
+# TODO: there might be the others variables to be processed in the same way as "PATH" variable
+#       ie MANPATH, INFOPATH, LD_*, etc. In the current implementation the values from /etc/evironments
+#       replace the values of the current environment
+function  reloadEtcEnvironment {
+    # add `export ` to every variable of /etc/environemnt except PATH and eval the result shell script
+    eval $(grep -v '^PATH=' /etc/environment | sed -e 's%^%export %')
+    # handle PATH specially
+    etc_path=$(getEtcEnvironmentVar PATH)
+    export PATH="$PATH:$etc_path"
+}
+