-n作用

[ -n 字符串 ] or [ 字符串 ]  字符串的长度为非零（有内容）则为真。加-n与不加-n结果相同。

-z作用

[ -z 字符串 ] 字符串的长度为零则为真。 字符串为空即NULL时为真，与上面的-n相反。

-f作用

[ -f FILE ]  如果 FILE 存在且是一个普通文件则为真。 

ros系统环境为ros2  foxy。

系统自带的包source的第一个系统脚本文件

/opt/ros/foxy/setup.bash

# copied from ament_package/template/prefix_level/setup.bash

AMENT_SHELL=bash

# source setup.sh from same directory as this file
AMENT_CURRENT_PREFIX=$(builtin cd "`dirname "${BASH_SOURCE[0]}"`" && pwd)
# trace output
if [ -n "$AMENT_TRACE_SETUP_FILES" ]; then
  echo "# . \"$AMENT_CURRENT_PREFIX/setup.sh\""
fi
. "$AMENT_CURRENT_PREFIX/setup.sh"

通过在终端显示这个变量验证，$AMENT_TRACE_SETUP_FILES没有内容。

  [ -n "$AMENT_TRACE_SETUP_FILES" ]没有内容，if fi内代码不运行，直接跳到最后一句执行

. "$AMENT_CURRENT_PREFIX/setup.sh"

 这个脚本文件的意思是运行同一个文件夹内的setup.sh文件，转入运行下面的脚本。

/opt/ros/foxy/setup.sh

# generated from ament_package/template/prefix_level/setup.sh.in

# since this file is sourced use either the provided AMENT_CURRENT_PREFIX
# or fall back to the destination set at configure time
: ${AMENT_CURRENT_PREFIX:=/opt/ros/foxy}

# set type of shell if not already set
: ${AMENT_SHELL:=sh}

# function to append non-duplicate values to environment variables
# using colons as separators and avoiding leading separators
ament_append_unique_value() {
  # arguments
  _listname=$1
  _value=$2
  #echo "listname $_listname"
  #eval echo "list value \$$_listname"
  #echo "value $_value"

  # check if the list contains the value
  eval _values=\$$_listname
  _duplicate=
  _ament_append_unique_value_IFS=$IFS
  IFS=":"
  if [ "$AMENT_SHELL" = "zsh" ]; then
    ament_zsh_to_array _values
  fi
  for _item in $_values; do
    # ignore empty strings
    if [ -z "$_item" ]; then
      continue
    fi
    if [ $_item = $_value ]; then
      _duplicate=1
    fi
  done
  unset _item

  # append only non-duplicates
  if [ -z "$_duplicate" ]; then
    # avoid leading separator
    if [ -z "$_values" ]; then
      eval $_listname=\"$_value\"
      #eval echo "set list \$$_listname"
    else
      # field separator must not be a colon
      unset IFS
      eval $_listname=\"\$$_listname:$_value\"
      #eval echo "append list \$$_listname"
    fi
  fi
  IFS=$_ament_append_unique_value_IFS
  unset _ament_append_unique_value_IFS
  unset _duplicate
  unset _values

  unset _value
  unset _listname
}

# iterate over all parent_prefix_path files
_prefix_setup_IFS=$IFS
IFS="
"
# this variable contains the concatenated prefix paths in reverse order
_UNIQUE_PREFIX_PATH=""

# this check is used to skip parent prefix path in the Debian package
if [ -z "SKIP_PARENT_PREFIX_PATH" ]; then
  # find parent prefix path files for all packages under the current prefix
  _RESOURCES="$(\find "$AMENT_CURRENT_PREFIX/share/ament_index/resource_index/parent_prefix_path" -mindepth 1 -maxdepth 1 2> /dev/null | \sort)"

  if [ "$AMENT_SHELL" = "zsh" ]; then
    ament_zsh_to_array _RESOURCES
  fi
  for _resource in $_RESOURCES; do
    # read the content of the parent_prefix_path file
    _PARENT_PREFIX_PATH="$(\cat "$_resource")"
    # reverse the list
    _REVERSED_PARENT_PREFIX_PATH=""
    IFS=":"
    if [ "$AMENT_SHELL" = "zsh" ]; then
      ament_zsh_to_array _PARENT_PREFIX_PATH
    fi
    for _path in $_PARENT_PREFIX_PATH; do
      # replace placeholder of current prefix
      if [ "$_path" = "{prefix}" ]; then
        _path="$AMENT_CURRENT_PREFIX"
      fi
      # avoid leading separator
      if [ -z "$_REVERSED_PARENT_PREFIX_PATH" ]; then
        _REVERSED_PARENT_PREFIX_PATH=$_path
      else
        _REVERSED_PARENT_PREFIX_PATH=$_path:$_REVERSED_PARENT_PREFIX_PATH
      fi
    done
    unset _PARENT_PREFIX_PATH
    # collect all unique parent prefix path
    if [ "$AMENT_SHELL" = "zsh" ]; then
      ament_zsh_to_array _REVERSED_PARENT_PREFIX_PATH
    fi
    for _path in $_REVERSED_PARENT_PREFIX_PATH; do
      ament_append_unique_value _UNIQUE_PREFIX_PATH "$_path"
    done
    unset _REVERSED_PARENT_PREFIX_PATH
  done
  unset _resource
  unset _RESOURCES
fi

# append this directory to the prefix path
ament_append_unique_value _UNIQUE_PREFIX_PATH "$AMENT_CURRENT_PREFIX"
unset AMENT_CURRENT_PREFIX

# store AMENT_SHELL to restore it after each prefix
_prefix_setup_AMENT_SHELL=$AMENT_SHELL
# source local_setup.EXT or local_setup.sh file for each prefix path
IFS=":"
if [ "$AMENT_SHELL" = "zsh" ]; then
  ament_zsh_to_array _UNIQUE_PREFIX_PATH
fi
for _path in $_UNIQUE_PREFIX_PATH; do
  # trace output
  if [ -n "$AMENT_TRACE_SETUP_FILES" ]; then
    echo "# . \"$_path/local_setup.$AMENT_SHELL\""
  fi
  if [ -f "$_path/local_setup.$AMENT_SHELL" ]; then
    if [ "$AMENT_SHELL" = "sh" ]; then
      # provide AMENT_CURRENT_PREFIX to .sh files
      AMENT_CURRENT_PREFIX=$_path
    fi
    # restore IFS before sourcing other files
    IFS=$_prefix_setup_IFS
    . "$_path/local_setup.$AMENT_SHELL"
    # restore AMENT_SHELL after each prefix-level local_setup file
    AMENT_SHELL=$_prefix_setup_AMENT_SHELL
  fi
done
unset _path
IFS=$_prefix_setup_IFS
unset _prefix_setup_IFS
unset _prefix_setup_AMENT_SHELL
unset _UNIQUE_PREFIX_PATH
unset AMENT_SHELL

我们自己生成的包source环境变量脚本 

工作空间内 source install/setup.bash

# generated from colcon_bash/shell/template/prefix_chain.bash.em

# This script extends the environment with the environment of other prefix
# paths which were sourced when this file was generated as well as all packages
# contained in this prefix path.

# function to source another script with conditional trace output
# first argument: the path of the script
_colcon_prefix_chain_bash_source_script() {
  if [ -f "$1" ]; then
 
    if [ -n "$COLCON_TRACE" ]; then
    
      echo ". \"$1\""
    fi
    . "$1"
    
  else
    echo "not found: \"$1\"" 1>&2
  fi
}

# source chained prefixes
# setting COLCON_CURRENT_PREFIX avoids determining the prefix in the sourced script
COLCON_CURRENT_PREFIX="/opt/ros/foxy"
_colcon_prefix_chain_bash_source_script "$COLCON_CURRENT_PREFIX/local_setup.bash"

# source this prefix
# setting COLCON_CURRENT_PREFIX avoids determining the prefix in the sourced script
COLCON_CURRENT_PREFIX="$(builtin cd "`dirname "${BASH_SOURCE[0]}"`" > /dev/null && pwd)"
_colcon_prefix_chain_bash_source_script "$COLCON_CURRENT_PREFIX/local_setup.bash"

unset COLCON_CURRENT_PREFIX
unset _colcon_prefix_chain_bash_source_script

$0当前脚本名 $1 脚本的第一个参数 

_colcon_prefix_chain_bash_source_script(){}函数的意思是第一个参数文件存在就运行第一个参数，没有就报错。

所以我们在自己的工作空间source install/setup.bash相当于同时source /opt/ros/foxy/local_setup.bash

source install/local_setup.bash