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Linux Shell / User Environment Basics

The command shell operates between the core operating system and other programs. Consequently, a certain amount of start-up information must be configured for the shell to do its work. This page explains fundamental configuration and interactions for the different Linux environments and shells, in particular for Windows environments.

Location of Software Files

Software files for each Linux environment are located in standard folders depending on the environment. These files provide the functionality of the Linux environment. It may be unnecessary to deal with software files; however, it is helpful to know where files exist in order to troubleshoot, etc. The following table summarizes software locations for each Linux environment.

Environment Software File Location (from shell) Software File Location (from Windows)
Cygwin / C:\cygwin64
Git for Windows / C:\Program Files\Git
Linux Filesystem Hierarchy Standard on Wikipedia Not applicable
Linux Subsystem on Windows 10 / ?
MinGW ? C:\MinGW

Location of User Files

User files for each Linux environment are located in standard folders depending on the environment. The user files in the Linux environment are typically shared with the Windows file system (when installed on Windows) via links or logic built into the command shell programs. The following table summarizes user file locations for each Linux environment, where user is the name of the user. The Linux pwd command can be used to determine the present working directory, for example after opening a command shell.

Environment Linux User File Location (from Linux) Linux User File Location (from Windows) Windows User File Location (from Linux) Windows User File Location (from Windows)
Cygwin /home/user C:\cygwin64\home\user /cygdrive/C/Users/user C:\Users\user
Git for Windows (Git Bash) /c/Users/user C:\Users\user /c/Users/user C:\Users\user
Linux /home/user Not applicable Not applicable Not applicable
Linux Subsystem on Windows 10 /home/user C:\Users\sam\AppData\Local\Packages\CanonicalGroupLimited.UbuntuonWindows_abcd123etc\LocalState\rootfs\home\sam /mnt/c, etc. C:\Users\user
MinGW ? C:\MinGW\msys\1.0\home\user ? C:\Users\user

The Linux and Windows files may exist in the same location or may be separate. For example, Git Bash user files are in the same location as Windows user files. However, the Cygwin installation has one set of files under /home for user files specific to Cygwin and also provides the /cygdrive folder to access Windows user files (/cygdrive/C/Users/user).

The home folder can be accessed programmatically using an environment variable, as discussed in the Environment Variables section below.

Login Shell and Shell Script Shell

A Linux command shell can be used as the login shell or to run a shell script. A Linux command shell is used when a shell window is opened and provides the command shell prompt. The command shell that is being run can be listed by running the ps (process status) command:

$ ps
      PID    PPID    PGID     WINPID   TTY         UID    STIME COMMAND
    14148   14320   14148      11836  pty0        1001   Nov 28 /usr/bin/bash
     9032       1    9032       9032  ?           1001   Nov 28 /usr/bin/mintty
S   19264   14148   19264       5892  pty0        1001 23:47:44 /usr/bin/vim

In the above it is clear that the bash shell is being run.

Shell scripts are run from a parent command shell using the command shell indicated in the shell script. It is standard for the first line in the shell script to indicate which program should be used for the script, and the program can be a command shell or another program. For example, the following comment at the top of the shell script indicates to run the script with the sh command shell program located in the standard Linux software installation folder:

#!/bin/sh
#
# This script will be run with the Bourne shell

It is common to use the bash command shell as the login shell and other command shells such as sh to run other programs. For example, sh may be faster to load and run if it has fewer features. However, on some systems sh and bash may be equivalent and have no difference in performance.

Shell Configuration Files

Command shell programs can be configured for a user by modifying files in the user's home folder. Configuration files depend on the shell that is being used. Configuration changes are often required when installing software and such changes may occur automatically during installation or require the user to make such changes with a text editor.

bash Configuration Files

The bash command shell is often used for the login shell and provides features such as:

  • command completion - for example, the tab key will complete filenames
  • command prompt customization - for example show the current folder
  • command aliases - for example alias la to ls -a to list all files

The following bash configuration files are used to configure the bash shell environment:

  • .bash_profile - is executed for login shells
  • .bashrc - is executed for interactive non-login shells and when running a shell script

On Linux, files and folders with names that start with a period are treated as hidden and are not by default listed by the ls command (use ls -a to list all files). Both of these files can be created in the user's home folder. To avoid redundancy, the .bashrc file can be called by the .bash_profile file, as in:

# source the users bashrc if it exists
if [ -f "${HOME}/.bashrc" ] ; then
  source "${HOME}/.bashrc"
fi

Environment Variables

Command shell programs use environment variables to hold important configuration information. This simplifies access to important data such as the user's account name. Standard Linux defines standard environment variables and each shell also defines environment variables. For example, the HOME environment variable indicates the user's home folder.

Environment variables are separate from normal variables in a shell script. Normal variables are only visible in the shell script and are then discarded when the shell script is complete. The following illustrates setting a normal variable and environment variable in a shell script:

normalVariable="test value"
export ENVIRONMENT_VARIABLE="test value"

It is customary to name environment variables with all uppercase letters.

A list of all environment variables can be displayed using the following commands:

$ env
$ set

A single environment variable can be displayed using syntax similar to:

$ echo $HOME

A Microsoft Windows command shell also maintains a list of environment variables, which can be displayed with the following command:

> set
> echo %USERPROFILE%

The following table indicates whether the Linux environments are able to access Windows environment variables.

Scope of Environment Variables

Environment variables that are defined for a parent shell are typically inherited by child shell programs, such as those used to run shell scripts. Therefore, when a shell script is run from a command shell prompt, it is generally not necessary to reinitialize environment variables. However, if the shell script being run needs additional configuration, then such configuration needs to be done in the login shell before running the shell script.

Environment variables that are set in a script to do not propagate to other command shells that were previously started. For example, if a login window is opened and environment variables are set, they are active only for that window. Setting the environment variables in a startup configuration file will cause those variables to be set for every window that is opened.

Accessing Windows Environment Variables from Linux

Linux environments that run on Windows can access Windows environment variables. This includes Cygwin and Git Bash.

PATH Environment Variable

The PATH environment variable indicates to shell programs how to find programs when program names are typed at the command shell prompt. All folders in the PATH are searched to find the program name and if found the program is run.

If PATH environment variable does not contain the required folder, then the PATH environment variable can be modified to add the folder. This is typically done in shell startup files, such as the .bash_profile or .bashrc file. It is typical to add more specific folders at the beginning of the path so that programs are found before system copies of the same programs, using syntax like the following:

export PATH="/some/folder:${PATH}"

Another option is to edit the PATH environment variable in a "setup" shell script file that is run from the command line. The disadvantage to this approach is that it must be run each time the setting is configured. The advantage of this approach is that configuration can be isolated from the general user environment. To ensure that the PATH is not modified each time that the setup script is run, and eventually reach the limit of the PATH length, a separate environment variable can be set and checked, for example, as follows. This only makes sense if the shell is persistent and does not close after each run.

if [ -z "${ABC_SETUP}" ]; then
  # The setup has not been completed so update the PATH
  export PATH="/some/folder:${PATH}"
  # Set the variable to check next time
  export ABC_SETUP="YES"
fi

PATH Lookup Optimization using hash

Parsing the PATH environment variable and searching folders each time a command is entered on a command line requires computer resources and takes processing time. Consequently, Linux uses a hash table to track programs and absolute path. The hash command is used to manage this hash table and can be run with no options to list commands in the hash table, for example:

$ hash
hits    command
  15    /mingw64/bin/git
   1    /c/WINDOWS/py
  20    /usr/bin/vi
  13    /usr/bin/ls

The following is from the bash documentation:

If  the  name  is neither a shell function nor a builtin, and contains no slashes,
bash searches each element of the PATH for a directory containing an executable file by that name.
Bash uses a hash table to remember the full pathnames of executable files (see hash under SHELL BUILTIN COMMANDS below).
A full search of the directories in PATH is performed only if the command is not found in the hash table.  If the search is  unsuccessful,
the  shell  searches for a defined shell function named command_not_found_handle.
If that function exists, it is invoked with the original command and the original command's arguments as its arguments,
and the function's exit status becomes the exit status of the shell.
If that function is not defined, the shell prints an error message and returns an exit status of 127.

If a new version of a program is installed resulting in different location in the PATH it may be necessary to clear the hash so that the new location is used:

$ hash -r

File Paths

File paths in Linux and Windows can be absolute or relative, where absolute paths include the full path from root folder, / on Linux, and C:\ on Windows. Relative paths are relative to the current folder and can contain .. to move up a folder level.

File paths on Linux use the / separator for folder (directory) levels whereas Windows uses \ for the separator.

Spaces in file and folder names can cause issues because programs do not know whether the space indicates a break between command parameters. Therefore, names containing spaces must be surrounded by double or single quotes. Some environments, such as Cygwin, allow spaces to be handled by inserting a backslash before the space.

Modifying the PATH can simplify handling software installation folders that contain spaces because the spaces don't need to be dealt with every time that a program name is typed.