Linux shell is a user interface used for executing the commands. Shell is a program the user uses for executing the commands. In UNIX, any program can be the users shell.
Types of Shells in Linux
sh ( Bourne Shell ): The original shell still used on UNIX systems and in UNIX-related environments. This is the basic shell, a small program with few features. While this is not the standard shell, it is still available on every Linux system for compatibility with UNIX programs.
bash ( Bourne Again shell ): The standard GNU shell, intuitive and flexible. Probably most advisable for beginning users while being at the same time a powerful tool for the advanced and professional user. On Linux, bash is the standard shell for common users. This shell is a so-called superset of the Bourne shell, a set of add-ons and plug-ins. This means that the Bourne Again shell is compatible with the Bourne shell: commands that work in sh, also work in bash. However, the reverse is not always the case.
csh ( C shell ): the syntax of this shell resembles that of the C programming language sometimes asked for by programmers.
tcsh ( TENEX C shell ): a superset of the common C shell, enhancing user-friendliness and speed. That is why some also call it the Turbo C shell.
ksh ( the Korn shell ): sometimes appreciated by people with a UNIX background. A superset of the Bourne shell; with standard configuration a nightmare for beginning users.
The file /etc/shells gives an overview of known shells on a Linux system.
[root@armourinfosec /]# vim /etc/shells
To switch from one shell to another, just enter the name of the new shell in the active terminal.
[root@armourinfosec /]# sh
Reserved Bourne shell variables
| Variable name | Definition |
| CDPATH | A colon-separated list of directories used as a search path for thecdbuilt-in command. |
| HOME | The current user’s home directory; the default for thecdbuilt-in. The value of this variable is also used by tilde expansion. |
| IFS | A list of characters that separate fields; used when the shell splits words as part of expansion. |
| If this parameter is set to a file name and the MAILPATHvariable is not set, Bash informs the user of the arrival of mail in the specified file. | |
| MAILPATH | A colon-separated list of file names which the shell periodically checks for new mail. |
| OPTARG | The value of the last option argument processed by thegetoptsbuilt-in. |
| OPTIND | The index of the last option argument processed by thegetoptsbuilt-in. |
| PATH | A colon-separated list of directories in which the shell looks for commands. |
| PS1 | The primary prompt string. The default value is “‘\s-\v\$ ‘”. |
| PS2 | The secondary prompt string. The default value is “‘> ‘”. |
Bash reserved variables
| Variable name | Definition |
| auto_resume | This variable controls how the shell interacts with the user and job control. |
| BASH | The full pathname used to execute the current instance of Bash. |
| BASH_ENV | If this variable is set when Bash is invoked to execute a shell script, its value is expanded and used as the name of a startup file to read before executing the script. |
| BASH_VERSION | The version number of the current instance of Bash. |
| BASH_VERSINFO | A read-only array variable whose members hold version information for this instance of Bash. |
| COLUMNS | Used by the select built-in to determine the terminal width when printing selection lists. Automatically set upon receipt of a SIGWINCH signal. |
| COMP_CWORD | An index into ${COMP_WORDS}of the word containing the current cursor position. |
| COMP_LINE | The current command line. |
| COMP_POINT | The index of the current cursor position relative to the beginning of the currentCommand. |
| COMP_WORDS | An array variable consisting of the individual words in the current command line. |
| COMPREPLY | An array variable from which Bash reads the possible completions generated by a shell function invoked by the programmable completion facility. |
| DIRSTACK | An array variable containing the current contents of the directory stack. |
| EUID | The numeric effective user ID of the current user. |
| FCEDIT | The editor used as a default by the -eoption to the fcbuilt-in command. |
| FIGNORE | A colon-separated list of suffixes to ignore when performing file name completion. |
| FUNCNAME | The name of any currently-executing shell function. |
| GLOBIGNORE | A colon-separated list of patterns defining the set of file names to be ignored by filename expansion. |
| GROUPS | An array variable containing the list of groups of which the current user is amember. |
| histchars | Up to three characters which control history expansion, quick substitution, andtokenization. |
| HISTCMD | The history number, or index in the history list, of the current command. |
| HISTCONTROL | Defines whether a command is added to the history file. |
| HISTFILE | The name of the file to which the command history is saved. The default value is~/.bash_history. |
| HISTFILESIZE | The maximum number of lines contained in the history file, defaults to 500. |
| HISTIGNORE | A colon-separated list of patterns used to decide which command lines should besaved in the history list. |
| HISTSIZE | The maximum number of commands to remember on the history list, default is 500. |
| HOSTFILE | Contains the name of a file in the same format as /etc/hosts that should be readwhen the shell needs to complete a hostname. |
| HOSTNAME | The name of the current host. |
| HOSTTYPE | A string describing the machine Bash is running on. |
| IGNOREEOF | Controls the action of the shell on receipt of an EOF character as the sole input. |
| INPUTRC | The name of the Read line initialization file, overriding the default/etc/inputrc. |
| LANG | Used to determine the locale category for any category not specifically selected witha variable starting with LC_. |
| LC_ALL | This variable overrides the value of LAN Gand any other LC_variable specifying alocale category. |
| LC_COLLATE | This variable determines the collation order used when sorting the results of filename expansion, and determines the behavior of range expressions, equivalence
classes, and collating sequences within file name expansion and pattern matching. |
| LC_CTYPE | This variable determines the interpretation of characters and the behavior ofcharacter classes within file name expansion and pattern matching. |
| LC_MESSAGES | This variable determines the locale used to translate double-quoted strings precededby a “$” sign. |
| LC_NUMERIC | This variable determines the locale category used for number formatting. |
| LINENO | The line number in the script or shell function currently executing. |
| LINES | Used by the select built-in to determine the column length for printing selectionlists. |
| MACHTYPE | A string that fully describes the system type on which Bash is executing, in thestandard GNU CPU-COMPANY-SYSTEM format. |
| MAILCHECK | How often (in seconds) that the shell should check for mail in the files specified in the MAILPATH or MAIL variables. |
| OLDPWD | The previous working directory as set by the cd built-in |
| OPTERR | If set to the value 1, Bash displays error messages generated by the getopts built-in. |
| OSTYPE | A string describing the operating system Bash is running on. |
| PIPESTATUS | An array variable containing a list of exit status values from the processes in themost recently executed foreground pipeline (which may contain only a single
command). |
| POSIXLY_CORRECT | If this variable is in the environment when bash starts, the shell enters POSIXmode. |
| PPID | The process ID of the shell’s parent process. |
| PROMPT_COMMAND | If set, the value is interpreted as a command to execute before the printing of eachprimary prompt (PS1) |
| PS3 | The value of this variable is used as the prompt for the select command. Defaults to”‘#? ‘” |
| PS4 | The value is the prompt printed before the command line is echoed when the -xoption is set; defaults to “‘+ ‘”. |
| PWD | The current working directory as set by the cd built-in command. |
| RANDOM | Each time this parameter is referenced, a random integer between 0 and 32767 is generated. Assigning a value to this variable seeds the random number generator. |
| REPLY | The default variable for the read built-in. |
| SECONDS | This variable expands to the number of seconds since the shell was started. |
| SHELLOPTS | A colon-separated list of enabled shell options. |
| SHLVL | Incremented by one each time a new instance of Bash is started. |
| TIMEFORMAT | The value of this parameter is used as a format string specifying how the timinginformation for pipelines prefixed with the time reserved word should be displayed. |
| TMOUT | If set to a value greater than zero, TMOUT is treated as the default timeout for theRead built-in. In an inter ative shell, the value is interpreted as the number of
seconds to wait for input after issuing the primary prompt when the shell is interactive. Bash terminates after that number of seconds if input does not arrive. |
| UID | The numeric, real user ID of the current user. |
