Subshells

In UNIX, when a program starts another program (more exactly, when a process starts another process), the new process runs as a subprocess  or child process. When a shell starts another shell, the new shell is called a subshell.

Running a shell script launches another instance of the command processor. Just as your commands are interpreted at the command line prompt, similarly does a script batch process a list of commands in a file. Each shell script running is, in effect, a subprocess of the parent shell, the one that gives you the prompt at the console or in an xterm window.

A shell script can also launch subprocesses. These subshells let the script do parallel processing, in effect executing multiple subtasks simultaneously.

( command1; command2; command3; ... )

A command list embedded between parentheses   runs as a subshell.

Variables in a subshell are not visible outside the block of code in the subshell. These are, in effect, local variables. That means that subsell is useful instrument when  you need to work in a different current directory, reset environment variables, set a new home directory, reset some aliases, use a different PATH, whatever. When you end the subshell, the parent shell's environment will be the way it was.

Notes:

• This isn't true when the subprocess is execd from the parent process without a fork  first. 
• When you use the shell's exec   command, it does not start a subprocess.

So what? There are some important things to know about it: the child process gets a copy of its parent's environment. Any changes in the environment of the child process aren't passed to its parent. "Still," I hear you say, "so what??"

• Shell scripts are run in a subshell (unless you use the source  or . commands  to start the script). If the script makes changes to the environment of its (sub)shell, the parent shell won't see those changes. If the script uses cd, it doesn't change the current directory in the parent shell. If the script changes the value of the TZ  (or any) environment variable, that won't change TZ  in the parent shell. The script can set a different umask   than the parent shell - no problem.

NEWS CONTENTS

• Unix Power Tools, 3rd Edition
• Show Subshell Level with $SHLVL (Unix Power Tools, 3rd Edition)

Old News ;-)

Unix Power Tools, 3rd Edition

If your parent shell has job control, you can stop the subshell and pop back to your parent shell without losing the changes in the subshell. If the child shell has job control, too, the command suspend (22.22) (or kill -STOP $$ ) will stop it. Otherwise, just type CTRL-z at the subshell's prompt. For example:

prompt %csh	myprompt% csh myprompt% set prompt="project% " project% cd project-directory project% setenv PRINTER plotter project% set path=($path some-new-directories) project% setenv EXINIT "se ts=4 sw=4 aw wm=0" ...do some work... project% suspend Stopped myprompt% ...back to parent shell... myprompt% fg %csh ...back to subshell... %

I use suspend so much that I've made a CTRL-z-like alias named z.

A shell escape starts a subshell. Do whatever you want to the subshell's environment. When you end the shell escape, the changes go away.

The su command starts a subshell. cd anywhere, change environment variables, and so on...

If you use the exit command, a subshell (or any shell) will terminate. In a script, when the shell reads the end of file, that does an implicit exit. On the command line, an end-of-input character (usually CTRL-d) will do the same thing.

Show Subshell Level with $SHLVL (Unix Power Tools, 3rd Edition)

If you're like me, when you start a shell escape (Section 17.21) or any subshell (Section 24.4), you can forget that you aren't in your login shell. Your shell history (Section 30.1) might get confused, shell variables (Section 35.9) may not be set, and other problems may come up. zsh and bash have a built-in SHLVL environment variable (Section 35.3) that lets you track how many subshells deep your current shell is. tcsh has a shlvl shell variable that's automatically set from (and sets) SHLVL. So, all three shells cooperate with each other to set the right value, even if you start one shell from another. (For other shells that don't have SHLVL -- ksh and csh -- you can set up something similar with a bit of arithmetic in the ENV (Section 35.5) file or the .cshrc file, respectively.)

In your top-level shell, the value of $shlvl is 1 (one). In the first subshell, it's 2; in a sub-subshell, it's 3; and so on. You can use this to control your shell startup files -- for example, have some commands in your .cshrc that run when you first log in (and $shlvl is 1), but don't run in subshells. You can also put $shlvl in your prompt (but only during subshells, if you'd like -- as a reminder that you aren't in your top-level shell). You can set your prompt to mike% in top-level shells, (1) mike% in a first-level subshell, (2) mike% in a second-level subshell, and so on. Here's some sample prompt-setting code for your .tcshrc:

# If this is a subshell, put shell level in prompt:
if ($shlvl == 1) then
    set prompt="${USER}% "
else
    set prompt="($SHLVL) ${USER}% "
endif

bash doesn't need an if because login shells read your .bash_profile (or .profile) and subshells read your .bashrc. Here are commands to set the prompts I mentioned earlier:

PS1='\u\$ '             ...for the .bash_profile
PS1='($SHLVL) \u\$ '    ...for the .bashrc

Does your account run a windowing system that's started from your top-level shell startup file (like .login)? If it does, lines like the following examples (these are for .login) will reset SHLVL so that the shell in the window will start at a SHLVL of 1 -- and act like a top-level shell. This code assumes that your first login shell starts on a tty named /dev/tty1 through /dev/tty6 (which are the Linux virtual consoles (Section 23.12)) and that the windows that open won't have a tty with the same name (which is true on Linux). (If you aren't sure, check who (Section 2.8).) You may need to adapt this. The trick is to make SHLVL 0 (zero) before you start the windowing system. When the windows' shells start, they'll raise SHLVL to 1:

# If on a virtual console, bury this shell and start X right away:
if ("`tty`" =~ /dev/tty[1-6]) then
   setenv SHLVL 0
   startx
endif

Getting this to work right in every situation (rsh (Section 1.21), ssh, su, shell escapes (Section 17.21) -- both interactive and noninteractive (Section 3.4) -- subshells, window systems, at jobs (Section 25.5), and so on) can be a challenge (Section 3.8)! It takes a little planning. Sit down and think about all the ways you start subshells -- which subshells are interactive and which aren't -- and whether they'll get SHLVL passed from their parent process. (If you aren't sure, test that with an env or printenv command (Section 35.3).) Then plan which kind of shell needs which SHLVL settings. If it gets too complicated, make it work in most cases! If you use many subshells, this system can be too handy to ignore.

--JP and SJC

Recommended Links

Etc

Society

Groupthink : Two Party System as Polyarchy : Corruption of Regulators : Bureaucracies : Understanding Micromanagers and Control Freaks : Toxic Managers :   Harvard Mafia : Diplomatic Communication : Surviving a Bad Performance Review : Insufficient Retirement Funds as Immanent Problem of Neoliberal Regime : PseudoScience : Who Rules America : Neoliberalism  : The Iron Law of Oligarchy : Libertarian Philosophy

Quotes

War and Peace : Skeptical Finance : John Kenneth Galbraith :Talleyrand : Oscar Wilde : Otto Von Bismarck : Keynes : George Carlin : Skeptics : Propaganda  : SE quotes : Language Design and Programming Quotes : Random IT-related quotes :  Somerset Maugham : Marcus Aurelius : Kurt Vonnegut : Eric Hoffer : Winston Churchill : Napoleon Bonaparte : Ambrose Bierce :  Bernard Shaw : Mark Twain Quotes

Bulletin:

Vol 25, No.12 (December, 2013) Rational Fools vs. Efficient Crooks The efficient markets hypothesis : Political Skeptic Bulletin, 2013 : Unemployment Bulletin, 2010 :  Vol 23, No.10 (October, 2011) An observation about corporate security departments : Slightly Skeptical Euromaydan Chronicles, June 2014 : Greenspan legacy bulletin, 2008 : Vol 25, No.10 (October, 2013) Cryptolocker Trojan (Win32/Crilock.A) : Vol 25, No.08 (August, 2013) Cloud providers as intelligence collection hubs : Financial Humor Bulletin, 2010 : Inequality Bulletin, 2009 : Financial Humor Bulletin, 2008 : Copyleft Problems Bulletin, 2004 : Financial Humor Bulletin, 2011 : Energy Bulletin, 2010 : Malware Protection Bulletin, 2010 : Vol 26, No.1 (January, 2013) Object-Oriented Cult : Political Skeptic Bulletin, 2011 : Vol 23, No.11 (November, 2011) Softpanorama classification of sysadmin horror stories : Vol 25, No.05 (May, 2013) Corporate bullshit as a communication method  : Vol 25, No.06 (June, 2013) A Note on the Relationship of Brooks Law and Conway Law

History:

Fifty glorious years (1950-2000): the triumph of the US computer engineering : Donald Knuth : TAoCP and its Influence of Computer Science : Richard Stallman : Linus Torvalds  : Larry Wall  : John K. Ousterhout : CTSS : Multix OS Unix History : Unix shell history : VI editor : History of pipes concept : Solaris : MS DOS :  Programming Languages History : PL/1 : Simula 67 : C : History of GCC development :  Scripting Languages : Perl history   : OS History : Mail : DNS : SSH : CPU Instruction Sets : SPARC systems 1987-2006 : Norton Commander : Norton Utilities : Norton Ghost : Frontpage history : Malware Defense History : GNU Screen : OSS early history

Classic books:

The Peter Principle : Parkinson Law : 1984 : The Mythical Man-Month :  How to Solve It by George Polya : The Art of Computer Programming : The Elements of Programming Style : The Unix Hater’s Handbook : The Jargon file : The True Believer : Programming Pearls : The Good Soldier Svejk : The Power Elite

Most popular humor pages:

Manifest of the Softpanorama IT Slacker Society : Ten Commandments of the IT Slackers Society : Computer Humor Collection : BSD Logo Story : The Cuckoo's Egg : IT Slang : C++ Humor : ARE YOU A BBS ADDICT? : The Perl Purity Test : Object oriented programmers of all nations : Financial Humor : Financial Humor Bulletin, 2008 : Financial Humor Bulletin, 2010 : The Most Comprehensive Collection of Editor-related Humor : Programming Language Humor : Goldman Sachs related humor : Greenspan humor : C Humor : Scripting Humor : Real Programmers Humor : Web Humor : GPL-related Humor : OFM Humor : Politically Incorrect Humor : IDS Humor : "Linux Sucks" Humor : Russian Musical Humor : Best Russian Programmer Humor : Microsoft plans to buy Catholic Church : Richard Stallman Related Humor : Admin Humor : Perl-related Humor : Linus Torvalds Related humor : PseudoScience Related Humor : Networking Humor : Shell Humor : Financial Humor Bulletin, 2011 : Financial Humor Bulletin, 2012 : Financial Humor Bulletin, 2013 : Java Humor : Software Engineering Humor : Sun Solaris Related Humor : Education Humor : IBM Humor : Assembler-related Humor : VIM Humor : Computer Viruses Humor : Bright tomorrow is rescheduled to a day after tomorrow : Classic Computer Humor

The Last but not Least Technology is dominated by two types of people: those who understand what they do not manage and those who manage what they do not understand ~Archibald Putt. Ph.D

The statements, views and opinions presented on this web page are those of the author (or referenced source) and are not endorsed by, nor do they necessarily reflect, the opinions of the Softpanorama society. We do not warrant the correctness of the information provided or its fitness for any purpose. The site uses AdSense so you need to be aware of Google privacy policy. You you do not want to be tracked by Google please disable Javascript for this site. This site is perfectly usable without Javascript.

Last modified: December, 06, 2015