Penguin
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A crude, simple means of InterProcessCommunication, often used for trivial configuration needs.

AddToMe: inherited by child processes, cannot be changed at runtime from outside process, etc.

Invoke printenv(1) to print a list of the current values of all the EnvironmentVariables in the invoking process.

Since make(1) often uses EnvironmentVariables named after commands to store the path to command in question, such names should be avoided for other purposes. Examples include CC, BISON, YACC, LEX, FLEX, F77, LD etc. It also uses the FLAGS suffix for default flags to pass a command, eg: CFLAGS, LDFLAGS, etc.


A list of EnvironmentVariables consulted by commonly used programs1?

APT_CONFIG
see apt.conf(5)
ASPELL_CONF
Configuration file for aspell, see aspell(1)
BROWSER
The users prefered browser (see http://www.tuxedo.org/esr/BROWSER/)
BZCAT
Command line arguments for bzcat
BZIP2
Command line arguments for BZIP2
COLUMNS
Number of columns on the screen
DEAD
used by Mail(1) apparently.
DISPLAY
Current X(1) display
EDITOR
Users prefered program to edit text files
ftp_proxy
ftp proxy server, used by various programs (eg see apt.conf(5), lynx(1), wget(1), ...)
http_proxy
http proxy server, used by various programs (eg see apt.conf(5), lynx(1), wget(1), ...)
HOME
Home directory
HOSTALIASES
Path to host aliases file, see hostname(7).
HOSTFILE
Path to /etc/hosts
HZ
Kernel timer mechanism
IFS
Internal Field Seperator, used for word splitting in shells, see bash(1)
INPUTRC
Path to .inputrc file. see bash(1)
LANG
The name of the locale to use for locale categories. See locale(5)
LC_ADDRESS
Locale settings, see locale(1)
LC_ALL
Locale settings, see locale(1)
LC_COLLATE
Locale settings, see locale(1)
LC_CTYPE
Locale settings, see locale(1)
LC_IDENTIFICATION
Locale settings, see locale(1)
LC_MEASUREMENT
Locale settings, see locale(1)
LC_MESSAGES
Locale settings, see locale(1)
LC_MONETARY
Locale settings, see locale(1)
LC_NAME
Locale settings, see locale(1)
LC_NUMERIC
Locale settings, see locale(1)
LC_PAPER
Locale Paper size, see locale(1)
LC_TELEPHONE
Locale settings, see locale(1)
LD_AOUT_LIBRARY_PATH
LD_LIBRARY_PATH for a.out binaries
LD_AOUT_PRELOAD
LD_PRELOAD for a.out binaries
LD_BIND_NOW
Disable lazy linking, see ld.so(8)
LD_KEEPDIR
Don't ignore directory names of a.out libraries. see ld.so(8)
LD_LIBRARY_PATH
Path to search for libraries, see ld.so(8)
LD_NOWARN
Suppress warnings of a.out minor version differences, see ld.so(8)
LD_PRELOAD
Libraries to load before all others, see ld.so(8)
LD_TRACE_LOADED_OBJECTS
Give ldd output instead of running, see ld.so(8)
LESSANSIENDCHARS
see less(1)
LESSBINFMT
see less(1)
LESSCHARDEF
Charset definition for less(1)
LESSCHARSET
Charset used by less(1) to display.
LESSCLOSE
see less(1)
LESSECHO
see less(1)
LESSEDIT
Editor used by less(1)
LESSGLOBALTAGS
see less(1)
LESSKEY
Name of the lesskey(1) file
LESSKEY_SYSTEM
Name of the system wide lesskey(1) file
LESSMETAESCAPE
see less(1)
LESSOPEN
less(1) preprocessor
LESS
options passed to less(1) automatically
LESSSECURE
Put less(1) into secure mode.
LESSSEPERATOR
used by less(1)
LINES
Number of lines on the screen
LISTER
Used by Mail(1), obsolete version of PAGER?
LOCALDOMAIN
Local domain, see hostname(7)
LOGNAME
The name of the logged in user
LPDEST
Default printer
MAILCHECK
How often to check for mail (in seconds), see bash(1)
MAIL
File for the shell to check for mail, see bash(1)
MAILPATH
Files for the shell to check for mail, see bash(1)
MALLOC_CHECK_
Change the implementation of malloc(3) to catch more bugs.
MANPATH
Path to search for man pages
MBOX
Path to mbox file? see Mail(1)
NGPRNTER
Default printer, see lpr(1)
NPRINTER
Default printer, see lpr(1)
PAGER
The users prefered pager program
PATH
Path to search for programs
POSIXLY_CORRECT
Makes programs/libraries adhere to strict POSIX requirements.
PRINTER
Default printer
PROMPTCOMMAND
Command for the shell to run every time it displays a prompt, see bash(1)
PS1
Shell prompt, see bash(1)
PS2
Shell prompt, see bash(1)
PS3
Shell prompt, see bash(1)
PS4
Shell prompt, see bash(1)
PWD
Process Working Directory (set by some shells)
REPLYTO
Used by Mail(1), reply to address?
RESOLV_ADD_TRIM_DOMAINS
Adds to Trim Domains, see resolv(8)?
RESOLV_HOST_CONF
Filename of /etc/host.conf. see resolv(8)?, host.conf(5)
RESOLV_MULTI
Overrides "multi" keyword from /etc/host.conf, see resolv(8)?
RESOLV_OVERRIDE_TRIM_DOMAINS
Overrides Tirm Domains from /etc/host.conf, see resolv(8)?
RESOLV_REORDER
Overrides the "reorder" keyword from /etc/host.conf, see resolv(8)?
RESOLV_SERV_ORDER
Overrides "order" keyword in /etc/host.conf, see resolv+(8)
RESOLV_SPOOF_CHECK
Sets Spoof Checking see resolv(8)?
SHELL
The user's shell
SSH_ASKPASS
Used by ssh(1)
SSH_AUTH_SOCK
Used by ssh(1)
SSH_CLIENT
Used by ssh(1)
SSH_ORIGINAL_COMMAND
Used by ssh(1)
SSH_TTY
Used by ssh(1)
SYSTEM
Operating System, used by apropos(1)
TERMCAP
Terminal type capabilities file
TERM
The terminal type
TIMEFORMAT
format of the time(1) command, see time(1) and bash(1)
TMOUT
Timeout for shells to log you out, see bash(1)
TMPDIR
Directory for temp files, see tempnam(3), sort(1) etc
TZDIR
Set TimeZone information
TZ
Set TimeZone information, tzset(1)?
USER
The currently logged in user
VISUAL
The users prefered visual editor

1? No claims as to comprehensiveness or correctness made.