3.7. Variables Revisited

environmental variables affecting bash script behavior

input field separator

This defaults to white space, but may be changed, for example, to parse a comma-separated data file.

home directory of the user (usually /home/username)

path to binaries (usually /usr/bin/, /usr/X11R6/bin/, /usr/local/bin, etc.)

Note that the "working directory", ./, is usually omitted from the $PATH as a security measure.

This is the main prompt, seen at the command line.

The secondary prompt, seen when additional input is expected. It is represented as an ">".

working directory (directory you are in at the time)

the default editor invoked by a script, usually vi or emacs.

If the TMOUT environmental variable is set to a non-zero value time, then the shell prompt will time out after time seconds. This will cause a logout.

Implementing timed input in a script is certainly possible, but hardly seems worth the effort. It requires setting up a timing loop to signal the script when it times out. Additionally, a signal handling routine is necessary to trap (see Example 3-82) the interrupt generated by the timing loop (whew!).

#!/bin/bash # TMOUT=3 useless in a script TIMELIMIT=3 # Three seconds in this instance, may be set to different value. PrintAnswer() { if [ $answer = TIMEOUT ] then echo $answer else # Don't want to mix up the two instances. echo "Your favorite veggie is $answer" kill $! # Kills no longer needed TimerOn function running in background. # $! is PID of last job running in background. fi } TimerOn() { sleep $TIMELIMIT && kill -s 14 $$ & # Waits XXX seconds, then sends sigalarm to script. } Int14Vector() { answer="TIMEOUT" PrintAnswer exit 14 } trap Int14Vector 14 # Timer interrupt - 14 - subverted for our purposes. echo "What is your favorite vegetable " TimerOn read answer PrintAnswer # Admittedly, this is a kludgy implementation of timed input, # but pretty much as good as can be done with Bash. # (Challenge to reader: come up with something better.) # If you need something a bit more elegant... # consider writing the application in C or C++, # using appropriate library functions, such as 'alarm' and 'setitimer'. exit 0

The number of seconds the script has been running.

#!/bin/bash ENDLESS_LOOP=1 echo echo "Hit Control-C to exit this script." echo while [ $ENDLESS_LOOP ] do if [ $SECONDS -eq 1 ] then units=second else units=seconds fi echo "This script has been running $SECONDS $units." sleep 1 done exit 0

The default value when a variable is not supplied to read. Also applicable to select menus, but only supplies the item number of the variable chosen, not the value of the variable itself.

#!/bin/bash echo echo -n "What is your favorite vegetable? " read echo "Your favorite vegetable is $REPLY." # REPLY holds the value of last "read" if and only if no variable supplied. echo echo -n "What is your favorite fruit? " read fruit echo "Your favorite fruit is $fruit." echo "but..." echo "Value of \$REPLY is still $REPLY." # $REPLY is still set to its previous value because # the variable $fruit absorbed the new "read" value. echo exit 0

the path to the bash binary itself, usually /bin/bash

an environmental variable pointing to a bash startup file to be read when a script is invoked

positional parameters (passed from command line to script, passed to a function, or set to a variable)

number of command line arguments or positional parameters

process id of script, often used in scripts to construct temp file names

exit status of command, function, or the script itself

All of the positional parameters

Same as $*, but each parameter is a quoted string, that is, the parameters are passed on intact, without interpretation or expansion

Flags passed to script

PID of last job run in background

Initializing or changing the value of a variable

the assignment operator (no space before & after)

Do not confuse this with = and -eq, which test, rather than assign!

#!/bin/bash

echo

# When is a variable "naked", i.e., lacking the '$' in front?

# Assignment
a=879
echo "The value of \"a\" is $a"

# Assignment using 'let'
let a=16+5
echo "The value of \"a\" is now $a"

echo

# In a 'for' loop (really, a type of disguised assignment)
echo -n "The values of \"a\" in the loop are "
for a in 7 8 9 11
do
  echo -n "$a "
done

echo
echo

# In a 'read' statement
echo -n "Enter \"a\" "
read a
echo "The value of \"a\" is now $a"

echo

exit 0

#!/bin/bash

a=23
# Simple case
echo $a
b=$a
echo $b

# Now, getting a little bit fancier...

a=`echo Hello!`
# Assigns result of 'echo' command to 'a'
echo $a

a=`ls -l`
# Assigns result of 'ls -l' command to 'a'
echo $a

exit 0

Variable assignment using the $() mechanism (a newer method than back quotes)

# From /etc/rc.d/rc.local R=$(cat /etc/redhat-release) arch=$(uname -m)

variables visible only within a code block or function (see Section 3.18)

variables that affect the behavior of the shell and user interface, such as the path and the prompt

If a script sets environmental variables, they need to be "exported", that is, reported to the environment local to the script. This is the function of the export command.

---

arguments passed to the script from the command line - $0, $1, $2, $3... ($0 is the name of the script itself, $1 is the first argument, etc.)

#!/bin/bash

echo

echo The name of this script is $0
# Adds ./ for current directory
echo The name of this script is `basename $0`
# Strip out path name info (see 'basename')

echo

if [ $1 ]
then
 echo "Parameter #1 is $1"
 # Need quotes to escape #
fi 

if [ $2 ]
then
 echo "Parameter #2 is $2"
fi 

if [ $3 ]
then
 echo "Parameter #3 is $3"
fi 

echo

exit 0

Some scripts can perform different operations, depending on which name they are invoked with. For this to work, the script needs to check $0, the name it was invoked by. There also have to be symbolic links present to all the alternate names of the same script.

variable1x=$1x
# This will prevent an error, even if positional parameter is absent.

# The extra character can be stripped off later, if desired, like so.
variable1=${variable1x/x/}
# This uses one of the parameter substitution templates previously discussed.
# Leaving out the replacement pattern results in a deletion.

---

#!/bin/bash

# Does a 'whois domain-name' lookup
# on any of 3 alternate servers:
# ripe.net, cw.net, radb.net

# Place this script, named 'wh' in /usr/local/bin

# Requires symbolic links:
# ln -s /usr/local/bin/wh /usr/local/bin/wh-ripe
# ln -s /usr/local/bin/wh /usr/local/bin/wh-cw
# ln -s /usr/local/bin/wh /usr/local/bin/wh-radb


if [ -z $1 ]
then
  echo "Usage: `basename $0` [domain-name]"
  exit 1
fi

case `basename $0` in
# Checks script name and calls proper server
    "wh"     ) whois [email protected];;
    "wh-ripe") whois [email protected];;
    "wh-radb") whois [email protected];;
    "wh-cw"  ) whois [email protected];;
    *        ) echo "Usage: `basename $0` [domain-name]";;
esac    

exit 0

---

The shift command reassigns the positional parameters, in effect shifting them to the left one notch.

$1 <--- $2, $2 <--- $3, $3 <--- $4, etc.

The old $1 disappears, but $0 does not change. If you use a large number of positional parameters to a script, shift lets you access those past 10.

#!/bin/bash

# Name this script something like shift000,
# and invoke it with some parameters, for example
# ./shift000 a b c def 23 skidoo

# Demo of using 'shift'
# to step through all the positional parameters.

until [ -z "$1" ]
do
  echo -n "$1 "
  shift
done

echo
# Extra line feed.

exit 0