9.1. Internal Variables

    the top value in the directory stack (affected by pushd and popd)

    This builtin variable corresponds to the dirs command, however dirs shows

    the entire contents of the directory stack.

    "effective" user ID number

    Identification number of whatever identity the current user has assumed,

    perhaps by means of su.

    Caution The $EUID is not necessarily the same as the $UID.

    A list of filename patterns to be excluded from matching in globbing.

     bash$ echo $IFS | cat -vte

     $

     bash$ bash -c 'set w x y z; IFS=":-;"; echo "$*"'

     w:x:y:z

IFS=" "

-------

the arg is: [a]

the arg is: [b]

the arg is: [c]

IFS=:

-----

the arg is: []

the arg is: [a]

the arg is: []

the arg is: [b]

the arg is: [c]

the arg is: []

the arg is: []

[mdx@localhost abs-exercises]$

    This variable is the line number of the shell script in which this

variable

    appears. It has significance only within the script in which it appears,

    and is chiefly useful for debugging purposes.

       1 # *** BEGIN DEBUG BLOCK ***

       2 last_cmd_arg=$_  # Save it.

       3

       4 echo "At line number $LINENO, variable \"v1\" = $v1"

       5 echo "Last command argument processed = $last_cmd_arg"

       6 # *** END DEBUG BLOCK ***

     bash$ ls -al | bogus_command

     bash: bogus_command: command not found

     bash$ echo $PIPESTATUS

     141

     bash$ ls -al | bogus_command

     bash: bogus_command: command not found

     bash$ echo $?

     127

...

          bash$ echo ${PIPESTATUS[@]}

    ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

    Example 9-7. Inconsistent $* and $@ behavior

LOG=$0.log

COMMAND1="sleep 100"

echo "Logging PIDs background commands for script: $0" >> "$LOG"

# So they can be monitored, and killed as necessary.

echo >> "$LOG"

# Logging commands.

echo -n "PID of \"$COMMAND1\":  " >> "$LOG"

${COMMAND1} &

echo $! >> "$LOG"

# PID of "sleep 100":  1506

# Thank you, Jacques Lederer, for suggesting this.

#下面这句不大懂：eval, kill -9

#possibly_hanging_job & { sleep ${TIMEOUT}; eval 'kill -9 $!' &> /dev/null; }      

# Forces completion of an ill-behaved program.

# Useful, for example, in init scripts.

# Thank you, Sylvain Fourmanoit, for this creative use of the "!" varia