Advanced Bash-Scripting Guide（三）
Advanced Bash-Scripting Guide（三）
Chapter 11. Loops and Branches
• for loops
for arg in [list]
This is the basic looping construct. It differs significantly from its C counterpart.
for arg in [list]
do
command(s)...
done
During each pass through the loop, arg takes on the value of each successive variable in the list.
# The argument list may contain wild cards.
. Arguments in [list] quoted to prevent possible word splitting.
for planet in Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto
do
echo $planet # Each planet on a separate line.
done
for planet in "Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto"
# All planets on same line.
# Entire 'list' enclosed in quotes creates a single variable.
# Why? Whitespace incorporated into the variable.
do
echo $planet
done
. Each [list] element may contain multiple parameters. This is useful when processing parameters
in groups. In such cases, use the set command (see Example 15-16) to force parsing of each [list]
element and assignment of each component to the positional parameters.
# Associate the name of each planet with its distance from the sun.
for planet in "Mercury 36" "Venus 67" "Earth 93" "Mars 142" "Jupiter 483"
do
set -- $planet # Parses variable "planet"
#+ and sets positional parameters.
# The "--" prevents nasty surprises if $planet is null or
#+ begins with a dash.
# May need to save original positional parameters,
#+ since they get overwritten.
# One way of doing this is to use an array,
# original_params=("$@")
echo "$1 $2,000,000 miles from the sun"
#-------two tabs---concatenate zeroes onto parameter $2
done
. If the [list] in a for loop contains wild cards (* and ?) used in filename expansion, then globbing
takes place.
for file in *
# ^ Bash performs filename expansion
#+ on expressions that globbing recognizes.
do
ls -l "$file"| awk '{ print $8 " file size: " $5 }' # Lists all files in $PWD (current directory) and Print 2 fields..
# Recall that the wild card character "*" matches every filename,
#+ however, in "globbing," it doesn't match dot-files.
# If the pattern matches no file, it is expanded to itself.
# To prevent this, set the nullglob option
#+ (shopt -s nullglob).
done
echo; echo
for file in [jx]*
do
rm -f $file # Removes only files beginning with "j" or "x" in $PWD.
echo "Removed file \"$file\"".
done
echo
Advanced Bash-Scripting Guide
Chapter
. Omitting the in [list] part of a for loop causes the loop to operate on $@ -- the positional
parameters.
#!/bin/bash
# Invoke this script both with and without arguments,
#+ and see what happens.
for a
do
echo -n "$a "
done
# The 'in list' missing, therefore the loop operates on '$@'
#+ (command-line argument list, including whitespace).
echo
exit 0
. It is possible to use command substitution to generate the [list] in a for loop.
NUMBERS="9 7 3 8 37.53"
for number in `echo $NUMBERS` # for number in 9 7 3 8 37.53
do
echo -n "$number "
done
echo
Example 11-8. Listing all users on the system
#!/bin/bash
# userlist.sh
PASSWORD_FILE=/etc/passwd
n=1 # User number
for name in $(awk 'BEGIN{FS=":"}{print $1}' < "$PASSWORD_FILE" )
# Field separator = : ^^^^^^
# Print first field ^^^^^^^^
# Get input from password file ^^^^^^^^^^^^^^^^^
do
echo "USER #$n = $name"
let "n += 1"
done
# USER #1 = root
# USER #2 = bin
# USER #3 = daemon
# ...
# USER #30 = bozo
. A final example of [list] / command substitution, but this time the "command" is a function.
generate_list ()
{
echo "one two three"
}
for word in $(generate_list) # Let "word" grab output of function.
do
echo "$word"
done
# one
# two
# three
. Example 11-12. A C-style for loop
# +==========================================+
# Now, let's do the same, using C-like syntax.
LIMIT=10
for ((a=1; a <= LIMIT ; a++)) # Double parentheses, and "LIMIT" with no "$".
do
echo -n "$a "
done # A construct borrowed from 'ksh93'.
echo; echo
# +=========================================================================+
# Let's use the C "comma operator" to increment two variables simultaneously.
for ((a=1, b=1; a <= LIMIT ; a++, b++))
do # The comma chains together operations.
echo -n "$a-$b "
done
echo; echo
• while
This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is true (returns a 0 exit status).
while [ condition ]
do
command(s)...
done
The bracket construct in a while loop is nothing more than our old friend, the test brackets used in an
if/then test. In fact, a while loop can legally use the more versatile double-brackets construct (while [[ condition ]]).
A while loop may have multiple conditions. Only the final condition determines when the loop
terminates. This necessitates a slightly different loop syntax, however.
#!/bin/bash
var1=unset
previous=$var1
while echo "previous-variable = $previous"
echo
previous=$var1
[ "$var1" != end ] # Keeps track of what $var1 was previously.
# Four conditions on "while", but only last one controls loop.
# The *last* exit status is the one that counts.
do
echo "Input variable #1 (end to exit) "
read var1
echo "variable #1 = $var1"
done
. As with a for loop, a while loop may employ C-style syntax by using the double-parentheses construct
#!/bin/bash
# wh-loopc.sh: Count to 10 in a "while" loop.
LIMIT=10 # 10 iterations.
a=1
while [ "$a" -le $LIMIT ]
do
echo -n "$a "
let "a+=1"
done # No surprises, so far.
echo; echo
# +=================================================================+
# Now, we'll repeat with C-like syntax.
((a = 1)) # a=1
# Double parentheses permit space when setting a variable, as in C.
while (( a <= LIMIT )) # Double parentheses,
do #+ and no "$" preceding variables.
echo -n "$a "
((a += 1)) # let "a+=1"
# Yes, indeed.
# Double parentheses permit incrementing a variable with C-like syntax.
done
. Inside its test brackets, a while loop can call a function.
t=0
condition ()
{
((t++))
if [ $t -lt 5 ]
then
return 0 # true
else
return 1 # false
fi
}
while condition
# ^^^^^^^^^
# Function call -- four loop iterations.
do
echo "Still going: t = $t"
done
. By coupling the power of the read command with a while loop, we get the handy while read construct,
useful for reading and parsing files.
cat $filename | # Supply input from a file.
while read line # As long as there is another line to read ...
do
...
done
# =========== Snippet from "sd.sh" example script ========== #
while read value # Read one data point at a time.
do
rt=$(echo "scale=$SC; $rt + $value" | bc)
(( ct++ ))
done
am=$(echo "scale=$SC; $rt / $ct" | bc)
echo $am; return $ct # This function "returns" TWO values!
# Caution: This little trick will not work if $ct > 255!
# To handle a larger number of data points,
#+ simply comment out the "return $ct" above.
} <"$datafile" # Feed in data file.
. A while loop may have its stdin redirected to a file by a < at its end.
. A while loop may have its stdin supplied by a pipe.
• until
This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is
false (opposite of while loop).
until [ condition-is-true ]
do
command(s)...
done
• break, continue
The break and continue loop control commands [52] correspond exactly to their counterparts in other
programming languages. The break command terminates the loop (breaks out of it), while continue
causes a jump to the next iteration of the loop, skipping all the remaining commands in that particular
loop cycle.
. The break command may optionally take a parameter. A plain break terminates only the innermost
loop in which it is embedded, but a break N breaks out of N levels of loop.
#!/bin/bash
# break-levels.sh: Breaking out of loops.
# "break N" breaks out of N level loops.
for outerloop in 1 2 3 4 5
do
echo -n "Group $outerloop: "
# --------------------------------------------------------
for innerloop in 1 2 3 4 5
do
echo -n "$innerloop "
if [ "$innerloop" -eq 3 ]
then
break # Try break 2 to see what happens.
# ("Breaks" out of both inner and outer loops.)
fi
done
# --------------------------------------------------------
echo
done
. The continue command, similar to break, optionally takes a parameter. A plain continue cuts short
the current iteration within its loop and begins the next. A continue N terminates all remaining
iterations at its loop level and continues with the next iteration at the loop, N levels above.
#!/bin/bash
# The "continue N" command, continuing at the Nth level loop.
for outer in I II III IV V # outer loop
do
echo; echo -n "Group $outer: "
# --------------------------------------------------------------------
for inner in 1 2 3 4 5 6 7 8 9 10 # inner loop
do
if [[ "$inner" -eq 7 && "$outer" = "III" ]]
then
continue 2 # Continue at loop on 2nd level, that is "outer loop".
# Replace above line with a simple "continue"
# to see normal loop behavior.
fi
echo -n "$inner " # 7 8 9 10 will not echo on "Group III."
done
# --------------------------------------------------------------------
done
• Testing and Branching
. case (in) / esac
The case construct is the shell scripting analog to switch in C/C++. It permits branching to one of a
number of code blocks, depending on condition tests. It serves as a kind of shorthand for multiple
if/then/else statements and is an appropriate tool for creating menus.
case "$variable" in
"$condition1" )
command...
;;
"$condition2" )
command...
;;
esac
# Quoting the variables is not mandatory, since word splitting does not take place.
# Each test line ends with a right paren ). [53]
# Each condition block ends with a double semicolon ;;.
# If a condition tests true, then the associated commands execute and the case block terminates.
# The entire case block ends with an esac (case spelled backwards).
#!/bin/bash
# Testing ranges of characters.
echo; echo "Hit a key, then hit return."
read Keypress
case "$Keypress" in
[[:lower:]] ) echo "Lowercase letter";;
[[:upper:]] ) echo "Uppercase letter";;
[0-9] ) echo "Digit";;
* ) echo "Punctuation, whitespace, or other";;
esac # Allows ranges of characters in [square brackets],
#+ or POSIX ranges in [[double square brackets.
. An exceptionally clever use of case involves testing for command-line parameters.
#! /bin/bash
while [ $# -gt 0 ]; do # Until you run out of parameters . . .
case "$1" in
-d|--debug)
# "-d" or "--debug" parameter?
DEBUG=1
;;
-c|--conf)
CONFFILE="$2"
shift
if [ ! -f $CONFFILE ]; then
echo "Error: Supplied file doesn't exist!"
exit $E_CONFFILE # File not found error.
fi
;;
esac
shift # Check next set of parameters.
done
. A case construct can filter strings for globbing patterns.
#!/bin/bash
# match-string.sh: Simple string matching.
match_string ()
{ # Exact string match.
MATCH=0
E_NOMATCH=90
PARAMS=2 # Function requires 2 arguments.
E_BAD_PARAMS=91
[ $# -eq $PARAMS ] || return $E_BAD_PARAMS
case "$1" in
"$2") return $MATCH;;
* ) return $E_NOMATCH;;
esac
}
a=one
b=two
c=three
d=two
match_string $a # wrong number of parameters
echo $? # 91
match_string $a $b # no match
echo $? # 90
match_string $b $d # match
echo $? # 0
• select
The select construct, adopted from the Korn Shell, is yet another tool for building menus.
select variable [in list]
do
command...
break
done
# This prompts the user to enter one of the choices presented in the variable list. Note that select uses
the $PS3 prompt (#? ) by default, but this may be changed.
. If in list is omitted, then select uses the list of command line arguments ($@) passed to the script
or the function containing the select construct.
#!/bin/bash
PS3='Choose your favorite vegetable: '
echo
choice_of()
{
select vegetable
# [in list] omitted, so 'select' uses arguments passed to function.
do
echo
echo "Your favorite veggie is $vegetable."
echo "Yuck!"
echo
break
done
}
choice_of beans rice carrots radishes tomatoes spinach
# $1 $2 $3 $4 $5 $6
# passed to choice_of() function
Chapter 12.Command Substitution
Command substitution reassigns the output of a command [54] or even multiple commands; it literally plugs
the command output into another context.
# Command substitution invokes a subshell.
. Command substitution may result in word splitting.
COMMAND `echo a b` # 2 args: a and b
COMMAND "`echo a b`" # 1 arg: "a b"
COMMAND `echo` # no arg
COMMAND "`echo`" # one empty arg
. Command substitution permits setting a variable to the output of a loop. The key to this is grabbing the output
of an echo command within the loop.
variable1=`for i in 1 2 3 4 5
do
echo -n "$i" # The 'echo' command is critical
done` #+ to command substitution here.
echo "variable1 = $variable1" # variable1 = 12345
i=0
variable2=`while [ "$i" -lt 10 ]
do
echo -n "$i" # Again, the necessary 'echo'.
let "i += 1" # Increment.
done`
echo "variable2 = $variable2" # variable2 = 0123456789
. Command substitution makes it possible to extend the toolset available to Bash. It is simply a matter of
writing a program or script that outputs to stdout (like a well-behaved UNIX tool should) and assigning
that output to a variable.
#include <stdio.h>
/* "Hello, world." C program */
int main()
{
printf( "Hello, world.\n" );
return (0);
}
bash$ gcc -o hello hello.c
#!/bin/bash
# hello.sh
greeting=`./hello`
echo $greeting
bash$ sh hello.sh
Hello, world.
Chapter 13. Arithmetic Expansion
Arithmetic expansion provides a powerful tool for performing (integer) arithmetic operations in scripts.
Translating a string into a numerical expression is relatively straightforward using backticks, double
parentheses, or let.
. Arithmetic expansion with backticks (often used in conjunction with expr)
z=`expr $z + 3` # The 'expr' command performs the expansion.
. Arithmetic expansion with double parentheses, and using let
The use of backticks (backquotes) in arithmetic expansion has been superseded by double parentheses
-- ((...)) and $((...)) -- and also by the very convenient let construction.
z=$(($z+3))
z=$((z+3)) # Also correct.
# Within double parentheses,
#+ parameter dereferencing
#+ is optional.
# $((EXPRESSION)) is arithmetic expansion. # Not to be confused with
#+ command substitution.
# You may also use operations within double parentheses without assignment.
n=0
echo "n = $n" # n = 0
(( n += 1 )) # Increment.
# (( $n += 1 )) is incorrect!
echo "n = $n" # n = 1
let z=z+3
let "z += 3" # Quotes permit the use of spaces in variable assignment.
# The 'let' operator actually performs arithmetic evaluation,
#+ rather than expansion.