36.7. Assorted Tips. Comtrol DeviceMaster Software Development Kit
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code sections in C, or even in assembler.
Try to minimize file I/O. Bash is not particularly efficient at handling files, so consider using more
appropriate tools for this within the script, such as awk or Perl.
Write your scripts in a modular and coherent form, [127] so they can be reorganized and tightened up as necessary. Some of the optimization techniques applicable to high-level languages may work for scripts, but others, such as loop unrolling, are mostly irrelevant. Above all, use common sense.
For an excellent demonstration of how optimization can dramatically reduce the execution time of a script, see
36.7. Assorted Tips
36.7.1. Ideas for more powerful scripts
•
You have a problem that you want to solve by writing a Bash script. Unfortunately, you don't know quite where to start. One method is to plunge right in and code those parts of the script that come easily, and write the hard parts as pseudo-code.
#!/bin/bash
ARGCOUNT=1 # Need name as argument.
E_WRONGARGS=65 if [ number-of-arguments is-not-equal-to "$ARGCOUNT" ]
# ^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^
# Can't figure out how to code this . . .
#+ . . . so write it in pseudo-code.
then
echo "Usage: name-of-script name"
# ^^^^^^^^^^^^^^ More pseudo-code.
exit $E_WRONGARGS fi
. . .
exit 0
# Later on, substitute working code for the pseudo-code.
# Line 6 becomes: if [ $# -ne "$ARGCOUNT" ]
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# Line 12 becomes:
echo "Usage: `basename $0` name"
•
For an example of using pseudo-code, see the Square Root exercise.
To keep a record of which user scripts have run during a particular session or over a number of sessions, add the following lines to each script you want to keep track of. This will keep a continuing file record of the script names and invocation times.
# Append (>>) following to end of each script tracked.
whoami>> $SAVE_FILE # User invoking the script.
echo $0>> $SAVE_FILE # Script name.
date>> $SAVE_FILE # Date and time.
echo>> $SAVE_FILE # Blank line as separator.
# Of course, SAVE_FILE defined and exported as environmental variable in ~/.bashrc
#+ (something like ~/.scripts-run)
•
The >> operator appends lines to a file. What if you wish to prepend a line to an existing file, that is, to paste it in at the beginning?
file=data.txt
title="***This is the title line of data text file***" echo $title | cat - $file >$file.new
# "cat -" concatenates stdout to $file.
# End result is
#+ to write a new file with $title appended at *beginning*.
• this.
A shell script may act as an embedded command inside another shell script, a Tcl or wish script, or
even a Makefile. It can be invoked as an external shell command in a C program using the
system()
call, i.e.,
system("script_name");
.
•
Setting a variable to the contents of an embedded sed or awk script increases the readability of the
•
surrounding shell wrapper. See Example A-1 and Example 15-20.
Put together files containing your favorite and most useful definitions and functions. As necessary,
"include" one or more of these "library files" in scripts with either the dot (.) or source command.
# SCRIPT LIBRARY
# ------ -------
# Note:
# No "#!" here.
# No "live code" either.
# Useful variable definitions
ROOT_UID=0 # Root has $UID 0.
E_NOTROOT=101 # Not root user error.
MAXRETVAL=255 # Maximum (positive) return value of a function.
SUCCESS=0
FAILURE=-1
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# Functions
Usage () # "Usage:" message.
{
if [ -z "$1" ] # No arg passed.
then
msg=filename
else
msg=$@
fi
echo "Usage: `basename $0` "$msg""
}
Check_if_root () # Check if root running script.
{ # From "ex39.sh" example.
if [ "$UID" -ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
exit $E_NOTROOT
fi
}
CreateTempfileName () # Creates a "unique" temp filename.
{ # From "ex51.sh" example.
prefix=temp
suffix=`eval date +%s`
Tempfilename=$prefix.$suffix
} isalpha2 () # Tests whether *entire string* is alphabetic.
{ # From "isalpha.sh" example.
[ $# -eq 1 ] || return $FAILURE
case $1 in
*[!a-zA-Z]*|"") return $FAILURE;;
*) return $SUCCESS;;
esac # Thanks, S.C.
} abs () # Absolute value.
{ # Caution: Max return value = 255.
E_ARGERR=-999999
if [ -z "$1" ] # Need arg passed.
then
return $E_ARGERR # Obvious error value returned.
fi
if [ "$1" -ge 0 ] # If non-negative,
then #
absval=$1 # stays as-is.
else # Otherwise,
let "absval = (( 0 - $1 ))" # change sign.
fi
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return $absval
} tolower () # Converts string(s) passed as argument(s)
{ #+ to lowercase.
if [ -z "$1" ] # If no argument(s) passed,
then #+ send error message
echo "(null)" #+ (C-style void-pointer error message)
return #+ and return from function.
fi
echo "$@" | tr A-Z a-z
# Translate all passed arguments ($@).
return
# Use command substitution to set a variable to function output.
# For example:
# oldvar="A seT of miXed-caSe LEtTerS"
# newvar=`tolower "$oldvar"`
# echo "$newvar" # a set of mixed-case letters
#
# Exercise: Rewrite this function to change lowercase passed argument(s)
# to uppercase ... toupper() [easy].
}
•
Use special-purpose comment headers to increase clarity and legibility in scripts.
## Caution.
rm -rf *.zzy ## The "-rf" options to "rm" are very dangerous,
##+ especially with wild cards.
#+ Line continuation.
# This is line 1
#+ of a multi-line comment,
#+ and this is the final line.
#* Note.
#o List item.
#> Another point of view.
while [ "$var1" != "end" ] #> while test "$var1" != "end"
•
Dotan Barak contributes template code for a progress bar in a script.
Example 36-17. A Progress Bar
#!/bin/bash
# progress-bar.sh
# Author: Dotan Barak (very minor revisions by ABS Guide author).
# Used in ABS Guide with permission (thanks!).
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BAR_WIDTH=50
BAR_CHAR_START="["
BAR_CHAR_END="]"
BAR_CHAR_EMPTY="."
BAR_CHAR_FULL="="
BRACKET_CHARS=2
LIMIT=100 print_progress_bar()
{
# Calculate how many characters will be full.
let "full_limit = ((($1 - $BRACKET_CHARS) * $2) / $LIMIT)"
# Calculate how many characters will be empty.
let "empty_limit = ($1 - $BRACKET_CHARS) - ${full_limit}"
# Prepare the bar.
bar_line="${BAR_CHAR_START}"
for ((j=0; j<full_limit; j++)); do
bar_line="${bar_line}${BAR_CHAR_FULL}"
done
for ((j=0; j<empty_limit; j++)); do
bar_line="${bar_line}${BAR_CHAR_EMPTY}"
done
bar_line="${bar_line}${BAR_CHAR_END}"
printf "%3d%% %s" $2 ${bar_line}
}
# Here is a sample of code that uses it.
MAX_PERCENT=100 for ((i=0; i<=MAX_PERCENT; i++)); do
#
usleep 10000
# ... Or run some other commands ...
#
print_progress_bar ${BAR_WIDTH} ${i}
echo -en "\r" done echo "" exit
•
A particularly clever use of if-test constructs is for comment blocks.
#!/bin/bash
COMMENT_BLOCK=
# Try setting the above variable to some value
#+ for an unpleasant surprise.
if [ $COMMENT_BLOCK ]; then
Comment block --
=================================
This is a comment line.
This is another comment line.
This is yet another comment line.
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================================= echo "This will not echo."
Comment blocks are error-free! Whee!
fi echo "No more comments, please." exit 0
Compare this with using here documents to comment out code blocks.
•
treated as an integer.
#!/bin/bash
SUCCESS=0
E_BADINPUT=85 test "$1" -ne 0 -o "$1" -eq 0 2>/dev/null
# An integer is either equal to 0 or not equal to 0.
# 2>/dev/null suppresses error message.
if [ $? -ne "$SUCCESS" ] then
echo "Usage: `basename $0` integer-input"
exit $E_BADINPUT fi let "sum = $1 + 25" # Would give error if $1 not integer.
echo "Sum = $sum"
# Any variable, not just a command-line parameter, can be tested this way.
exit 0
• The 0 - 255 range for function return values is a severe limitation. Global variables and other workarounds are often problematic. An alternative method for a function to communicate a value back to the main body of the script is to have the function write to
stdout
"return value," and assign this to a variable. This is actually a variant of command substitution.
Example 36-18. Return value trickery
#!/bin/bash
# multiplication.sh
multiply () # Multiplies params passed.
{ # Will accept a variable number of args.
local product=1
until [ -z "$1" ] # Until uses up arguments passed...
do
let "product *= $1"
shift
done
echo $product # Will not echo to stdout,
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} #+ since this will be assigned to a variable.
mult1=15383; mult2=25211 val1=`multiply $mult1 $mult2`
# Assigns stdout (echo) of function to the variable val1.
echo "$mult1 X $mult2 = $val1" # 387820813 mult1=25; mult2=5; mult3=20 val2=`multiply $mult1 $mult2 $mult3` echo "$mult1 X $mult2 X $mult3 = $val2" # 2500 mult1=188; mult2=37; mult3=25; mult4=47 val3=`multiply $mult1 $mult2 $mult3 $mult4` echo "$mult1 X $mult2 X $mult3 X $mult4 = $val3" # 8173300 exit 0
The same technique also works for alphanumeric strings. This means that a function can "return" a non-numeric value.
capitalize_ichar () # Capitalizes initial character
{ #+ of argument string(s) passed.
string0="$@" # Accepts multiple arguments.
firstchar=${string0:0:1} # First character.
string1=${string0:1} # Rest of string(s).
FirstChar=`echo "$firstchar" | tr a-z A-Z`
# Capitalize first character.
echo "$FirstChar$string1" # Output to stdout.
} newstring=`capitalize_ichar "every sentence should start with a capital letter."` echo "$newstring" # Every sentence should start with a capital letter.
It is even possible for a function to "return" multiple values with this method.
Example 36-19. Even more return value trickery
#!/bin/bash
# sum-product.sh
# A function may "return" more than one value.
sum_and_product () # Calculates both sum and product of passed args.
{
echo $(( $1 + $2 )) $(( $1 * $2 ))
# Echoes to stdout each calculated value, separated by space.
} echo echo "Enter first number " read first echo echo "Enter second number " read second echo
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sum=`echo "$retval" | awk '{print $1}'` # Assigns first field.
product=`echo "$retval" | awk '{print $2}'` # Assigns second field.
echo "$first + $second = $sum" echo "$first * $second = $product" echo exit 0
There can be only one echo statement in the function for this to work. If you alter the previous example: sum_and_product ()
{
echo "This is the sum_and_product function." # This messes things up!
echo $(( $1 + $2 )) $(( $1 * $2 ))
}
...
retval=`sum_and_product $first $second` # Assigns output of function.
# Now, this will not work correctly.
•
back to the main body of the script.
Passing an array involves loading the space-separated elements of the array into a variable with
mentioned strategem of echoing the array in the function, then invoking command substitution and
the ( ... ) operator to assign it to an array.
Example 36-20. Passing and returning arrays
#!/bin/bash
# array-function.sh: Passing an array to a function and ...
# "returning" an array from a function
Pass_Array ()
{
local passed_array # Local variable!
passed_array=( `echo "$1"` )
echo "${passed_array[@]}"
# List all the elements of the new array
#+ declared and set within the function.
} original_array=( element1 element2 element3 element4 element5 ) echo echo "original_array = ${original_array[@]}"
# List all elements of original array.
# This is the trick that permits passing an array to a function.
# ********************************** argument=`echo ${original_array[@]}`
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# **********************************
# Pack a variable
#+ with all the space-separated elements of the original array.
#
# Attempting to just pass the array itself will not work.
# This is the trick that allows grabbing an array as a "return value".
# ***************************************** returned_array=( `Pass_Array "$argument"` )
# *****************************************
# Assign 'echoed' output of function to array variable.
echo "returned_array = ${returned_array[@]}" echo "============================================================="
# Now, try it again,
#+ attempting to access (list) the array from outside the function.
Pass_Array "$argument"
# The function itself lists the array, but ...
#+ accessing the array from outside the function is forbidden.
echo "Passed array (within function) = ${passed_array[@]}"
# NULL VALUE since the array is a variable local to the function.
echo
############################################
# And here is an even more explicit example: ret_array ()
{
for element in {11..20}
do
echo "$element " # Echo individual elements
done #+ of what will be assembled into an array.
} arr=( $(ret_array) ) # Assemble into array.
echo "Capturing array \"arr\" from function ret_array () ..." echo "Third element of array \"arr\" is ${arr[2]}." # 13 (zero-indexed) echo -n "Entire array is: " echo ${arr[@]} # 11 12 13 14 15 16 17 18 19 20 echo exit 0
# Nathan Coulter points out that passing arrays with elements containing
#+ whitespace breaks this example.
•
For a more elaborate example of passing arrays to functions, see Example A-10.
Using the double-parentheses construct, it is possible to use C-style syntax for setting and
incrementing/decrementing variables and in for and while loops. See Example 11-13 and Example
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•
a "foreign" machine whose user may have bollixed up the
$PATH
and umask.
#!/bin/bash
PATH=/bin:/usr/bin:/usr/local/bin ; export PATH umask 022 # Files that the script creates will have 755 permission.
# Thanks to Ian D. Allen, for this tip.
•
A useful scripting technique is to repeatedly feed the output of a filter (by piping) back to the same
# From "wstrings.sh" example.
wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \ tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '`
Example 36-21. Fun with anagrams
#!/bin/bash
# agram.sh: Playing games with anagrams.
# Find anagrams of...
LETTERSET=etaoinshrdlu
FILTER='.......' # How many letters minimum?
# 1234567 anagram "$LETTERSET" | # Find all anagrams of the letterset...
grep "$FILTER" | # With at least 7 letters, grep '^is' | # starting with 'is' grep -v 's$' | # no plurals grep -v 'ed$' # no past tense verbs
# Possible to add many combinations of conditions and filters.
# Uses "anagram" utility
#+ that is part of the author's "yawl" word list package.
# http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
# http://bash.deta.in/yawl-0.3.2.tar.gz
exit 0 # End of code.
bash$ sh agram.sh
islander isolate isolead isotheral
# Exercises:
# ---------
# Modify this script to take the LETTERSET as a command-line parameter.
# Parameterize the filters in lines 11 - 13 (as with $FILTER),
#+ so that they can be specified by passing arguments to a function.
# For a slightly different approach to anagramming,
#+ see the agram2.sh script.
See also Example 29-4, Example 16-25, and Example A-9.
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•
Use "anonymous here documents" to comment out blocks of code, to save having to individually
•
comment out each line with a #. See Example 19-11.
Running a script on a machine that relies on a command that might not be installed is dangerous. Use
whatis to avoid potential problems with this.
CMD=command1 # First choice.
PlanB=command2 # Fallback option.
command_test=$(whatis "$CMD" | grep 'nothing appropriate')
# If 'command1' not found on system , 'whatis' will return
#+ "command1: nothing appropriate."
#
# A safer alternative is:
# command_test=$(whereis "$CMD" | grep \/)
# But then the sense of the following test would have to be reversed,
#+ since the $command_test variable holds content only if
#+ the $CMD exists on the system.
# (Thanks, bojster.) if [[ -z "$command_test" ]] # Check whether command present.
then
$CMD option1 option2 # Run command1 with options.
else # Otherwise,
$PlanB #+ run command2. fi
•
An if-grep test may not return expected results in an error case, when text is output to
stderr
, rather that
stdout
.
if ls -l nonexistent_filename | grep -q 'No such file or directory'
then echo "File \"nonexistent_filename\" does not exist." fi
stderr
to
stdout
fixes this.
if ls -l nonexistent_filename 2>&1 | grep -q 'No such file or directory'
# ^^^^
then echo "File \"nonexistent_filename\" does not exist." fi
# Thanks, Chris Martin, for pointing this out.
• If you absolutely must access a subshell variable outside the subshell, here's a way to do it.
TMPFILE=tmpfile # Create a temp file to store the variable.
( # Inside the subshell ...
inner_variable=Inner echo $inner_variable echo $inner_variable >>$TMPFILE # Append to temp file.
)
# Outside the subshell ...
echo; echo "-----"; echo echo $inner_variable # Null, as expected.
echo "-----"; echo
# Now ...
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