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" ]

# 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 is a simplified variant of the Example 19.13, “Prepending a line to a file” script given earlier. And, of course, sed can also do 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, “mailformat: Formatting an e-mail message” and Example 15.20, “Using export to pass a variable to an embedded awk script”.

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



# 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  

  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.

#!/bin/bash
# progress-bar.sh

# Author: Dotan Barak (very minor revisions by ABS Guide author).
# Used in ABS Guide with permission (thanks!).


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.
=================================

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.

Using the $? exit status variable, a script may test if a parameter contains only digits, so it can be 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 (usually with echo) the “return value,” and assign this to a variable. This is actually a variant of command substitution.

#!/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,
}                               #+ 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.

#!/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

retval=`sum_and_product $first $second`      # Assigns output of function.
sum=`echo "$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

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.

Next in our bag of tricks are techniques for passing an array to a function, then “returning” an array back to the main body of the script.

Passing an array involves loading the space-separated elements of the array into a variable with command substitution. Getting an array back as the “return value” from a function uses the previously mentioned strategem of echoing the array in the function, then invoking command substitution and the ( ... ) operator to assign it to an array.

#!/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[@]}`
# **********************************
#  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, “Game of Life”.

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, “A C-style for loop” and Example 11.18, “C-style syntax in a while loop”.

Setting the path and umask at the beginning of a script makes it more portable -- more likely to run on 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 filter, but with a different set of arguments and/or options. Especially suitable for this are tr and grep.

# 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 ' '`

#!/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, “On-line connect status”, Example 16.25, “Generating “Crypto-Quote” Puzzles”, and Example A.9, “Soundex conversion”.

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, “Commenting out a block of code”.

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

Redirecting 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 ...
read inner_variable <$TMPFILE    # Read back shell variable.
rm -f "$TMPFILE"                 # Get rid of temp file.
echo "$inner_variable"           # It's an ugly kludge, but it works.

The run-parts command is handy for running a set of command scripts in a particular sequence, especially in combination with cron or at.

For doing multiple revisions on a complex script, use the rcs Revision Control System package.

Among other benefits of this is automatically updated ID header tags. The co command in rcs does a parameter replacement of certain reserved key words, for example, replacing # $Id$ in a script with something like:

# $Id$