CodeMagic: bashtail - a script version of the Unix tail command

#!/bin/bash
#
# bashtail simulates a simple version of tail written in bash.
# A simple way to run the script is by invoking:
#
#     bashtail -n NUMBER_OF_LINES_TO_SHOW SOME_INPUT_FILE
#
# bashtail uses a circular buffer to retain the last n lines
# read from stdin or from a file. The buffer is written out in
# proper order when the input stream is finished.
#
# The script demonstrates the proper use of local namespaces
# and it modularizes the work of the program through
# functional decomposition. To allow a function to pass
# back a return value, a technique similar to globbing in
# Perl is used. The function wishing to set a return value
# evals a variable name, provided as a string by the caller,
# into the caller's symbol table. As a side effect, eval sets
# the value for the variable returned to the caller.
#
# bashtail includes a small self-test harness which can be
# run using the -t argument. The output of bashtail is
# compared to that of tail and a short report of the tests
# passed is produced.
#
# Copyright (c) 2008 Technetra Corp
#
# Permission is hereby granted, free of charge, to any person
# obtaining a copy of this software and associated documentation
# files (the "Software"), to deal in the Software without
# restriction, including without limitation the rights to use,
# copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the
# Software is furnished to do so, subject to the following
# conditions:
#
# The above copyright notice and this permission notice shall be
# included in all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
# OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
# HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
# WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
# OTHER DEALINGS IN THE SOFTWARE.
#
RB_ITER=
_read_lines() {
  local rbc=$1
  local N=$2
  local i=0
  local line_count=0
  local oldIFS="$IFS"; IFS=
  while read l; do
    line_count=$((line_count+1))
    i=$((i%N))
    ary[i++]="$l"
  done
  IFS="$oldIFS"
  RB_ITER=$i
  eval "$rbc=$line_count"
}
 
_print_buf() {
  local line_count=$1
  local N=$2
  local j=0
  # $RB_ITER points to start of ring buffer when line_count >= N
  # else to one past end of ring buffer when line_count < N
  if [[ line_count -lt N ]]; then
    RB_ITER=0
    N=$line_count
  fi
  while [[ j++ -lt N ]]; do
    echo "${ary[RB_ITER++%N]}"
  done
}
 
usage() {
  echo "SYNOPSIS:"
  echo -e "\t$0 [OPTION] [FILE]"
  echo "DESCRIPTION:"
  echo -e "\tOutput last part of file FILE. "
  echo -e "\tWhen no FILE is given or when FILE is -, read standard input"
  echo -en "\tExample: echo -e "
  echo "\"a\\nb\\nc\\n\\nd\"|$0 -n 3"
  echo "OPTIONS:"
  echo -e "\t-h\tThis help message"
  echo -e "\t-n N\tOutput last N lines"
  echo -e "\t-t\tTest against real tail using above example while"
  echo -e "\t\tvarying n from 6 down to 0"
  exit
}
 
test_script() {
  local passed=0
  local total=0
  local TAIL=/usr/bin/tail
  local result
  local target
  echo "testing tabs, spaces and newlines from standard input with simple echo..."
  for i in $(seq 6 -1 0); do
    total=$((total+1))
    result=$(echo -e " a\\n\\tb\\nc\\n\\nd"|$0 -n $i)
    target=$(echo -e " a\\n\\tb\\nc\\n\\nd"|$TAIL -n $i)
    if [[ "$result" == "$target" ]]; then
      passed=$((passed+1))
    else
      echo "failed simple echo test $i \"echo -e " a\\n\\tb\\nc\\n\\nd"|$0 -n $i\""
    fi
  done
  echo "testing file input..."
  for i in $(seq 10 -1 0); do
    total=$((total+1))
    result=$($0 -n $i /etc/passwd)
    target=$($TAIL -n $i /etc/passwd)
    if [[ "$result" == "$target" ]]; then
      passed=$((passed+1))
    else
      echo "failed file input test $i"
    fi
  done
  echo "testing standard input from file..."
  for i in $(seq 10 -1 0); do
    total=$((total+1))
    result=$(cat /etc/passwd|$0 -n $i )
    target=$(cat /etc/passwd|$TAIL -n $i )
    if [[ "$result" == "$target" ]]; then
      passed=$((passed+1))
    else
      echo "failed file input from stdin, test $i"
    fi
  done
  echo "passed $passed out of $total tests"
  exit
}
 
get_args() {
  local N_RET=$1; shift
  local FN_RET=$1; shift
  local arg=$1
  local N
  local FN
  case "$arg" in
    -h) usage;;
    -n) shift; N=$1; shift; FN=$1;;
    -t) test_script;;
  esac
  [[ -z "$N" ]] &amp;&amp; usage
  [[ $N -lt 1 ]] &amp;&amp; exit
  eval "$N_RET=$N; $FN_RET=$FN"
}
 
read_lines() {
  local result=$1
  local rb_count
  if [ "$FILE_NAME" -a "$FILE_NAME" != "-" ]; then
    _read_lines rb_count $HOW_MANY_LINES &lt; $FILE_NAME
  else
    _read_lines rb_count $HOW_MANY_LINES
  fi
  eval "$result=$rb_count"
}
 
print_buf() {
  local rb_count=$1
  _print_buf $rb_count $HOW_MANY_LINES
}
 
#
# MAIN
#
get_args HOW_MANY_LINES FILE_NAME $@
read_lines count
print_buf $count