#!/bin/bash#  Tip:#  If you're unsure how a certain condition might evaluate,#+ test it in an if-test.echoecho "Testing "0"" if [ 0 ]  # zerothen  echo "0 is true." else  # Or else ...  echo "0 is false." fi # 0 is true.echoecho "Testing "1"" if [ 1 ]  # onethen  echo "1 is true." else  echo "1 is false." fi # 1 is true.echoecho "Testing "-1"" if [ -1 ] # minus onethen  echo "-1 is true." else  echo "-1 is false." fi # -1 is true.echoecho "Testing "NULL"" if [ ] # NULL (empty condition)then  echo "NULL is true." else  echo "NULL is false." fi # NULL is false.echoecho "Testing "xyz"" if [ xyz ] # stringthen  echo "Random string is true." else  echo "Random string is false." fi # Random string is true.echoecho "Testing "$xyz"" if [ $xyz ]   # Tests if $xyz is null, but...  # it's only an uninitialized variable.then  echo "Uninitialized variable is true." else  echo "Uninitialized variable is false." fi # Uninitialized variable is false.echoecho "Testing "-n $xyz"" if [ -n "$xyz" ] # More pedantically correct.then  echo "Uninitialized variable is true." else  echo "Uninitialized variable is false." fi # Uninitialized variable is false.echoxyz=  # Initialized, but set to null value.echo "Testing "-n $xyz"" if [ -n "$xyz" ]then  echo "Null variable is true." else  echo "Null variable is false." fi # Null variable is false.echo# When is "false" true?echo "Testing "false"" if [ "false" ]  #  It seems that "false" is just a string ...then  echo ""false" is true." #+ and it tests true.else  echo ""false" is false." fi # "false" is true.echoecho "Testing "$false""  # Again, uninitialized variable.if [ "$false" ]then  echo ""$false" is true." else  echo ""$false" is false." fi # "$false" is false.  # Now, we get the expected result.#  What would happen if we tested the uninitialized variable "$true"?echoexit 0

#!/bin/bashechoif test -z "$1" then  echo "No command-line arguments." else  echo "First command-line argument is $1." fiechoif /usr/bin/test -z "$1"  # Equivalent to "test" builtin.#  ^^^^^^^^^^^^^  # Specifying full pathname.then  echo "No command-line arguments." else  echo "First command-line argument is $1." fiechoif [ -z "$1" ] # Functionally identical to above code blocks.#   if [ -z "$1" should work, but...#+  Bash responds to a missing close-bracket with an error message.then  echo "No command-line arguments." else  echo "First command-line argument is $1." fiechoif /usr/bin/[ -z "$1" ]   # Again, functionally identical to above.# if /usr/bin/[ -z "$1"   # Works, but gives an error message.# # Note:#   This has been fixed in Bash, version 3.x.then  echo "No command-line arguments." else  echo "First command-line argument is $1." fiechoexit 0

#!/bin/bash# arith-tests.sh# Arithmetic tests.# The (( ... )) construct evaluates and tests numerical expressions.# Exit status opposite from [ ... ] construct!(( 0 ))echo "Exit status of "(( 0 ))" is $?." # 1(( 1 ))echo "Exit status of "(( 1 ))" is $?." # 0(( 5 > 4 ))  # trueecho "Exit status of "(( 5 > 4 ))" is $?." # 0(( 5 > 9 ))  # falseecho "Exit status of "(( 5 > 9 ))" is $?." # 1(( 5 == 5 )) # trueecho "Exit status of "(( 5 == 5 ))" is $?." # 0# (( 5 = 5 ))  gives an error message.(( 5 - 5 ))  # 0echo "Exit status of "(( 5 - 5 ))" is $?." # 1(( 5 / 4 ))  # Division o.k.echo "Exit status of "(( 5 / 4 ))" is $?." # 0(( 1 / 2 ))  # Division result < 1.echo "Exit status of "(( 1 / 2 ))" is $?." # Rounded off to 0. # 1(( 1 / 0 )) 2>/dev/null  # Illegal division by 0.#   ^^^^^^^^^^^echo "Exit status of "(( 1 / 0 ))" is $?." # 1# What effect does the "2>/dev/null" have?# What would happen if it were removed?# Try removing it, then rerunning the script.# ======================================= ## (( ... )) also useful in an if-then test.var1=5var2=4if (( var1 > var2 ))then #^  ^  Note: Not $var1, $var2. Why?  echo "$var1 is greater than $var2" fi # 5 is greater than 4exit 0

#!/bin/bash# broken-link.sh# Written by Lee bigelow <[email protected]># Used in ABS Guide with permission.#  A pure shell script to find dead symlinks and output them quoted#+ so they can be fed to xargs and dealt with :)#+ eg. sh broken-link.sh /somedir /someotherdir|xargs rm##  This, however, is a better method:##  find "somedir" -type l -print0|#  xargs -r0 file|#  grep "broken symbolic"|#  sed -e 's/^|: *broken symbolic.*$/"/g'##+ but that wouldn't be pure Bash, now would it.#  Caution: beware the /proc file system and any circular links!#################################################################  If no args are passed to the script set directories-to-search #+ to current directory.  Otherwise set the directories-to-search #+ to the args passed.######################[ $# -eq 0 ] && directorys=`pwd` || directorys=$@#  Setup the function linkchk to check the directory it is passed #+ for files that are links and don't exist, then print them quoted.#  If one of the elements in the directory is a subdirectory then #+ send that subdirectory to the linkcheck function.##########linkchk () { for element in $1/*; do  [ -h "$element" -a ! -e "$element" ] && echo "$element"   [ -d "$element" ] && linkchk $element # Of course, '-h' tests for symbolic link, '-d' for directory. done}#  Send each arg that was passed to the script to the linkchk() function#+ if it is a valid directoy.  If not, then print the error message#+ and usage info.##################for directory in $directorys; do if [ -d $directory ]then linkchk $directoryelse echo "$directory is not a directory" echo "Usage: $0 dir1 dir2 ..." fidoneexit $?

#!/bin/basha=4b=5#  Here "a" and "b" can be treated either as integers or strings.#  There is some blurring between the arithmetic and string comparisons,#+ since Bash variables are not strongly typed.#  Bash permits integer operations and comparisons on variables#+ whose value consists of all-integer characters.#  Caution advised, however.echoif [ "$a" -ne "$b" ]then  echo "$a is not equal to $b"   echo "(arithmetic comparison)" fiechoif [ "$a" != "$b" ]then  echo "$a is not equal to $b."   echo "(string comparison)"   # "4"  != "5"   # ASCII 52 != ASCII 53fi# In this particular instance, both "-ne" and "!=" work.echoexit 0

#!/bin/bash#  str-test.sh: Testing null strings and unquoted strings,#+ but not strings and sealing wax, not to mention cabbages and kings . . .# Using   if [ ... ]# If a string has not been initialized, it has no defined value.# This state is called "null" (not the same as zero!).if [ -n $string1 ] # string1 has not been declared or initialized.then  echo "String "string1" is not null." else echo "String "string1" is null." fi # Wrong result.# Shows $string1 as not null, although it was not initialized.echo# Let's try it again.if [ -n "$string1" ]  # This time, $string1 is quoted.then  echo "String "string1" is not null." else echo "String "string1" is null." fi # Quote strings within test brackets!echoif [ $string1 ]   # This time, $string1 stands naked.then  echo "String "string1" is not null." else echo "String "string1" is null." fi # This works fine.# The [ ... ] test operator alone detects whether the string is null.# However it is good practice to quote it (if [ "$string1" ]).## As Stephane Chazelas points out,# if [ $string1 ] has one argument, "]" # if [ "$string1" ]  has two arguments, the empty "$string1" and "]" echostring1=initializedif [ $string1 ]   # Again, $string1 stands unquoted.then  echo "String "string1" is not null." else echo "String "string1" is null." fi # Again, gives correct result.# Still, it is better to quote it ("$string1"), because . . .string1="a = b" if [ $string1 ]   # Again, $string1 stands unquoted.then  echo "String "string1" is not null." else echo "String "string1" is null." fi # Not quoting "$string1" now gives wrong result!exit 0   # Thank you, also, Florian Wisser, for the "heads-up".

#!/bin/bash# zmore# View gzipped files with 'more' filter.E_NOARGS=85E_NOTFOUND=86E_NOTGZIP=87if [ $# -eq 0 ] # same effect as:  if [ -z "$1" ]# $1 can exist, but be empty:  zmore "" arg2 arg3then  echo "Usage: `basename $0` filename" >&2  # Error message to stderr.  exit $E_NOARGS  # Returns 85 as exit status of script (error code).fi  filename=$1if [ ! -f "$filename" ]   # Quoting $filename allows for possible spaces.then  echo "File $filename not found!" >&2   # Error message to stderr.  exit $E_NOTFOUNDfi  if [ ${filename##*.} != "gz" ]# Using bracket in variable substitution.then  echo "File $1 is not a gzipped file!"   exit $E_NOTGZIPfi  zcat $1 | more# Uses the 'more' filter.# May substitute 'less' if desired.exit $?   # Script returns exit status of pipe.#  Actually "exit $?" is unnecessary, as the script will, in any case,#+ return the exit status of the last command executed.