“All the useful stuff on a single card”

 

#!/bin/bash

$ chmod ugo+x shell_script.sh

 

$ bash [options] [file]

Options

-x show execution of [file]

-v echo lines as they are read

 

Variables

 

$ var=”some value” # declare a variable

$ echo $var # access contents of variable

$ echo ${var} # access contents of variable

$ echo ${var:-”default value”} # with default

$ var= # delete a variable

$ unset var # delete a variable

Quoting - “$variable” - preserves whitespace

 

Positional Variables

 

$0	Name of script
$1-$9	Positional parameters #1 - #9
${10}	to access positional parameter #10 onwards
$#	Number of positional parameters
"$*"	All the positional parameters (as a single word) *
"$@"	All the positional parameters (as separate strings)
$?	Return value

 

set [values] - sets positional params to [values]

set -- - deletes all positional parameters

shift [n]- move positional params n places to the left

 

Command Substitution

 

$ var=`ls *.txt` # Variable contains output

$ var=$(ls *.txt)   # Alternative form

$ cat myfile >/dev/null # suppress stdout

$ rm nofile 2>/dev/null # suppress stderr

$ cat nofile 2>/dev/null >/dev/null # suppress both

 

Arithmetic Operators

 

$ var=$(( 20 + 5 ))

$ expr 1 + 3# 4

$ expr 2 - 1# 1

$ expr 10 / 3# 3

$ expr 20 % 3# 2 (remainder)

$ expr 10 \* 3# 30 (multiply)

 

String Operators

 

Expression	Meaning
${#str}	Length of $str
${str:pos}	Extract substring from $str at $pos
${str:pos:len}	Extract $len chars from $str at $pos
${str/sub/rep}	Replace first match of $sub with $rep
${str//sub/rep}	Replace all matches of $sub with $rep
${str/#sub/rep}	If $sub matches front end of $str, substitute $rep for $sub
${str/%sub/rep}	If $sub matches back end of $str, substitute $rep for $sub

 

Relational Operators

 

Num	String	Test
-eq	=	Equal to
	==	Equal to
-ne	!=	Not equal to
-lt	\<	Less than
-le		Less than or equal to
-gt	\>	Greater than
-ge		Greater than or equal to
	-z	is empty
	-n	is not empty

 

File Operators

 

	True if file exists and...
-f file	...is a regular file
-r file	...is readable
-w file	...is writable
-x file	...is executable
-d file	...is a directory
-s file	...has a size greater than zero.

Control Structures

if [ condition ] # true = 0

  then

# condition is true

elif [ condition1 ]

  then

# condition1 is true

 elif condition2

   then

# condition2 is true

else

# None of the conditions is true

fi

 

case expression in

  pattern1) execute commands ;;

  pattern2) execute commands ;;

esac

 

while [ true ]

do

#  execute commands

done

 

until [ false ]

do

#   execute commands

done

 

for x in 1 2 3 4 5 # or for x in {1..5}

do

    echo "The value of x is $x";

done

 

LIMIT=10

for ((x=1; x <= LIMIT ; x++))

do

  echo -n "$x "

done

 

for file in *~

do

  echo "$file"

done

 

break [n] # exit n levels of loop

continue [n] # go to next iteration of loop n up

 

Function Usage

function-name arg1 arg2 arg3 argN

n.b. functions must be defined before use...

Function Definition

 

function function-name ()

{

# statement1

# statement2

# statementN

  return [integer] # optional

}

Functions have access to script variables, and may have local variables:

$ local var=value

 

Arrays

 

$ vars[2]=”two” # declare an array

$ echo ${vars[2]} # access an element

$ fruits=(apples oranges pears) # populate array

$ echo ${fruits[0]} # apples - index from 0

$ declare -a fruits # creates an array

echo "Enter your favourite fruits: "

read -a fruits

echo You entered ${#fruits[@]} fruits

for f in "${fruits[@]}"

do

  echo "$f"

done

$ copy=( "${fruits[@]}" ) # copy an array

$ array=( "${fruits[@]}" "grapes" ) # add to end

$ unset fruits[1] # delete one element

$ unset fruits # delete array

Array elements do not have to be sequential - indices are listed in {!fruits[@]}:

for i in ${!fruits[@]}

do

  echo fruits[$i]=${fruits[i]}

done

All variables are single element arrays:

$ var=”The quick brown fox”

$ echo {var[0]} #  The quick brown fox

 

String operators can be applied to all the string elements in an array using ${name[@] ... } notation, e.g.:

$ echo ${arrayZ[@]//abc/xyz} # Replace all occurrences of abc with xyz

 

User Interaction

 

echo -n "Prompt: "

read

echo "You typed $REPLY."

 

echo -n "Prompt: "

read response

echo "You typed $response."

 

PS3="Choose a fruit: "

select fruit in "apples" "oranges" "pears"

do

  if [ -n "$fruit" ]

  then

    break

  fi

  echo "Invalid choice"

done

 

$ dialog --menu "Choose" 10 20 4 1 apples 2 \ oranges 3 pears 4 bananas 2>/tmp/ans

$ fruit=`cat /tmp/ans`

$ echo $fruit

 

$ zenity --list --radiolist --column "Choose" \

--column "Fruit" 0 Apples 0 Oranges 0 Pears 0 \ Bananas > /tmp/ans

$ fruit=`cat /tmp/ans`

$ echo $fruit

 

Reading Input from a File

 

exec 6<&0        # ‘Park’ stdin on #6

exec < temp.txt# stdin=file "temp.txt"

read  # from stdin

until [ -z "$REPLY" ]

do

  echo “$REPLY”# lists temp.txt

  read

done

exec 0<&6 6<&-# restore stdin

echo -n "Press any key to continue"

read

 

Trapping Exceptions

 

TMPFILE=`mktemp`

on_break()

{

  rm -f $TMPFILE

  exit 1

}

trap on_break 2 # catches Ctrl+C

 

Data and Time

 

$ start=`date +%s`

$ end=`date +%s`

$ echo That took $((end-start)) seconds

$ date +"%c" -d19540409

Fri 09 Apr 1954 12:00:00 AM GMT

 

Case Conversion

 

$ in="The quick brown fox"

$ out=`echo $in | tr [:lower:] [:upper:]`

$ echo “$out”

THE QUICK BROWN FOX

 

Preset Variables

 

$HOME	User’s home directory
$HOSTNAME	Name of host
$HOSTTYPE	Type of host (e.g. i486)
$PWD	Current directory
$REPLY	default variable for READ and SELECT
$SECONDS	Elapsed time of script
$TMOUT	Max. script elapsed time or wait time for read

 

References

 

Linux Shell Scripting Tutorial  - A Beginner's handbook http://www.cyberciti.biz/nixcraft/linux/docs/uniqlinuxfeatures/lsst/

BASH Programming Introduction, Mike G http://www.tldp.org/HOWTO/Bash-Prog-Intro-HOWTO.html

Advanced BASH Scripting Guide, Mendel Cooper http://tldp.org/LDP/abs/html/

 

Copyright & Licence

 

This Reference Card is Copyright (c)2007 John McCreesh jpmcc@users.sf.net and is licensed under the Creative Commons Attribution-Noncommercial-Share Alike 2.5 UK: Scotland License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/2.5/scotland/ or send a letter to Creative Commons, 543 Howard Street, 5th Floor, San Francisco, California, 94105, USA.