Scripting

   Basics | Script example | awk | sed | Regular Expressions | useful commands
   The Bourne shell (/bin/sh) is present on all Unix installations and scripts written in this
   language are (quite) portable; man 1 sh is a good reference.

Basics

Variables and arguments

   Assign with variable=value and get content with $variable
MESSAGE="Hello World"                        # Assign a string
PI=3.1415                                    # Assign a decimal number
N=8
TWON=`expr $N * 2`                           # Arithmetic expression (only integers)
TWON=$(($N * 2))                             # Other syntax
TWOPI=`echo "$PI * 2" | bc -l`               # Use bc for floating point operations
ZERO=`echo "c($PI/4)-sqrt(2)/2" | bc -l`

   The command line arguments are
$0, $1, $2, ...                              # $0 is the command itself
$#                                           # The number of arguments
$*                                           # All arguments (also $@)

Special Variables

$$                                           # The current process ID
$?                                           # exit status of last command
  command
  if [ $? != 0 ]; then
    echo "command failed"
  fi
mypath=`pwd`
mypath=${mypath}/file.txt
echo ${mypath##*/}                           # Display the filename only
echo ${mypath%%.*}                           # Full path without extention
foo=/tmp/my.dir/filename.tar.gz
path = ${foo%/*}                             # Full path without extention
var2=${var:=string}                          # Use var if set, otherwise use string
                                             # assign string to var and then to var2.
size=$(stat -c%s "$file")                    # get file size in bourne script
filesize=${size:=-1}

Constructs

for file in `ls`
do
    echo $file
done

count=0
while [ $count -lt 5 ]; do
    echo $count
    sleep 1
    count=$(($count + 1))
done

myfunction() {
    find . -type f -name "*.$1" -print       # $1 is first argument of the function
}
myfunction "txt"

Generate a file

MYHOME=/home/colin
cat > testhome.sh << _EOF
# All of this goes into the file testhome.sh
if [ -d "$MYHOME" ] ; then
    echo $MYHOME exists
else
    echo $MYHOME does not exist
fi
_EOF
sh testhome.sh

Bourne script example

   As a small example, the script used to create a PDF booklet from this xhtml document:
#!/bin/sh
# This script creates a book in pdf format ready to print on a duplex printer
if [ $# -ne 1 ]; then                        # Check the argument
  echo 1>&2 "Usage: $0 HtmlFile"
  exit 1                                     # non zero exit if error
fi

file=$1                                      # Assign the filename
fname=${file%.*}                             # Get the name of the file only
fext=${file#*.}                              # Get the extension of the file

prince $file -o $fname.pdf                   # from www.princexml.com
pdftops -paper A4 -noshrink $fname.pdf $fname.ps # create postscript booklet
cat $fname.ps |psbook|psnup -Pa4 -2 |pstops -b "2:0,1U(21cm,29.7cm)" > $fname.book.ps

ps2pdf13 -sPAPERSIZE=a4 -sAutoRotatePages=None $fname.book.ps $fname.book.pdf
                                             # use #a4 and #None on Windows!
exit 0                                       # exit 0 means successful

Some awk commands

   Awk is useful for field stripping, like cut in a more powerful way. Search this document
   for other examples. See for example gnulamp.com and one-liners for awk for some nice
   examples.
awk '{ print $2, $1 }' file                  # Print and inverse first two columns
awk '{printf("%5d : %s\n", NR,$0)}' file     # Add line number left aligned
awk '{print FNR "\t" $0}' files              # Add line number right aligned
awk NF test.txt                              # remove blank lines (same as grep '.')
awk 'length > 80'                            # print line longer than 80 char)

Some sed commands

   Here is the one liner gold minehttp://student.northpark.edu/pemente/sed/sed1line.txt. And a
   good introduction and tutorial to sedhttp://www.grymoire.com/Unix/Sed.html.
sed 's/string1/string2/g'                    # Replace string1 with string2
sed -i 's/wroong/wrong/g' *.txt              # Replace a recurring word with g
sed 's/\(.*\)1/\12/g'                        # Modify anystring1 to anystring2
sed '/<p>/,/<\/p>/d' t.xhtml                 # Delete lines that start with <p>
                                             # and end with </p>
sed '/ *#/d; /^ *$/d'                        # Remove comments and blank lines
sed 's/[ \t]*$//'                            # Remove trailing spaces (use tab as \t)
sed 's/^[ \t]*//;s/[ \t]*$//'                # Remove leading and trailing spaces
sed 's/[^*]/[&]/'                            # Enclose first char with [] top->[t]op
sed = file | sed 'N;s/\n/\t/' > file.num     # Number lines on a file

Regular Expressions

   Some basic regular expression useful for sed too. See Basic Regex
   Syntaxhttp://www.regular-expressions.info/reference.html for a good primer.
[\^$.|?*+()                          # special characters any other will match themselves
\                                    # escapes special characters and treat as literal
*                                    # repeat the previous item zero or more times
.                                    # single character except line break characters
.*                                   # match zero or more characters
^                                    # match at the start of a line/string
$                                    # match at the end of a line/string
.$                                   # match a single character at the end of line/string
^ $                                  # match line with a single space
[^A-Z]                               # match any line beginning with any char from A to Z

Some useful commands

   The following commands are useful to include in a script or as one liners.
sort -t. -k1,1n -k2,2n -k3,3n -k4,4n         # Sort IPv4 ip addresses
echo 'Test' | tr '[:lower:]' '[:upper:]'     # Case conversion
echo foo.bar | cut -d . -f 1                 # Returns foo
PID=$(ps | grep script.sh | grep bin | awk '{print $1}')    # PID of a running script
PID=$(ps axww | grep [p]ing | awk '{print $1}')             # PID of ping (w/o grep pid)
IP=$(ifconfig $INTERFACE | sed '/.*inet addr:/!d;s///;s/ .*//')   # Linux
IP=$(ifconfig $INTERFACE | sed '/.*inet /!d;s///;s/ .*//')        # FreeBSD
if [ `diff file1 file2 | wc -l` != 0 ]; then [...] fi       # File changed?
cat /etc/master.passwd | grep -v root | grep -v \*: | awk -F":" \ # Create http passwd
'{ printf("%s:%s\n", $1, $2) }' > /usr/local/etc/apache2/passwd

testuser=$(cat /usr/local/etc/apache2/passwd | grep -v \    # Check user in passwd
root | grep -v \*: | awk -F":" '{ printf("%s\n", $1) }' | grep ^user$)
:(){ :|:& };:                                # bash fork bomb. Will kill your machine
tail +2 file > file2                         # remove the first line from file

   I use this little trick to change the file extension for many files at once. For example
   from .cxx to .cpp. Test it first without the | sh at the end. You can also do this with the
   command rename if installed. Or with bash builtins.
# ls *.cxx | awk -F. '{print "mv "$0" "$1".cpp"}' | sh
# ls *.c | sed "s/.*/cp & &.$(date "+%Y%m%d")/" | sh # e.g. copy *.c to *.c.20080401
# rename .cxx .cpp *.cxx                             # Rename all .cxx to cpp
# for i in *.cxx; do mv $i ${i%%.cxx}.cpp; done      # with bash builtins