UNIX for Advanced & Expert Users

#!/bin/ksh93

TMP=file.$$

cat <<EOT >$TMP
aaa
bbb
ccc
ddd
eee
fff
EOT

# open file descriptor 3 for read/write
command exec 3<> $TMP || exit 1

# check file descriptor 3 position
print
print "At offset: $(3<#)"
if (($(3<#) != 0))
then
   print "Not at offset 0"
   exit 1
fi

# read in the first line and print it
read -u3
print $REPLY
print "At offset $(3<#) after reading line"
print

# search forward for string "ddd"
3<#"ddd"
print "At offset $(3<#) after search forward for 'ddd'"
read -u3
print $REPLY
print

# check that we are at offset 8 and, if so, read line
if (( $(3<# ((8))) != 8))
then
  print "Not at offset 8"
  exit 1
fi
print "At offset $(3<#) after specifying absolute offset of 8"
read -u3
print $REPLY
print

# go on that is at offset 24, so check.
if (( $(3<#((EOF))) != 4*6 ))
then
   print "Not at EOF"
   exit 1
fi
print "At offset $(3<#) after specifying EOF"
print

# backup one line i.e. 4 characters
3<#((CUR - 4))
print "At offset $(3<#) after backing up 4 characters"
read -u3
print $REPLY
print

redirect 3<&- || echo 'cannot close FD 3'

rm $TMP

At offset: 0 
aaa 
At offset 4 after reading line 

At offset 12 after search forward for 'ddd' 
ddd 

At offset 8 after specifying absolute offset of 8 
ccc 

At offset 24 after specifying EOF 

At offset 20 after backing up 4 characters 
fff