[SOLVED] A simple question with sed and pipe with shell script

Issue

A sed and script problem needs to save

i want to write a script and do something like this the whole task is write a shell script and convert parameters to the command used by sed ,Then call sed to print the specified line from stdin .

seq 1 100 | ./script.sh 1 9 34-35

and get the answer with :

1 9 34 35

below is my code

#!/bin/bash

str="sed -n -e '"

data=$(cat)
#echo "$data"

for arg in $@;do
 if [ `expr index $arg -` -eq 0 ]; then
   str="$str${arg}p;"
 else
   len=`expr length $arg`
   i=`expr index $arg -`
   i=`expr $i - 1`
   tmp1=`expr substr $arg 1 $i`
   str="$str${tmp1},"
   i=`expr $i + 2`
   tmp2=`expr substr $arg $i $len`
   str="$str${tmp2}p;"
 fi
done
str="$str'"
echo "$str"
`echo $data | $str`
#echo `$str`
~                

and i got the error sed: -e expression #1, char 1: unknown command: `''

what is the problem and how can i do this correct?

Solution

Your code builds a string.

With arguments 1 9 34-35, you will get: sed -n -e '1p;9p;34,35p;'

You call:

`echo $data | $str`

This becomes two commands piped together:

• echo with arguments: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100

• sed with arguments: -n -e '1p;9p;34,35p;'

Note that the shell does no further manipulation of the words. In particular, quote-stripping has already happened (there were none before expansion).

So sed receives a script argument starting literally with '.

' is not a valid sed command, so the error is printed.

Note that $data should have been quoted, otherwise the shell will strip out the newlines as shown above, and echo when run will output only a single line.

Note also that if the sed had succeeded, the backticks will cause the shell to try to run its output as a new command. This seems very unlikely to be what you intended.

To fix your code, it would make more sense to avoid confusion by separating commands and arguments:

#!/bin/bash

str=

data=$(cat)
#echo "$data"

for arg in $@;do
 if [ `expr index $arg -` -eq 0 ]; then
   str="$str${arg}p;"
 else
   len=`expr length $arg`
   i=`expr index $arg -`
   i=`expr $i - 1`
   tmp1=`expr substr $arg 1 $i`
   str="$str${tmp1},"
   i=`expr $i + 2`
   tmp2=`expr substr $arg $i $len`
   str="$str${tmp2}p;"
 fi
done
echo `echo "$data" | sed -n -e "$str"`

And you can simplify / modernise to:

#!/bin/bash

str=

for arg; do
 if [ $(expr index $arg -) -eq 0 ]; then
   str="$str${arg}p;"
 else
   len=$(expr length $arg)
   i=$(expr index $arg -)
   i=$(expr $i - 1)
   tmp1=$(expr substr $arg 1 $i)
   str="$str${tmp1},"
   i=$(expr $i + 2)
   tmp2=$(expr substr $arg $i $len)
   str="$str${tmp2}p;"
 fi
done

# this command will already read from stdin, so no need to buffer the data
echo $(sed -n -e "$str")

expr is rather tedious to use. Making use of bash's built-in functionality you can simplify much further:

#!/bin/bash

str=

for arg; do
    str=$str${arg/-/,}"p;"
done

echo $(sed -n "$str") # output is combined into single line
                      # for separate lines, just do: sed -n "$str"

Or with argument validation:

#!/bin/bash

str=

for arg; do
    if ! [[ $arg =~ ^[0-9]+(-[0-9]+)?$ ]]; then
        echo bad args 1>&2
        exit 1
    fi
    str=$str${arg/-/,}"p;"
done

echo $(sed -n "$str")

Any of these versions produce the sample output from the question.

Answered By - jhnc

Answer Checked By - Gilberto Lyons (WPSolving Admin)