bash script and awk

phonebooth · 03-24-2010, 10:38 AM

Below is the part of my bash script which uses awk to to fetch a line from a file. Choice is set by a case, and i know it is receiving a proper number because of the echo statement. The problem is with the syntax of the awk command it says the error is with one of the ', but when I run the command at the command line and replace "$choice" with a number it works properly. So I am not sure what is going on.

echo "announcing choice $choice"
command="awk 'NR = "$choice"' beerpong.txt | say"
$command

Thanks for the help

grail · 03-24-2010, 10:46 AM

Let me ask you this, if we changed this slightly to:

command="ls"
$command

What would you expect to be the output from this?
If you answer this you will be able to figure out what is wrong, unless you have skipped some of the code?

catkin · 03-24-2010, 10:47 AM

The pipe symbol in $command will not be actioned because bash parses for pipelines before expanding variables.

You could get around this by using eval but it would be messy regards double parsing the quotes so easier to go with

Code:

echo "announcing choice $choice"
command="awk 'NR = "$choice"' beerpong.txt"
$command | say

slinx · 03-24-2010, 10:48 AM

Quote:

Originally Posted by phonebooth

Below is the part of my bash script which uses awk to to fetch a line from a file. Choice is set by a case, and i know it is receiving a proper number because of the echo statement. The problem is with the syntax of the awk command it says the error is with one of the ', but when I run the command at the command line and replace "$choice" with a number it works properly. So I am not sure what is going on.

Code:

        echo "announcing choice $choice"
        command="awk 'NR = "$choice"' beerpong.txt | say"
        $command

Thanks for the help

The single quotes prevent expansion of shell variables. Try this:

Code:

        echo "announcing choice $choice"
        command="awk -vchoice=$choice 'NR = "choice"' beerpong.txt | say"
        eval $command

It would help to explain what you are trying to accomplish...

phonebooth · 03-26-2010, 10:07 AM

The only method I could kinda get to work was slinx's although instead of reading a single line it reads the entire file. The script takes an input from the user and then translastes the corresponding input into a line number (the line number refers to a second file). The script should read that line and pipe it to the say command so the computer will speak it.

Here is the entire script.

Code:

#!/bin/bash 
 input=a

while [ "$input" != "qq" ]
do
        read input

        case "$input" in
        "b" | "B")
          choice=1;;
        "i" | "I")
          choice=2;;
        "d" | "D")
           choice=3;;
        "t" | "T")
         choice=4;;
        "s" | "S")
          choice=5;;
        "y" | "Y")
          choice=6;;
        "h" | "H")
          choice=7;;
        "t" | "T")
          choice=8;;
        "l" | "L")
          choice=9;;
        "r" | "R" )
          choice=10;;
        "g" | "G")
          choice=11;;

        esac

        echo "announcing choice $choice"
        command="awk -v choice=$choice 'NR = "choice"' beerpong.txt"
        eval $command | say
done
exit 0

catkin · 03-26-2010, 10:13 AM

Quote:

Originally Posted by phonebooth

The only method I could kinda get to work was slinx's ...

Please post the output when trying my suggestion.

grail · 03-26-2010, 10:41 AM

@catkin Am I missing something or should yours at least have an echo at the front of the last line:

Quote:

echo $command | say

catkin · 03-26-2010, 02:54 PM

Quote:

Originally Posted by grail

@catkin Am I missing something or should yours at least have an echo at the front of the last line:

Maybe! I was guessing from the OP's

Code:

command="awk 'NR = "$choice"' beerpong.txt | say"

that the intention was for awk to print line number $choice from beerpong.txt to stdout and the say command would take it from standard in and do ... whatever say does.

catkin · 03-27-2010, 12:51 AM

First an analysis of what bash does with

Code:

command="awk 'NR = "$choice"' beerpong.txt | say"
$command

The steps taken by bash are given here where it says:

... the shell does the following:
[snip]

Breaks the input into words and operators, obeying the quoting rules described in Quoting. These tokens are separated by metacharacters. Alias expansion ... [snip].

Parses the tokens into simple and compound commands (see Shell Commands).

Performs the various shell expansions (see Shell Expansions), breaking the expanded tokens into lists of filenames (see Filename Expansion) and commands and arguments.

Performs any necessary redirections ... [snip]

Executes the command

[snip]

Assuming the value of $choice is 4 and no aliases apply then the value of $command is

Code:

awk 'NR = 4' beerpong.txt | say

When the second line of the code snippet is run by bash, this is what bash does

[snip]
Examines the command (line) and finds a single token (word-or-operator) that is a word. The command is unchanged; it is still $command.
Examines the command for
1. Pipelines identified by "|"
2. Lists identified by ";", "&", "&&" or "||"
3. Compound commands identified by "until", "while" or "for"
4. Conditional commands identified by "if", "case", "select", "(( ))" or "[[ ]]"
5. Grouping commands identified by "( )" or "{ }"
6. Coprocesses identified by "coproc"
None of the above apply. The command is unchanged; it is still $command.
Expands $command. The command is now
Code:
```
awk 'NR = 4' beerpong.txt | say
```
Looks for redirection and finds none. The command is not changed.
Executes the command: runs awk with arguments
1. 'NR = 4'
2. beerpong.txt
3. |
4. say
[snip]

When the awk command runs, it borks on the first argument. Note: when awk 'NR = 4' beerpong.txt is run at the command prompt it works because bash strips off the single quotes and runs awk with two arguments: NR = 4 and beerpong.txt.

How to fix?

"NR = 4" can be corrected to "NR == 4"
"NR == 4" can conveniently be made a single word by changing it to NR==4.
Move the pipe symbol and the say command out of $command
Code:
```
command="awk NR==$choice beerpong.txt"
$command | say
```