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up vote 2 down vote favorite
1

Well, this apparently is not possible the way I'm trying it.

This alternate approach to obtain bar as a resulting string works, though:

#!/bin/bash

path1=/usr/tmp/foo/bar/baz

awk -F/ '{print $5}' <<< "$path1"

So far so good, but what if I want to do without the <<< operator as well as those notorious echo | ... pipes? In a nutshell, what I'm trying to do is passing path1 as a variable with the -v pa="$path1" directive and using both the field separator -F/ and the field identifiers (e. g. $5) to parse the awk-internal pa variable, which got its value assigned from the external path1 shell variable. Can this be done "inside" awk, too?

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4 Answers 4

up vote 6 down vote accepted

The problem with the -v option or with the var=value arguments to awk is that they can't be used for arbitrary data since ANSI C escape sequences (like \n, \b...) are expanded in them.

The alternative is to use the ARGV or ENVIRON awk arrays:

awk -F/ 'BEGIN{$0 = ARGV[1]; print $5}' "$path1"

Or:

export path1
awk -F/ 'BEGIN{$0 = ENVIRON["path1"]; print $5}'

Or:

path1="$path1" awk -F/ 'BEGIN{$0 = ENVIRON["path1"]; print $5}'

Now, if all you want is split a shell variable, you may not need awk.

In all POSIX shells:

IFS=/; set -f
set -- $path1
printf '%s\n' "$5"
share|improve this answer
    
Thank you, this looks interesting as well! Just one thing always confuses me again: awk expects a file after the '...', right? So why does it accept a string in this case without complaining? -- To your second approach: As I normally use local variables in my bash scripts, I normally do not make them available from outside using export. And not to be forgotten: these variables will have to be un`export`ed again to reduce clutter, whilst local variables are destroyed after the script ends (or exits prematurely). –  syntaxerror Aug 19 '13 at 20:04
2  
@syntaxerror, See the 3rd variant then, or use ARGV. If there's only a BEGIN section, files or standard input are not read. –  Stéphane Chazelas Aug 19 '13 at 20:10
    
Nice, thanks, I'll remember that from now on! Besides, your logic also works the other way round: that is to say, if you do NOT set a BEGIN section and use a variable like this here (and not a file), awk may "stall" and you have to CTRL-C it to get back to your shell prompt. Didn't happen to me only once when I did my first awk steps some moons ago. –  syntaxerror Aug 19 '13 at 20:16
    
Thanks for showing how to use ENVIRON. I tried that but didn't realize to drop the $ bit from the variable name. Very nice and elegant solutions! –  slm♦ Aug 19 '13 at 20:22
up vote 3 down vote

You could split pa into an array

awk -F'/' -v pa=$path1 'BEGIN{split(pa, arr, FS); print(arr[5]); exit}'
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This is sneaky, thanks! :) –  syntaxerror Aug 19 '13 at 18:33
1  
That fails if $path1 contains blanks. May fail if it contains wildcard characters or backslashes. –  Stéphane Chazelas Aug 19 '13 at 20:11
    
@StephaneChazelas, good point. And +1 for your solution that addresses these issues –  1_CR Aug 19 '13 at 20:35
up vote 1 down vote

There are a couple of ways you can pass environment variables into an awk script/command:

Method #1

This gets the shell to expand the variable, $path1, prior to running the awk command.

$ echo $path1
/usr/tmp/foo/bar/baz
$ awk -F'/' 'END{a="'$path1'"; split(a,arr,FS); print(arr[5])}' /dev/null
bar

Method #2

Pass env. variable in as a awk variable, a.

$ awk -F '/' 'END{split(a,arr,FS); print(arr[5])}' a=$path1 /dev/null
bar

Method #3

Explicitly pass awk a variable using the -v switch.

$ awk -F'/' -v a=$path1 'END{split(a,arr,FS); print(arr[5])}' /dev/null
bar

Debugging

You can enable the shell's verbosity by setting set -x before running any of these commands to see what's going on. Here's method #2 as an example:

$ set -x
$ awk -F '/' 'END{split(a,arr,FS); print(arr[5])}' a=$path1 /dev/null
+ awk -F / 'END{split(a,arr,FS); print(arr[5])}' a=/usr/tmp/foo/bar/baz /dev/null
bar

The above shows set -x being executed followed by the awk line. You can see that when executed the variable a has the value of $path1 already expanded when it executes.

Another example, this time method #1:

$ awk -F'/' 'END{a="'$path1'";split(a,arr,FS); print(arr[5])}' /dev/null
+ awk -F/ 'END{a="/usr/tmp/foo/bar/baz";split(a,arr,FS); print(arr[5])}' 
bar

Here you can see that the shell is definitely expanding the $path1 variable prior to executing awk.

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Thanks a ton for your efforts! Indeed, but there is obviously no way you can make direct use of those $3, $4 field identifiers. You will always have to work around it by using split() and arrays. -- However, if I had the possibility of using field identifiers, it would make the line appear significantly shorter for sure (which is always a criteria to care upon :)) –  syntaxerror Aug 19 '13 at 19:13
1  
Method 1: not sure why you use both a BEGIN and END section, may fail (possibly badly) if $path1 contains double quotes, blanks, backslashes, globbing characters. For instance, try with path1='";system("rm\040-rf\040~");"' –  Stéphane Chazelas Aug 19 '13 at 20:06
    
Method2 2 and 3: may fail if $path1 contains blanks or wildcards (as $path1 is still not quoted) or backslashes. –  Stéphane Chazelas Aug 19 '13 at 20:08
    
@StephaneChazelas - thanks I've removed the BEGIN section. –  slm♦ Aug 19 '13 at 20:18
up vote 0 down vote

How about:

basename "$(dirname "$path1")"

or, if your shell supports process substitution and you need to use awk:

awk -F'/' '{print $5}' <(printf '%s\n' "$path1")
share|improve this answer
1  
To your first line: OK for this purpose, but these examples should not be taken too literally. I usually use these with the intention to do something way more complex later. I was just asking because I needed the right technique to get string variables from outside parsed inside awk. -- To your second line: This is not much different from my <<< approach. This will as well have to be done from outside awk. But thanks anyway for your input. –  syntaxerror Aug 19 '13 at 19:04

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