Bash – Shell expansion and positional parameters

argumentsbash

I want to make an script equivalent to zgrep or bzgrep, but using tar.

I made two very similar attempts and I would like to know if there is a better one and if they could be improved or they are gonna crash for some reason I can't see. I know how use tar and bzip2, but I use to have some troubles with shell expansion.

First using $@:

#!/bin/bash
FILE_IN="${@: -1}"; ## Take last arg (file name) 
echo "FILE_IN: $FILE_IN"; 

INPUT_PARAMS="${@%%$FILE_IN}"; ## Take all args, but last (options and pattern)
echo "INPUT_PARAMS: $INPUT_PARAMS"; 

### Here I have to treat $INPUT_FILE before using grep (skipped) ###  

grep "$INPUT_PARAMS" "$FILE_IN"; 
exit $?;

Second using variable (almost same above, just a couple of differences):

#!/bin/bash
ARGS="$@" 
FILE_IN="${!ARGS[@]: -1}"; ## Take last arg (file name) 

INPUT_PARAMS=${ARGS%%$FILE_IN};

### From here is identical

They both seem to work, but then I took a look at script for bzgrep and it's much more complicated. So, it makes me think my scripts won't work.

Any thoughts? Can they be improved?

Best Answer

You need an array if you want to store more than one argument:

file_in="${@: -1}"
input_params=("${@:1:$#-1}")

Or:

file_in="${@: -1}"
input_params=("$@")
unset 'input_params[-1]'

Or, with just sh syntax:

n=$#
for file_in do
  shift
  [ "$((n = n - 1))" -gt 0 ] || break
  set -- "$@" "$file_in"
done
# input_params in "$@"

(all uppercase variables should be reserved for environment variables).

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Bash – Using Bash -c with Positional Parameters

That gives you an opportunity to set/choose $0 when using an inline script. Otherwise, $0 would just be bash.

Then you can do for instance:

$ echo foo > foo
$ bash -c 'wc -c < "${1?}"' getlength foo
4
$ rm -f bar
$ bash -c 'wc -c < "${1?}"' getlength bar
getlength: bar: No such file or directory
$ bash -c 'wc -c < "${1?}"' getlength
getlength: 1: parameter not set

Not all shells used to do that. The Bourne shell did. The Korn (and Almquist) shell chose to have the first parameter go to $1 instead. POSIX eventually went for the Bourne way, so ksh and ash derivatives reverted to that later (more on that at http://www.in-ulm.de/~mascheck/various/find/#shell). That meant that for a long time for sh (which depending on the system was based on the Bourne, Almquist or Korn shell), you didn't know whether the first argument went into $0 or $1, so for portability, you had to do things like:

sh -c 'echo foo in "$1"' foo foo

Or:

sh -c 'shift "$2"; echo txt files are "$@"' tentative-arg0 3 2 *.txt

Thankfully, POSIX has specified the new behavior where the first argument goes in $0, so we can now portably do:

sh -c 'echo txt files are "$@"' meaningful-arg0-for-error *.txt

Bash – Exit bash when find gets to a folder with permission denied

find will set its return code to non-zero if it saw an error. So you can do:

if ! find ... 
then    echo had an error >&2
fi |
while ...

(I'm not sure what you want to do with the find output).

To collect all the error messages from find on stderr (file descriptor 2) you can redirect 2 to a file. Eg:

if ! find ... 2>/tmp/errors
then     zenity --error --text "$(</tmp/errors)"
fi |
while ...

Best Answer

Related Solutions

Bash – Using Bash -c with Positional Parameters

Bash – Exit bash when find gets to a folder with permission denied

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