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i have a simple script

wget -O file.zip www.site.com/asdqwdkjhasd.zip
unzip file.zip

and i call the script

./script.sh

now my problem is if zip file is corrupt then it outputs

  End-of-central-directory signature not found.  Either this file is not
  a zipfile, or it constitutes one disk of a multi-part archive.  In the
  latter case the central directory and zipfile comment will be found on
  the last disk(s) of this archive.

how can i detect this text in script? If the output of the program contains the string "End-of-central-directory signature not found." then do something.

sometimes the downloaded zip is corrupt so it has to be redownloaded. How can i check if the output is "End-of-central-directory signature not found." and if so redownload i.e execute wget and again unzip. In other words how to loop until download and extract succeeds?

How can i achieve this in bash scripting?

I appreciate any help! Thanks!

UPDATE:

Sorry i have made a slight change to the problem. This is the actual problem i am trying to solve. Thanks for any feedbacks!

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    If unzip is well-written, the error will be reflected in exit status, not just output. Thus, you'll be able to use if unzip "$1"; then echo "unzip succeeded"; else echo "unzip failed"; fi, which is the best-practice approach. Checking for a specific string is fragile: It can fail if a future version of the program changes the message's text, if the user has a different language/locale and thus a different translation table; etc. Commented Feb 4, 2018 at 5:26
  • ...if you want to check not just for failure but for a specific failure, encourage the program's author to use a distinct exit status for that failure. (See man rsync for an example fo this being done well -- every major failure mode has a distinct, well-documented exit status). Commented Feb 4, 2018 at 5:34
  • @CharlesDuffy: Why don't you answer in an answer, but as a comment? However - about duplicate: Output of a command is not the same as an error message. Commented Feb 4, 2018 at 5:39
  • @userunknown, I flagged two duplicates here: One that speaks to comparing stdout (since that's what the OP literally asked), and a second one that speaks to comparing stderr (since that's what the OP actually needs). Between them, I believe we've got everything covered. Commented Feb 4, 2018 at 5:41
  • @userunknown, ...and, well, you know as well as I did why I'm not adding an answer: This is a duplicate, and adding an answer would fall afoul of site rules as described in How to Answer -- specifically the section "Answer well-asked questions", specifically the bullet point regarding questions which "have already been asked and answered many times before". Commented Feb 4, 2018 at 5:42

1 Answer 1

Reset to default
1

It's possible, but you normally don't.

You can pick the exit code of the last command, 0 is 0 Errors, every other value indicates some kind of error, application specific or just a general error. With $? you get the exit code from the last command, but you have to use it immediately after the command in question. 

((3 < 1)) ; err=$?; echo $err
1
((3 > 1)) ; err=$?; echo $err
0

Error messages often depend on LOCALE settings:

erg=$(LC_ALL=C ls nosuchsamplefile 2>&1)
echo "$erg"
ls: cannot access 'nosuchsamplefile': No such file or directory

erg=$(ls nosuchsamplefile 2>&1)
echo $erg
ls: Zugriff auf 'nosuchsamplefile' nicht möglich: Datei oder Verzeichnis nicht gefunden

Comparing words can fail and is inherently insecure. Even by setting the command to a specific setting can't prevent updates from changing the wording and surely, the user had set his Locale to some different value for a reason.

However, 2 > &1 redirects the error stream to the output, which is the default for the screen, but on capturing, it makes a difference. As long as you just script for yourself and in ad hoc situations, approaches can be useful, which don't scale with multiple users and when to be maintained over years.

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15 Comments

a && b || c is an antipattern, because it changes whether c is run based not just on the exit status of a but also on the exit status of b. Best practice is to teach people to write it out: if a; then b; else c; fi. This is BashPitfalls #22.
Ok, how can 'echo file found' fail? What would your if block in such a case produce?
Also, echo "$erg", not echo $erg -- without the quotes string-splitting and glob expansion take place, so if it were ls ' * ' 2>&1 for which we'd captured an error, then unquoted echo $erg would be replacing that * with a list of filenames. This is BashPitfalls #14.
An echo can fail if the file handle is closed, or if the write fails due to a problem with whatever stdout is connected to. I/O failures are things that exist. More to the point, though, if you teach a practice, people can be expected to assume it's safe to leverage more widely unless it's taught with explicit caveats ("only safe for use in conditions X/Y/Z").
sorry for the update. i have changed the problem a bit. this is the exact problem i am trying to solve.
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