09-23-2004
There are several differences between the two types of tests. The double bracket demands a logical expression while the single bracket does not.
[ $string ]
is true if string contains something. And
[ ]
if always false. You are using a variation on that. This depends on $string completely disappearing before [ sees its arguments. That will happen, but it's why
[ $string = "cat" ]
doesn't work. You must put quotes around $string for that to work. [[ is immune to that effect so
[[ $string = "cat" ]]
works fine. You can use -n with both of them to achieve the same effect:
[ -n "$string" ]
[[ -n $string ]]
But beware of
[ -n $string ]
If string is empty, it will disappear and the single bracket is back to seeing a single argument which will be true if that argument is not empty. "-n" is not empty so the test will be true.
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ZGREP(1) General Commands Manual ZGREP(1)
NAME
zgrep - search possibly compressed files for a regular expression
SYNOPSIS
zgrep [ grep_options ] [ -e ] pattern filename...
DESCRIPTION
Zgrep invokes grep on compressed or gzipped files. These grep options will cause zgrep to terminate with an error code:
(-[drRzZ]|--di*|--exc*|--inc*|--rec*|--nu*). All other options specified are passed directly to grep. If no file is specified, then the
standard input is decompressed if necessary and fed to grep. Otherwise the given files are uncompressed if necessary and fed to grep.
If the GREP environment variable is set, zgrep uses it as the grep program to be invoked.
EXIT CODE
2 - An option that is not supported was specified.
AUTHOR
Charles Levert (charles@comm.polymtl.ca)
SEE ALSO
grep(1), gzexe(1), gzip(1), zdiff(1), zforce(1), zmore(1), znew(1)
ZGREP(1)