Bash Expressions

Internal

• bash
• if

Conditional Expressions

Difference between -a and &&

If want to combine the results of multiple command executions in an if condition, use &&, not -a.

Example:

if grep "something" /file1.txt && grep "something" /file2.txt' then
    # "something" exists in both files ...
fi

-a should be used in test expressions:

if [ -f /file.txt -a -x /file.txt ]; then 
  ...
fi

[[...]] Extended Test Command

http://mywiki.wooledge.org/BashFAQ/031

[ and "test" are equivalent: [...] is part of the shell built-in command test. [[ is a keyword rather than a program, and it is called the extended test command. [ and [[ have much in common and share many expression operators like "-f", "-s", "-n", and "-z".

Notable differences:

• [[ allows =~ and regular expression matching.
• Variables do not have to be quoted inside [[...]] because [[ handles empty strings or strings with spaces more intuitively.
• < and > can be used for string comparison.

This is a systematic comparison of the [[...]]'s feature compared with [...]:

topleft	topmiddle	topright
middleleft	middlemiddle	middleright
bottomleft	bottommiddle	bottomright

((...))

http://tldp.org/LDP/abs/html/dblparens.html

The double parentheses construct.

test

test