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 [...]:

String Comparison

>

[[ a > b ]] || echo "a does not come after b"

<

[[ az < za ]] && echo "az comes before za"

= (or ==)

a = a  && echo "a equals a"

!=

a != b  && echo "a is not equal to b"

Integer Comparison

-gt

5 -gt 10  || echo "5 is not bigger than 10"

-lt

8 -lt 9  && echo "8 is less than 9"

-ge

3 -ge 3  && echo "3 is greater than or equal to 3"

-le

3 -le 8  && echo "3 is less than or equal to 8"

-eq

5 -eq 05  && echo "5 equals 05"

-ne

6 -ne 20  && echo "6 is not equal to 20"

Conditional Evaluation

&&

[[ -n ${var} && -f ${var} ]] && echo "${var} is a file"

||

[[ -b ${var} || -c ${var} ]] && echo "${var} is a device"

Combining Conditional Evaluation with Command Result

A test expression can be combined with a regular command result by using [[...]] around the test expression and combining the resulted expression with the command, as follows:

if [[ -f ./a ]] && ! diff -q ./a ./b >/dev/null; then
  # ...
fi

Expression Grouping

[[ ${var} = img* && (${var} = *.png || ${var} = *.jpg) ]] && echo "${var} starts with img and ends with .jpg or .png"

Pattern Matching

[[ ${name} = a* ]] || echo "name does not start with an 'a': ${name}"

Regular Expression Matching

$(date) =~ ^Fri\ ...\ 13  && echo "It's Friday the 13th!"

((...))

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

The double parentheses construct.

test

test