Download e-book for iPad: Advanced bash-scripting guide by Mendel Cooper

By Mendel Cooper

This record is either an instructional and a reference on shell scripting with Bash. It assumes no past wisdom of scripting or programming, yet progresses swiftly towards an intermediate/advanced point of guideline. The routines and heavily-commented examples invite energetic reader participation. nonetheless, it's a paintings in growth. The purpose is so as to add a lot supplementary fabric in destiny updates to this rfile, because it evolves right into a entire booklet that fits or surpasses any of the shell scripting manuals in print.

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No filename expansion or word splitting takes place between [[ and ]], but there is parameter expansion and command substitution. " fi Using the [[ ... ]] test construct, rather than [ ... ] can prevent many logic errors in scripts. For example, The &&, ||, <, and > operators work within a [[ ]] test, despite giving an error within a [ ] construct. Following an if, neither the test command nor the test brackets ( [ ] or [[ ]] ) are strictly necessary. " fi # "2>/dev/null" hides error message. The "if COMMAND" construct returns the exit status of COMMAND.

Bash Variables Are Untyped Unlike many other programming languages, Bash does not segregate its variables by "type". Essentially, Bash variables are character strings, but, depending on context, Bash permits integer operations and comparisons on variables. The determining factor is whether the value of a variable contains only digits. Example 5−4. Integer or string? sh # Integer or string? a=2334 let "a += 1" echo "a = $a " echo # Integer. b=${a/23/BB} echo "b = $b" declare −i b # Transform into a string.

Extended test command, which performs comparisons in a manner more familiar to programmers from other languages. Note that [[ is a keyword, not a command. Bash sees [[ $a −lt $b ]] as a single element, which returns an exit status. The (( ... )) and let ... constructs also return an exit status of 0 if the arithmetic expressions they evaluate expand to a non−zero value. These arithmetic expansion constructs may therefore be used to perform arithmetic comparisons. let "1<2" returns 0 (as "1<2" expands to "1") (( 0 && 1 )) returns 1 (as "0 && 1" expands to "0") • An if can test any command, not just conditions enclosed within brackets.

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Advanced bash-scripting guide by Mendel Cooper

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