Advanced bash-scripting guide by Mendel Cooper

By Mendel Cooper

This record is either an educational and a reference on shell scripting with Bash. It assumes no prior wisdom of scripting or programming, yet progresses speedily towards an intermediate/advanced point of guideline. The workouts and heavily-commented examples invite lively 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 record, because it evolves right into a complete publication that fits or surpasses any of the shell scripting manuals in print.

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This is an important use of quoting, protecting a command−line parameter from the shell, but still letting the calling program expand it. txt. txt. txt would not work. When referencing a variable, it is generally advisable in enclose it in double quotes (" "). This preserves all special characters within the variable name, except $, ` (backquote), and \ (escape). Keeping $ as a special character permits referencing a quoted variable ("$variable"), that is, replacing the variable with its value (see Example 5−1, above).

Echo "\$hello (null value) = $hello" # Note that setting a variable to a null value is not the same as #+ unsetting it, although the end result is the same (see below). # −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− # It is permissible to set multiple variables on the same line, #+ if separated by white space. # Caution, this may reduce legibility, and may not be portable. var1=variable1 var2=variable2 var3=variable3 echo echo "var1=$var1 var2=$var2 var3=$var3" # May cause problems with older versions of "sh".

Let "z=5**3" echo "z = $z" # z = 125 % modulo, or mod (returns the remainder of an integer division operation) bash$ echo `expr 5 % 3` 2 This operator finds use in, among other things, generating numbers within a specific range (see Example 9−20 and Example 9−21) and formatting program output (see Example 26−6). It can even be used to generate prime numbers, (see Example A−11). += "plus−equal" (increment variable by a constant) let "var += 5" results in var being incremented by 5. −= "minus−equal" (decrement variable by a constant) *= "times−equal" (multiply variable by a constant) let "var *= 4" results in var being multiplied by 4.

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