Download Pro Bash Programming: Scripting the GNU/Linux Shell by Chris Johnson, Jayant Varma PDF
By Chris Johnson, Jayant Varma
The bash shell is a whole programming language, now not in simple terms a glue to mix exterior Linux instructions. by means of taking complete good thing about shell internals, shell courses can practice as snappily as utilities written in C or different compiled languages. and you may see how, with no assuming Unix lore, you could write specialist bash 4.0 courses via usual programming techniques.
- Complete bash assurance
- Teaches bash as a programming language
- Helps you grasp bash 4.0 positive factors
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Sample text
Inside single quotes, a double quote is not special. $ sa "a double-quoted single quote, '" "a double-quoted double quote, \"" :a double-quoted single quote, ': :a double-quoted double quote, ": $ sa 'a single-quoted double quotation mark, "' :a single-quoted double quotation mark, ": All characters inside a single-quoted word are taken literally. A single-quoted word cannot contain a single quote even if it is escaped; the quotation mark will be regarded as closing the preceding one, and another single quote opens a new quoted section.
Whenever an external command is executed (creating a child process), whether it is a compiled, binary command or an interpreted script, this array is passed to it behind the scenes. In a shell script, these strings are available as variables. ). Exporting a variable doesn’t make it visible anywhere except child processes. Listing 5-1 tells you whether the variable $x is in the environment and what it contains, if anything. Listing 5-1. showvar, Print Value of Variable x if [ "${x+X}" = X ] ## If $x is set then if [ -n "$x" ] ## if $x is not empty 43 CHAPTER 5 PARAMETERS AND VARIABLES then printf " \$x = %s\n" "$x" else printf " \$x is set but empty\n" fi else printf " %s is not set\n" "\$x" fi Once a variable is exported, it remains in the environment until it is unset: $ unset x $ showvar $x is not set $ x=3 $ showvar $x is not set $ export x $ showvar $x = 3 $ x= ## in bash, reassignment doesn't remove a variable from the environment $ showvar $x is set but empty Variables set in a subshell are not visible to the script that called it.
KornShell 93 added more expansions that have not been incorporated into the standard but that bash has adopted. 0 has added two new expansions of its own. Bourne Shell The Bourne shell and its successors have expansions to replace an empty or unset variable with a default, to assign a default value to a variable if it is empty or unset, and to halt execution and print an error message if a variable is empty or unset. bashrc ## parse options here filename=${filename:-"$defaultfile"} ${var:+alternate}, ${var+alternate}: Use Alternate Values The complement to the previous expansion substitutes an alternate value if the parameter is not empty or, without a colon, if it is set.