Download Advanced Bash−Scripting Guide. An in−depth exploration of by Cooper M. PDF

By Cooper M.

This instructional assumes no prior wisdom of scripting or programming, yet progresses quickly towards an intermediate/advanced point of guide ...all the whereas sneaking in little snippets of UNIX knowledge and lore. It serves as a textbook, a guide for self-study, and a reference and resource of information on shell scripting concepts. The routines and heavily-commented examples invite energetic reader participation, less than the idea that the single strategy to particularly examine scripting is to put in writing scripts.

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Chapter 4. ). Using a variable before assigning a value to it will usually cause problems. It is nevertheless possible to perform arithmetic operations on an uninitialized variable. echo "$uninitialized" let "uninitialized += 5" echo "$uninitialized" # # #+ # # (blank line) # Add 5 to it. # 5 Conclusion: An uninitialized variable has no value, however it acts as if it were 0 in an arithmetic operation. This is undocumented (and probably non−portable) behavior. See also Example 11−19. 2. Variable Assignment = the assignment operator (no space before & after) Do not confuse this with = and −eq, which test, rather than assign!

Example 7−3. /bin/bash # Arithmetic tests. # The (( ... )) construct evaluates and tests numerical expressions. # Exit status opposite from [ ... ] construct! k. # 0 Chapter 7. " # Division result < 1. # Rounded off to 0. " # Illegal division by 0. # 1 # What effect does the "2>/dev/null" have? # What would happen if it were removed? # Try removing it, then rerunning the script. 2. File test operators Returns true if... ) −p file is a pipe −h file is a symbolic link −L file is a symbolic link −S file is a socket −t file (descriptor) is associated with a terminal device This test option may be used to check whether the stdin ([ −t 0 ]) or stdout ([ −t 1 ]) in a given script is a terminal.

This is the extended test command, adopted from ksh88. No filename expansion or word splitting takes place between [[ and ]], but there is parameter expansion and command substitution. " fi Chapter 7. Tests 44 Advanced Bash−Scripting Guide 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.

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