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

By Cooper M.

This instructional assumes no prior wisdom of scripting or programming, yet progresses swiftly towards an intermediate/advanced point of guideline ...all the whereas sneaking in little snippets of UNIX knowledge and lore. It serves as a textbook, a handbook for self-study, and a reference and resource of data on shell scripting options. The routines and heavily-commented examples invite energetic reader participation, less than the idea that the single solution to rather examine scripting is to write down scripts.

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Is greater than, in ASCII alphabetical order if [[ "$a" > "$b" ]] if [ "$a" \> "$b" ] Note that the ">" needs to be escaped within a [ ] construct. See Example 26−5 for an application of this comparison operator. −z string is "null", that is, has zero length −n string is not "null". The −n test absolutely requires that the string be quoted within the test brackets. Using an unquoted string with ! −z, or even just the unquoted string alone within test brackets (see Example 7−6) normally works, however, this is an unsafe practice.

What is truth? " fi # 0 is true. " fi # 1 is true. " # −1 is true. " fi # NULL is false. " fi # Random string is true. echo echo "Testing \"\$xyz\"" if [ $xyz ] # Tests if $xyz is null, but... # it's only an uninitialized variable. " else Chapter 7. " # Uninitialized variable is false. fi echo echo "Testing \"−n \$xyz\"" if [ −n "$xyz" ] # More pedantically correct. " fi # Uninitialized variable is false. echo xyz= # Initialized, but set to null value. " fi # Null variable is false. echo # When is "false" true?

Compound comparison −a logical and exp1 −a exp2 returns true if both exp1 and exp2 are true. −o logical or exp1 −o exp2 returns true if either exp1 or exp2 are true. These are similar to the Bash comparison operators && and ||, used within double brackets. [[ condition1 && condition2 ]] The −o and −a operators work with the test command or occur within single test brackets. if [ "$exp1" −a "$exp2" ] Chapter 7. Tests 53 Advanced Bash−Scripting Guide Refer to Example 8−3 and Example 26−9 to see compound comparison operators in action.

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