Advanced Bash-Scripting Guide escape sequences. It substitutes a clean and logical syntax for the clumsy constructs just discussed.
Henry/teikedvl has likewise created a utility (http://scriptechocolor.sourceforge.net/) to simplify creation of colorized scripts.
36.6. Optimizations
Most shell scripts are quick 'n dirty solutions to non-complex problems. As such, optimizing them for speed is not much of an issue. Consider the case, though, where a script carries out an important task, does it well, but runs too slowly. Rewriting it in a compiled language may not be a palatable option. The simplest fix would be to rewrite the parts of the script that slow it down. Is it possible to apply principles of code optimization even to a lowly shell script?
Check the loops in the script. Time consumed by repetitive operations adds up quickly. If at all possible, remove time-consuming operations from within loops.
Use builtin commands in preference to system commands. Builtins execute faster and usually do not launch a
subshell when invoked.
Avoid unnecessary commands, particularly in a pipe.
cat "$file" | grep "$word" grep "$word" "$file"
# The above command-lines have an identical effect,
#+ but the second runs faster since it launches one fewer subprocess.
The cat command seems especially prone to overuse in scripts.
Disabling certain Bash options can speed up scripts.
As Erik Brandsberg points out:
If you don't need Unicode support, you can get potentially a 2x or more improvement in speed by simply
setting the
LC_ALL
variable.
export LC_ALL=C
[specifies the locale as ANSI C,
thereby disabling Unicode support]
[In an example script ...]
Without [Unicode support]: erik@erik-desktop:~/capture$ time ./cap-ngrep.sh
live2.pcap > out.txt
real 0m20.483s
user 1m34.470s
sys 0m12.869s
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Advanced Bash-Scripting Guide
With [Unicode support]: erik@erik-desktop:~/capture$ time ./cap-ngrep.sh
live2.pcap > out.txt
real 0m50.232s
user 3m51.118s
sys 0m11.221s
A large part of the overhead that is optimized is, I believe, regex match using [[ string =~ REGEX ]], but it may help with other portions of the code as well.
I hadn't [seen it] mentioned that this optimization helped with Bash, but I had seen it helped with "grep," so why not try?
Certain operators, notably expr, are very inefficient and might be replaced by double parentheses
arithmetic expansion. See Example A-59.
Math tests math via $(( )) real 0m0.294s
user 0m0.288s
sys 0m0.008s
math via expr: real 1m17.879s # Much slower!
user 0m3.600s
sys 0m8.765s
math via let: real 0m0.364s
user 0m0.372s
sys 0m0.000s
Condition testing constructs in scripts deserve close scrutiny. Substitute case for if-then constructs and
combine tests when possible, to minimize script execution time. Again, refer to Example A-59.
Test using "case" construct: real 0m0.329s
user 0m0.320s
sys 0m0.000s
Test with if [], no quotes: real 0m0.438s
user 0m0.432s
sys 0m0.008s
Test with if [], quotes: real 0m0.476s
user 0m0.452s
sys 0m0.024s
Test with if [], using -eq: real 0m0.457s
user 0m0.456s
Chapter 36. Miscellany 526
Advanced Bash-Scripting Guide sys 0m0.000s
Erik Brandsberg recommends using associative arrays in preference to conventional numeric-indexed
arrays in most cases. When overwriting values in a numeric array, there is a significant performance
penalty vs. associative arrays. Running a test script confirms this. See Example A-60.
Assignment tests
Assigning a simple variable real 0m0.418s
user 0m0.416s
sys 0m0.004s
Assigning a numeric index array entry real 0m0.582s
user 0m0.564s
sys 0m0.016s
Overwriting a numeric index array entry real 0m21.931s
user 0m21.913s
sys 0m0.016s
Linear reading of numeric index array real 0m0.422s
user 0m0.416s
sys 0m0.004s
Assigning an associative array entry real 0m1.800s
user 0m1.796s
sys 0m0.004s
Overwriting an associative array entry real 0m1.798s
user 0m1.784s
sys 0m0.012s
Linear reading an associative array entry real 0m0.420s
user 0m0.420s
sys 0m0.000s
Assigning a random number to a simple variable real 0m0.402s
user 0m0.388s
sys 0m0.016s
Assigning a sparse numeric index array entry randomly into 64k cells real 0m12.678s
user 0m12.649s
sys 0m0.028s
Reading sparse numeric index array entry real 0m0.087s
user 0m0.084s
sys 0m0.000s
Assigning a sparse associative array entry randomly into 64k cells real 0m0.698s
Chapter 36. Miscellany 527
