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.
Certain operators, notably expr, are very inefficient and might be replaced by double parentheses arithmetic expansion. See Example A.59, “Testing execution times of various commands”.
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, “Testing execution times of various commands”.
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 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, “Associative arrays vs. conventional arrays (execution times)”.
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 user 0m0.696s sys 0m0.004s Reading sparse associative index array entry real 0m0.083s user 0m0.084s sys 0m0.000s
Use the time and times tools to profile computation-intensive commands. Consider rewriting time-critical code sections in C, or even in assembler.
Try to minimize file I/O. Bash is not particularly efficient at handling files, so consider using more appropriate tools for this within the script, such as awk or Perl.
Write your scripts in a modular and coherent form, [127] so they can be reorganized and tightened up as necessary. Some of the optimization techniques applicable to high-level languages may work for scripts, but others, such as loop unrolling, are mostly irrelevant. Above all, use common sense.
For an excellent demonstration of how optimization can dramatically reduce the execution time of a script, see Example 16.47, “Monthly Payment on a Mortgage”.