Getting help — Examples
These are contrived examples, but let’s step through how we might approach getting help on the command line through getting and assessing the quality of help online.
Example: using
find to match multiple patterns
We previously used find for a few different reasons: to
help us find all files matching a pattern, to delete those files, and to
run commands on those files.
In one situation we wanted to delete both docx and
pdf files from our repositories.
Let’s ignore the first step: We know that running find
by itself will just print out the names of all files in the current
directory and below, so there’s no point trying with that.
Can find --help help us?
Usage: find [-H] [-L] [-P] [-Olevel] [-D debugopts] [path...] [expression]
default path is the current directory; default expression is -print
expression may consist of: operators, options, tests, and actions:
operators (decreasing precedence; -and is implicit where no others are given):
( EXPR ) ! EXPR -not EXPR EXPR1 -a EXPR2 EXPR1 -and EXPR2
EXPR1 -o EXPR2 EXPR1 -or EXPR2 EXPR1 , EXPR2
positional options (always true): -daystart -follow -regextype
normal options (always true, specified before other expressions):
-depth --help -maxdepth LEVELS -mindepth LEVELS -mount -noleaf
--version -xdev -ignore_readdir_race -noignore_readdir_race
tests (N can be +N or -N or N): -amin N -anewer FILE -atime N -cmin N
-cnewer FILE -ctime N -empty -false -fstype TYPE -gid N -group NAME
-ilname PATTERN -iname PATTERN -inum N -iwholename PATTERN -iregex PATTERN
-links N -lname PATTERN -mmin N -mtime N -name PATTERN -newer FILE
-nouser -nogroup -path PATTERN -perm [-/]MODE -regex PATTERN
-readable -writable -executable
-wholename PATTERN -size N[bcwkMG] -true -type [bcdpflsD] -uid N
-used N -user NAME -xtype [bcdpfls] -context CONTEXT
actions: -delete -print0 -printf FORMAT -fprintf FILE FORMAT -print
-fprint0 FILE -fprint FILE -ls -fls FILE -prune -quit
-exec COMMAND ; -exec COMMAND {} + -ok COMMAND ;
-execdir COMMAND ; -execdir COMMAND {} + -okdir COMMAND ; Actually yeah, maybe. It tells us one important thing: expressions
aren’t just one “-name” and a pattern. Expressions are
kinda closely related to something we’ve seen in programming: they’re
Boolean expressions that can have groupings with parentheses
(), negation with ! or -not,
AND-ing with -a or -and, OR-ing with
-o or -or. We want to find files that have a
name matching .docx or .pdf, so let’s try:
find . -name "*.docx" -or "*.pdf"… that didn’t work, and the message that find prints out
isn’t exactly helpful.
Right, let’s try the manual page, then:
man find # or man 1 findI think I know I want to use the -or option, that’s
literally what my question is, so I’ll search for it by typing
/, typing -or, then pressing
Enter.
The thing that I immediately see when I do this is:
expr1 -or expr2
Same as expr1 -o expr2, but not POSIX compliant.This… actually maybe isn’t not helpful: it’s telling me that
-or requires two expressions around it. When I
think back to find . -name "*.docx" and the
-name "*.docx" being the expression part of the command,
it’s a hint that maybe I can have two -name
expressions, one on each side of the -or.
Let’s keep looking through the manual page. I’m going to press
n on my keyboard to find the next instance of
-or. It’s in the examples section. Its hard to decipher,
but it does seem helpful:
Given the following directory of projects and their associated SCM ad‐
ministrative directories, perform an efficient search for the projects'
roots:
$ find repo/ \
\( -exec test -d '{}/.svn' \; \
-or -exec test -d '{}/.git' \; \
-or -exec test -d '{}/CVS' \; \
\) -print -prune
Sample output:
repo/project1/CVS
repo/gnu/project2/.svn
repo/gnu/project3/.svn
repo/gnu/project3/src/.svn
repo/project4/.gitThis example does more than I want, but it confirms what I thought
before: you can put multiple expressions (e.g., -name)
around -or. Let’s try that:
find . -name "*.docx" -or -name "*.pdf"It works! Or at least it doesn’t give us an error. We deleted all those files last time, so it’s hard to say if it works.
Re-generate those files using what
you learned in the previous topic (-exec), and confirm
if this command does indeed find those files.
Now let’s try delete those files using the same approach:
find . -name "*.docx" -or -name "*.pdf" -deleteUh, that sort of worked. Run that command again without
-delete. Let’s check the manual page again for
-delete. You can search for -delete with
/, but you have to press n multiple times to get
to the option’s documentation:
-delete
Delete files; true if removal succeeded. If the removal failed, an er‐
ror message is issued. If -delete fails, find's exit status will be
nonzero (when it eventually exits). Use of -delete automatically turns
on the `-depth' option.
Warnings: Don't forget that the find command line is evaluated as an ex‐
pression, so putting -delete first will make find try to delete every‐
thing below the starting points you specified. When testing a find com‐
mand line that you later intend to use with -delete, you should explic‐
itly specify -depth in order to avoid later surprises. Because -delete
implies -depth, you cannot usefully use -prune and -delete together.This is a lot, but it does tell us (implicitly) why this didn’t work:
the find command line is evaluated as an expression. This
tells me that find is only applying -delete to
the right side of -or.
Wow. This is really taking a long time. I’m going to give up with the manual page (Note: We could actually piece together what we need with the parts of the manual pages we’ve seen already, but it’s not immediately obvious).
I’ll open my search engine and search:
find delete with multiple extensions
and my top result is on StackOverflow:
The question is “Using find command to delete multiple extensions
older than certaint(sic) time”, and the question itself has an example
command that doesn’t include -delete.
Looking at the accepted answer, even if it doesn’t include the
-delete option, gives me a hint on something new to try:
parenthesis.
I remember this from basic Boolean expressions, we can combine or set
precedence for an entire expression by wrapping it with parenthesis.
Maybe that means that increasing precedence for the whole
-or expression with find will have the
-delete apply to the entire thing. The accepted answer also
copies part of the manual page explaining why I need to put
\ in front of the parenthesis, nice!
Let’s try this. Start by regenerating your .docx and
.pdf files, then:
find . \( -name "*.docx" -or -name "*.pdf" \) -delete🎉, it works!
Example: find the length of an array in C
This isn’t a programming course, but let’s see what we can find about finding the length of an array in C.
We uh, can’t run a command here, so we can immediately skip both
steps of running the command by itself, and running the command with
--help. We also have no idea what the command is
(.length in Java is not even a method!).
Let’s try using the search feature in the manual pages:
man -k array lengthWe do actually find some interesting things, but they aren’t helpful:
bsearch (3) - binary search of a sorted array
cciss (4) - HP Smart Array block driver
ftruncate (2) - truncate a file to a specified length
ftruncate64 (2) - truncate a file to a specified length
...
strcspn (3) - get length of a prefix substring
strlen (3) - calculate the length of a string
strnlen (3) - determine the length of a fixed-size string
strspn (3) - get length of a prefix substringstrlen or strnlen are actually sort
of related to what we’re looking for, but I’m trying to calculate
the length of an array of integers. Let’s forget about the manual
pages.
Here are two results that I get when I search for “find array length in C”:
- https://www.knowprogram.com/c-programming/find-length-of-array-in-c/
- https://stackoverflow.com/questions/37538/how-do-i-determine-the-size-of-my-array-in-c
Let’s focus on the StackOverflow post. The most upvoted answer, with
more than 1500 upvotes, and the one that’s marked as correct seems to
have a very short and to the point example of how you find the length of
an array in C. I’m itching to copy and paste that fragment into my code
(modifying it to use the name of my array, of course), but I can see
that there’s more text below. Let’s read. It seems to make sense! I even
think I can explain this in my own words (sizeof tells me
how many bytes an entire array has, and can also tell me how many bytes
one element is, so I can divide the number of bytes in the array by the
number of bytes of one element and get the size of the array. Neat 📷.)
Right, let’s copy it and see if it works.
#include <stdio.h>
int main(void)
{
int array[10];
int size = sizeof(array) / sizeof(array[0]);
printf("array size is %d\n", size);
return 0;
}I compile and run my program
(clang array_size.c -o array_size) and it works! Yes!
But then I also remember that I need to do this in a function:
void do_stuff(int my_array[])
{
int size = sizeof(my_array) / sizeof(my_array[0]);
printf("array size is %d\n", size);
}I compile and run my program… and it doesn’t work? If you keep reading the StackOverflow thread, the very replies to this tell you to keep reading. When you read the next answer, it tells you more about why this won’t work, and how you should pass arrays and their size to functions in C.
The second link is worse than the StackOverflow post because it doesn’t explain why this doesn’t work at all.
This example is contrived, but is a common example of an issue you can run into when finding help online.