Difference between revisions of "Perl"
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=Warnings= | =Warnings= | ||
===Put a ; at the end of each line=== | ===Put a ; at the end of each line=== | ||
+ | This is still what I forget most. | ||
===autovivification=== | ===autovivification=== | ||
Line 60: | Line 61: | ||
;$|++ | ;$|++ | ||
:Set standard output to unbuffered, every is put on screen immediately. | :Set standard output to unbuffered, every is put on screen immediately. | ||
+ | |||
+ | ;$username = getpwuid($<) | ||
+ | :Get the username of the one executing the script. $< is the uid of the current user. | ||
==Hashes== | ==Hashes== | ||
Line 65: | Line 69: | ||
;$newhashref = {$oldhasref1,$oldhashref3} | ;$newhashref = {$oldhasref1,$oldhashref3} | ||
:Merge 2 hashes, works if all keys are unique | :Merge 2 hashes, works if all keys are unique | ||
+ | |||
+ | ==Arrays== | ||
+ | ;arr1 = split(<sep>,$string) | ||
+ | :Split a string in to an array on <sep> | ||
+ | |||
+ | ;a = arr1[0] | ||
+ | :First element in arr1 | ||
+ | |||
+ | ;a = arr1[-1] | ||
+ | :Last element in arr1 | ||
+ | |||
+ | ;if ( grep(/^<value>$/,@arr1) ) | ||
+ | :Check if <value> exist in arr1 (Note the ^ and $ for an exact match | ||
==Files== | ==Files== | ||
Line 72: | Line 89: | ||
;@array = <FH> | ;@array = <FH> | ||
:Read the file opened on FH into @array (1 element per line | :Read the file opened on FH into @array (1 element per line | ||
+ | |||
+ | ;while ( line = (<FH>) ) | ||
+ | :Read the file opened on FH line by line | ||
==Flow control== | ==Flow control== | ||
Line 81: | Line 101: | ||
;|| do {codeblock}; | ;|| do {codeblock}; | ||
:Like die but not exiting the program, use to do additional actions on the failure of the previous command | :Like die but not exiting the program, use to do additional actions on the failure of the previous command | ||
+ | |||
+ | ==Compare== | ||
+ | ;==, !=, >=, >, <, etc | ||
+ | :Compare numeric data | ||
+ | ;eq, ne, ge, gt, lt, etc | ||
+ | :Compare strings |
Latest revision as of 12:43, 12 November 2020
Warnings
Put a ; at the end of each line
This is still what I forget most.
autovivification
- If testing for existence of a hash key beware that all intermediate keys will be created by the test. In below example
$hash1($key1}
will be created if it does not exist yet.
if (exists $hash1{$key1}{$key2) {
...
} else {
...
}
)
Things that do work
Searching and matching is explained in Regular Expressions.
- Print out the contents of $variable in a structured format
use Data::Dumper;
print Dumper($variable);
if-then-else:
if < expr > {
<codeblock>
} elsif < expr ) {
<codeblock>
} else {
<codeblock>
}
- $var += <value>
- Add <value> to $var (works for -, *, / too)
- $string .= $addstring
- Concatenate $string and $addstring
- $string =~ s/<regexp>/<newvalue>/[g]
- Substitute <regexp> with <newvalue> in $string. The g modifier makes the all occurrences of <regexp> are substituted.
- $newstring = substr($string,start,length)
- Return substring of $string
- sprintf(format,$string)
- Return formatted string. format is e.g. "%.3f". Check Python:Strings#Advanced for all formats.
- $exitcode = system("a command")
- Executed the command in a subprocess and return the exitcode
- $output = `a command`
- Execute the command and return all command output
- sleep(x)
- Sleep for x seconds
- $|++
- Set standard output to unbuffered, every is put on screen immediately.
- $username = getpwuid($<)
- Get the username of the one executing the script. $< is the uid of the current user.
Hashes
- %newhash = (%oldhas1,%oldhash3)
- $newhashref = {$oldhasref1,$oldhashref3}
- Merge 2 hashes, works if all keys are unique
Arrays
- arr1 = split(<sep>,$string)
- Split a string in to an array on <sep>
- a = arr1[0]
- First element in arr1
- a = arr1[-1]
- Last element in arr1
- if ( grep(/^<value>$/,@arr1) )
- Check if <value> exist in arr1 (Note the ^ and $ for an exact match
Files
- open (FH, "< filename") || die("Open failed for filename")
- Open filename for reading on filehandle FH. Exit the program when the open failed
- @array = <FH>
- Read the file opened on FH into @array (1 element per line
- while ( line = (<FH>) )
- Read the file opened on FH line by line
Flow control
The usual works (if-then-else, for , foreach)
- ||die("message")
- || causes the statement only to be executed if the previous failed. Stop the program and write messaged to standard error.
- || do {codeblock};
- Like die but not exiting the program, use to do additional actions on the failure of the previous command
Compare
- ==, !=, >=, >, <, etc
- Compare numeric data
- eq, ne, ge, gt, lt, etc
- Compare strings