CS303 UNIX: 5.4 Meta character, Pattern matching

5.4 Meta character, Pattern matching

A wildcard is a character that can be used as a substitute for any of a class of characters in a search, thereby greatly increasing the flexibility and efficiency of searches.

Wildcards are commonly used in shell commands in Linux and other Unix-like operating systems.

Shell's wildcards
Wildcard	Matches
*	Any number of characters including none
?	Any single character
[ijk]	A single character either i , j or k. This is a character class.
[x-z]	A single character between x and z
[!ijk]	A single character except i , j and k (not in C Shell)
[!x-z]	A Single character except x to z (not in C Shell)
{pat1, pat2, ........}	Pattern 1 , Pattern 2 etc. (not in Bourne Shell)

Note - Wildcards * and ? don't match a dot(.) character in file name and a / in file path. Therefore these two characters . and / must be matched explicitly.

* - any number of character in any filename / search string

e.g. $ lpr * (here * means all files, and this command will print all the files)

e.g. $ ls a*.txt (all file starting with a and .txt as extension)

? - any single character

e.g. $cp draft?.doc (it will copy draft1.doc draft2.doc draftb.doc etc. )

Character Class - set of characters enclosed by rectangular bracket [ ]

- It matches the single character in the class.

e.g. $ls data0[124] - it matches data01 data02 and data04
$ls chap[1-4] - it matches chap1, chap2, chap3 and chap4

! is used to reverse the searching criteria in character class. (It is not available in C Shell)

e.g. $ls *.[!co] - matches all filenames with single character extension not .c and .o

$ls [!a-zA-Z]* - matches all filenames that don't start with an alphabet.

$cp /home/srm/{progs, docs, data}/* . - all files from the three sub-directories progrs, docs, data will be copied to current directory. (Note- it doesn't work in Bourne Shell)

Q. What will be performed by the following commands -

ls *.c
mv * ../bin
cp ????? progs
rm *.[!l][!o][!g]
ls data[0-1][0-9]
cp -r /home/srm/{c,java} .

Expansion

किसी भी command को execute करने से पहले shell द्वारा command में प्रयुक्त meta-characters को resolve कर expand किया जाता है इस प्रक्रिया को expansion कहा जाता है |

$echo hello world
echo एक shell कमांड है यह दिए गए text को प्रिंट करता है |

$echo *
यहाँ * एक meta-character है तथा यह सभी files और directory name को व्यक्त करता है |

Path-name Expansion

expansion को निम्न प्रकार use किया जा सकता है -

expansion example 1

यहाँ *D का अर्थ है कि केवल वही file name या directory name show होंगे जिनका नाम अक्षर "D" से प्रारंभ होता है |

expansion example 2

यहाँ *s से अर्थ है कि केवल वही नाम show होंगे जिनके अंत में अक्षर "s" आता हो |

Tilde expansion

tilde (~) character - यह current user की home directory को व्यक्त करता है |

~foo = यहाँ foo किसी user का नाम है | तथा ~foo उस user की home directory को व्यक्त करता है |
नीचे दिए गए commands को run करके देखें -

$echo ~ यह command current user की home directory को show करेगा |

$echo ~foo यह command foo नाम के user account की home directory को show करेगा |