Wildcards: *, ? and [abc]

Typing twenty file names is tedious; describing them is quick. A wildcard (or *glob*) is a word containing *, ? or [...] that the shell replaces with every existing name that fits the description. You write rm *.tmp; the shell turns it into rm a.tmp b.tmp cache.tmp before rm even starts. rm never sees the star. That is why wildcards work identically with every command: ls, cp, mv, rm, cat, grep…

~% cd /root/lab/l18/logs
logs% ls
1.log   10.log  11.log  2.log   3.log
logs% echo ?.log
1.log 2.log 3.log
logs% echo 1*
1.log 10.log 11.log
logs% echo [23].log
2.log 3.log
logs% echo *.zip
*.zip

Rules worth remembering

• Hidden files are skipped. * does not match a leading dot, so rm * leaves .profile alone. To include them you must write the dot yourself, as in .* — careful, that also matches . and ...
• Case matters. *.jpg does not match PHOTO.JPG; write *.[jJ][pP][gG] to catch both.
• No match, no expansion. If nothing fits, this shell passes the pattern through unchanged, so ls *.zip complains about a file literally called *.zip.
• Quotes switch it off. '*.txt' or "*.txt" reaches the command as a plain string. You will need that with find -name in lesson 35, where the command itself does the matching.
• Wildcards only match existing names. cp a.txt new*.txt cannot invent a name for you.

Preview before you act

Because the shell does the expansion, you can see the exact list any command would receive by putting echo in front: echo rm /root/lab/l18/logs/?.log prints the full command without running it. ls -d PATTERN does the same and also shows directories as names instead of listing their contents. Make this a reflex before every rm or mv with a wildcard; it costs one second and it is the difference between "oops" and nothing.