| Metacharacters are special characters that are used to represent something other than themselves . As a rule of thumb, characters that are neither letters nor numbers may be metacharacters. Like grep , sed , and awk , the shell has its own set of metacharacters, often called shell wildcards . [4] Shell metacharacters can be used to group commands together, to abbreviate filenames and pathnames, to redirect and pipe input/output, to place commands in the background, and so forth. Table 9.3 presents a partial list of shell metacharacters. [4] Programs such as grep , sed , and awk have a set of metacharacters, called regular expression metacharacters , for pattern matching. These should not be confused with shell metacharacters. Table 9.3. Shell Metacharacters | Metacharacter | Purpose | Example | Meaning | | $ | Variable substitution | set name=Tom echo $name Tom
| Sets the variable name to Tom ; displays the value stored there. | | ! | History substitution | !3 | Re-executes the third event from the history list. | | * | Filename substitution | rm * | Removes all files. | | ? | Filename substitution | ls ?? | Lists all two-character files. | | [ ] | Filename substitution | cat f[123] | Displays contents of f1 , f2 , f3 . | | ; | Command separator | ls;date;pwd | Each command is executed in turn . | | & | Background processing | lp mbox& | Printing is done in the background. Prompt returns immediately. | | > | Redirection of output | ls > file | Redirects standard output to file . | | < | Redirection of input | ls < file | Redirects standard input from file . | | >& | Redirection of output and error | ls >& file | Redirects both output and errors to file . | | >! | If noclobber is set, override it | ls >! file | If file exists, truncate and overwrite it, even if noclobber is set. | | >>! | If noclobber is set, override it | ls >>! file | If file does not exist, create it; even if noclobber is set. | | ( ) | Groups commands to be executed in a subshell | (ls ; pwd) >tmp | Executes commands and sends output to tmp file. | | { } | Groups commands to be executed in this shell | { cd /; echo $cwd } | Changes to root directory and displays current working directory. |
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