9.7. Shell Metacharacters
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. 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.
Table 9.3. Shell MetacharactersMetacharacter | 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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