6.3. Syntax This section describes the many symbols peculiar to the Bash and Korn shells. The topics are arranged as follows: 6.3.1. Special Files Both shells read one or more startup files. Some of the files are read only when a shell is a login shell. The Korn shell reads these files: /etc/profile. Executed automatically at login, first. ~/.profile. Executed automatically at login, second. $ENV. Specifies the name of a file to read when a new Korn shell is created. (ksh88: all shells, ksh93: interactive shells only.) The value is a variable (ksh93: and command and arithmetic) substituted in order to determine the actual filename. Login shells read $ENV after processing /etc/profile and $HOME/.profile.
Bash reads these files: /etc/profile. Executed automatically at login, first. The first file found from this list: ~/.bash_profile, ~/.bash_login, or ~/.profile. Executed automatically at login, second. ~/.bashrc is read by every shell, after the login files. However, if invoked as sh, Bash instead reads $ENV, just as the Korn shell does.
For both shells, the getpwnam( ) and getpwuid( ) functions are are the sources of home directories for ~name abbreviations. (On single-user systems, the user database is stored in /etc/passwd. However on networked systems, this information may come from NIS, NIS+, or LDAPnot your workstation password file.) 6.3.2. Filename Metacharacters Characters | Meaning |
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* | Match any string of zero or more characters. | ? | Match any single character. | [abc...] | Match any one of the enclosed characters; a hyphen can specify a range (e.g., a-z, A-Z, 0-9). | [!abc...] | Match any character not enclosed as above. | ~ | Home directory of the current user. | ~name | Home directory of user name. | ~+ | Current working directory ($PWD). | ~- | Previous working directory ($OLDPWD). |
In the Korn shell, or Bash with the extglob option on: Characters | Meaning |
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?(pattern) | Match zero or one instance of pattern. | *(pattern) | Match zero or more instances of pattern. | +(pattern) | Match one or more instances of pattern. | @(pattern) | Match exactly one instance of pattern. | !(pattern) | Match any strings that don't match pattern. | \n | Match the text matched by the nth subpattern in (...). ksh93 only. |
This pattern can be a sequence of patterns separated by |, meaning that the match applies to any of the patterns. This extended syntax resembles that available in egrep and awk. In the Korn shell, but not in Bash, if & is used instead of |, all the patterns must match. & has higher precedence than |. ksh93 and Bash support the POSIX [[=c=]] notation for matching characters that have the same weight, and [[.c.]] for specifying collating sequences. In addition, character classes, of the form [[:class:]], allow you to match the following classes of characters. Class | Class | Class | Class |
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alnum | Alphanumeric characters | graph | Nonspace characters | alpha | Alphabetic characters | print | Printable characters | blank | Space or tab | punct | Punctuation characters | cntrl | Control characters | space | Whitespace characters | digit | Decimal digits | upper | Uppercase characters | lower | Lowercase characters | xdigit | Hexadecimal digits |
Bash and ksh93 also accept the [:word:] character class, which is not in POSIX. [[:word:]] is equivalent to [[:alnum:]_]. 6.3.2.1 Examples $ ls new* List new and new.1 $ cat ch? Match ch9 but not ch10 $ vi [D-R]* Match files that begin with uppercase D through R $ pr !(*.o|core) | lp Print files that are not object files or core dumps
NOTE On modern systems, ranges such as [D-R] are not portable; the system's locale may include more than just the uppercase letters from D to R in the range. 6.3.3. Quoting Quoting disables a character's special meaning and allows it to be used literally, as itself. The following table displays characters that have special meaning to the Bash and Korn shells. Character | Meaning |
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; | Command separator | & | Background execution | ( ) | Command grouping | | | Pipe | < > & | Redirection symbols | * ? [ ] ~ + - @ ! | Filename metacharacters | " ' \ | Used in quoting other characters | ' | Command substitution | $ | Variable substitution (or command or arithmetic substitution) | space tab newline | Word separators |
These characters can be used for quoting:
" " Everything between " and " is taken literally, except for the following characters that keep their special meaning:
$ Variable (or command and arithmetic) substitution will occur.
' Command substitution will occur.
" This marks the end of the double quote.
' ' Everything between ' and ' is taken literally except for another '. You cannot embed another ' within such a quoted string.
\ The character following a \ is taken literally. Use within " " to escape ", $, and '. Often used to escape itself, spaces, or newlines.
$" " Not ksh88. Just like " ", except that locale translation is done.
$' ' Not ksh88. Similar to ' ', but the quoted text is processed for the following escape sequences:
Sequence | Value | Sequence | Value |
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\a | Alert | \t | Tab | \b | Backspace | \v | Vertical tab | \cX | Control character X | \nnn | Octal value nnn | \e | Escape | \xnn | Hexadecimal value nn | \E | Escape | \' | Single quote | \f | Form feed | \" | Double quote | \n | Newline | \\ | Backslash | \r | Carriage return | | |
6.3.3.1 Examples $ echo 'Single quotes "protect" double quotes' Single quotes "protect" double quotes $ echo "Well, isn't that \"special\"?" Well, isn't that "special"? $ echo "You have `ls | wc -l` files in `pwd`" You have 43 files in /home/bob $ echo "The value of \$x is $x" The value of $x is 100
6.3.4. Command Forms Syntax | Effect |
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cmd & | Execute cmd in background. | cmd1 ; cmd2 | Command sequence; execute multiple cmds on the same line. | { cmd1 ; cmd2 ; } | Execute commands as a group in the current shell. | (cmd1 ; cmd2) | Execute commands as a group in a subshell. | cmd1 | cmd2 | Pipe; use output from cmd1 as input to cmd2. | cmd1 'cmd2' | Command substitution; use cmd2 output as arguments to cmd1. | cmd1 $(cmd2) | POSIX shell command substitution; nesting is allowed. | cmd $((expression)) | POSIX shell arithmetic substitution. Use the result of expression as argument to cmd. | cmd1 && cmd2 | AND; execute cmd1 and then (if cmd1 succeeds) cmd2. This is a "short-circuit" operation; cmd2 is never executed if cmd1 fails. | cmd1 || cmd2 | OR; execute either cmd1 or (if cmd1 fails) cmd2. This is a "short-circuit" operation; cmd2 is never executed if cmd1 succeeds. | ! cmd | NOT; execute cmd, and produce a zero exit status if cmd exits with a nonzero status. Otherwise, produce a nonzero status when cmd exits with a zero status. Not ksh88. |
6.3.4.1 Examples $ nroff file > file.txt & Format in the background $ cd; ls Execute sequentially $ (date; who; pwd) > logfile All output is redirected $ sort file | pr -3 | lp Sort file, page output, then print $ vi `grep -l ifdef *.c` Edit files found by grep $ egrep '(yes|no)' `cat list` Specify a list of files to search $ egrep '(yes|no)' $(cat list) POSIX version of previous $ egrep '(yes|no)' $(< list) Faster, not in POSIX $ grep XX file && lp file Print file if it contains the pattern; $ grep XX file || echo "XX not found" Otherwise, echo an error message
6.3.5. Redirection Forms File descriptor | Name | Common abbreviation | Typical default |
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0 | Standard input | stdin | Keyboard | 1 | Standard output | stdout | Screen | 2 | Standard error | stderr | Screen |
The usual input source or output destination can be changed, as seen in the following sections. 6.3.5.1 Simple redirection
cmd > file Send output of cmd to file (overwrite).
cmd >> file Send output of cmd to file (append).
cmd < file Take input for cmd from file.
cmd << text The contents of the shell script up to a line identical to text become the standard input for cmd (text can be stored in a shell variable). This command form is sometimes called a Here document. Input is usually typed at the keyboard or in the shell program. Commands that typically use this syntax include cat, ex, and sed. (If <<- is used, leading tabs are stripped from the contents of the here document, and the tabs are ignored when comparing input with the end-of-input text marker.) If any part of text is quoted, the input is passed through verbatim. Otherwise, the contents are processed for variable, command, and arithmetic substitutions.
cmd <<< word Supply text of word, with trailing newline, as input to cmd. (This is known as a here string, from the free version of the rc shell.) Not ksh88.
cmd <> file Open file for reading and writing on the standard input. The contents are not destroyed.[*] [*] With <, the file is opened read-only, and writes on the file descriptor will fail. With <>, the file is opened read-write; it is up to the application to actually take advantage of this.
cmd >| file Send output of cmd to file (overwrite), even if the shell's noclobber option is set.
6.3.5.2 Redirection using file descriptors Syntax | Effect |
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cmd >&n | Send cmd output to file descriptor n. | cmd m>&n | Same, except that output that would normally go to file descriptor m is sent to file descriptor n instead. | cmd >&- | Close standard output. | cmd <&n | Take input for cmd from file descriptor n. | cmd m<&n | Same, except that input that would normally come from file descriptor m comes from file descriptor n instead. | cmd <&- | Close standard input. | cmd <&n- | Move input file descriptor n instead of duplicating it. Not ksh88. | cmd >&n- | Move output file descriptor n instead of duplicating it. Not ksh88. |
6.3.5.3 Multiple redirection Syntax | Effect |
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cmd 2>file | Send standard error to file ; standard output remains the same (e.g., the screen). | cmd > file 2>&1 | Send both standard error and standard output to file. | cmd &> file | Same. Bash only, preferred form. | cmd >& file | Same. Bash only. | cmd > f1 2>f2 | Send standard output to file f1, standard error to file f2. | cmd | tee files | Send output of cmd to standard output (usually the terminal) and to files. (See the Example in Chapter 3 under tee.) | cmd 2>&1 | tee files | Send standard output and error output of cmd to standard output (usually the terminal) and to files. |
No space should appear between file descriptors and a redirection symbol; spacing is optional in the other cases. Bash allows multi-digit file descriptor numbers. The other shells do not. 6.3.5.4 Examples $ cat part1 > book $ cat part2 part3 >> book $ mail tim < report $ sed 's/^/XX /g' << END_ARCHIVE > This is often how a shell archive is "wrapped", > bundling text for distribution. You would normally > run sed from a shell program, not from the command line. > END_ARCHIVE XX This is often how a shell archive is "wrapped", XX bundling text for distribution. You would normally XX run sed from a shell program, not from the command line.
To redirect standard output to standard error: $ echo "Usage error: see administrator" 1>&2
The following command sends output (files found) to filelist and error messages (inaccessible files) to file no_access: $ find / -print > filelist 2>no_access
6.3.6. Coprocesses Coprocesses are a feature of the Korn shell only. Syntax | Syntax |
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cmd1 | cmd2 |& | Coprocess; execute the pipeline in the background. The shell sets up a two-way pipe, allowing redirection of both standard input and standard output. | read -p var | Read coprocess output into variable var. | print -p string | Write string to the coprocess. | cmd <&p | Take input for cmd from the coprocess. | cmd >&p | Send output of cmd to the coprocess. | exec n<&p | Move input from coprocess to file descriptor n. | exec n>&p | Move output for coprocess to file descriptor n. |
Moving the coprocess input and output file descriptors to standard file descriptors allows you to open multiple coprocesses. 6.3.6.1 Examples $ ed - memo |& Start coprocess $ print -p /word/ Send ed command to coprocess $ read -p search Read output of ed command into variable search $ print "$search" Show the line on standard output A word to the wise.
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