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Shell Command Summary

This section describes the syntax of the Bourne Shell. It lists special characters, variables, and built-in programming commands used by the shell.

Special Files

$ HOME/.profile      Executed at shell startup.

Special Characters for Filename Generation

* Match any string of characters.
? Match any single character.
[ . . . ] Match any of the enclosed characters. A pair of characters separated by a minus will match any character lexically between the pair.

Special Characters for Control Flow

| Perform pipeline (use output of preceding command as input of following command, e.g., cat file | lpr).
; Separate sequential commands on the same line.
& Run command in background (e.g., lpr file&).
&& Execute command if previous command was successful (e.g., grep string file && lpr file).
|| Execute command if previous command was unsuccessful (e.g., grep string 1 file | | grep string2 file).
( ) Execute commands enclosed in ( ) in a subshell; output from the entire set can then be redirected as a unit or placed in the background.
'...' Take all characters between single quotation marks literally. (Don’t allow special character meaning.)
\ Take following character literally.
". .. " Take enclosed characters literally but allow variable and command substitution.
'cmd' Use output of cmd as argument to another command.
# Begin a comment in a shell file.
<file Take input from file.
<<string Read standard input up to a line identical to string.
>file Redirect output to file (overwrite).
>>file Redirect output to end of file (append).
>&digit Duplicate standard input from digit e.g., 2>&1.
<&- Close standard input.
>&- Close standard output.

Variable Substitution

variable = value Set variable to value.
$ variable Use value of variable.
$ variable-value
$ variable[:]value
Use variable if set; otherwise set to value. For example: TERM=$ {1: -$TERM} will set the TERM variable to the value of the first argument to a shell script, if given, or else to the existing (default) value of TERM.
$ variable=value
$ variable[:]=value
Use variable if not set; otherwise set to value.
$ variable?value
$ variable[:]?value
Use variable if set; otherwise print value then exit.
$ variable+value
$ variable[:]+value
Use value if variable is set; otherwise nothing.

If the colon (:) is included in these expressions, a test is performed to see if the variable is non-null as well as set.

Shell Parameters Set by the Shell under Execution

$# Number of command-line arguments.
$– Options supplied in invocation or by the set command.
$? Return value of last executed command.
$$ Return process number of current process.
$! Return process number of last background command.

Shell Variables Initially Set By profile

$HOME Default (home directory) value for the cd command.
$IFS Internal field separators.
$MAIL Default mail file.
$PATH Default search path for commands.
$PS1 Primary prompt string; default is $.
$PS2 Secondary prompt string; default is >.
$TERM Specifies the type of terminal.

Shell Functions

name ( ) {command1; ...; commandn}
                Create a function called name that consists of the commands enclosed
                in braces. The function can be invoked by name within the current
                script.

Built-in Commands

file file
Execute contents of file.
break break [n]
Exit from a for, while, or until loop in n levels.
case case value in
  pattern1) commands ; ;
  .
  .
  .
     patternn) commands ; ;
esac
For each item in list that matches pattern, execute command.
cd cd [dir]
Change current directory to dir.
continue continue [n]
Resume nth iteration of a for, while, or until loop.
echo echo args
Print args on standard output.
eval eval [arg ... ]
Evaluate arguments, then execute results.
exec exec [cmd]
Execute cmd in place of current shell.
exit exit [n]
Exit the shell with exit status n, e.g., exit 1.
export export [var ...]
Export variable var to environment.
for for variable [in list ... ]
do
  commands
done
For variable x (in optional list) do commands.
if if condition
    then commands
    [elif condition2
    then commands2] ...
    [else commands3]
fi
If condition is met, do list of commands, or else if condition2 is met, do commands2, otherwise do commands3. (See test for a list of conditions.)
hash hash cmds
Temporarily add cmds to search path.
login login [user ... ]
Log in as another user.
newgrp newgrp [group ...]
Change your group ID to group; if no argument, change back to your default group.
pwd pwd
Print current working directory.
read read [var ...]
Read value of var from standard input.
readonly readonly [var ... ]
Mark variable var as read only.
return return
Stop execution of current shell function and return to calling level.
set set [t] [options] [arg ...]
With no arguments, set prints the values of all variables known to the current shell. The following options can be enabled (-option) or disabled (+option).
—- Don’t treat subsequent arguments beginning with – as options.
—a Automatically export all subsequently defined variables.
—e Exit shell if any command has a nonzero exit status.
—k Put keywords in an environment for a command.
—n Read but do not execute commands.
—t Exit after one command is executed.
—u Treat unset variables as an error.
—v Print commands as they are executed.
—x Turn on trace mode in current shell (echo commands in scripts as they are executed).

arg ... Assigned in order to $1, $2, ... $9.

shift shift
Perform a shift for arguments, e.g., $2 becomes $1.
test test exp | [exp]
Evaluate the expression exp. An alternate form of the command uses [ ] rather than the word test. The following primitives are used to construct expression.
—b file True if file exists and is a block special file.
—c file True if file exists and is a character special file.
-d file True if file exists and is a directory.
—f file True if file exists and is a regular file.
—g file True if file exists and its set-group-id bit is set.
—k file True if file exists and its sticky bit is set.
—n s1 True if the length of string s1 is nonzero.
—r file True if file exists and is readable.
—s file True if file exists and has a size greater than zero.
—t [n] True if the open file whose file descriptor number is n (default is 1) is associated with a terminal device.
—u file True if file exists and its set-user-id bit is set.
—w file True if file exists and is writable.
—x file True if file exists and is executable.
—z s1 True if the length of string s1 is zero.
s1 = S2 True if strings s1 and s2 are identical.
s1 != S2 True if strings s1 and s2 are not identical.
s1 True if string s1 is not the null string.
n1 -eq n2 True if the integers n1 and n2 are algebraically equal. Any of the comparisons -ne, -gt, -ge, -lt, and -le may be used in place of -eq.
times times
Print accumulated process times.
trap trap [cmd] [n]
Execute cmd if signal n is received. Useful signals include:
          0      Successful exit of command.
          1      Hangup of terminal line.
          2      Interrupt.
          15    Process is killed.
type type commands
Print information about commands.
until until condition
    [do commands]
done
Until condition is met, do commands (see test for conditions).
ulimit ulimit [size]
Set maximum size of file that can be created to size; if no arguments, print current limit.
umask umask [nnn]
Set file creation mask to octal value nnn.
unset unset vars ...
Remove definitions for variable var.
wait wait [n]
Wait for specified process with identification number (n) to terminate and report its status.
while while condition
    [do commands]
done
While condition is met, do commands (see test for conditions).
filename filename
Read and execute commands from executable file filename.

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