Bash operators

From LinuxIntro
(Redirected from &&)
Jump to: navigation, search

You have a shell script and wonder what these operators do? Then this article is for you.


* and ?

You may already know these used for so called "shell globbing" (pattern matching). * replaces an arbitrary amount of characters (including none), while ? replaces exactly one character.

These wildcards are actually bash operators! Imagine, we have the files a1, a2:

cp a*

now is a valid bash command which overrides one of these files with the other one.

已經改好了 請 smart update 後安裝 oxim其實不安裝 qt-immodule 或 gtk-immodule 也可以但是預設的設定檔環境變數有設GTK_IM_MODULE=oxim 和 QT_IM_MODULE=oxim若真不需要將 /etc/X11/xim.d/oxim 中將上面兩個變數改成*_IM_MODULE=xim


The operator $(()) in the bash shell is replaced by the arithmetic result of the expression enclosed in the parentheses.


# echo $((2*2))


The operator $$ delivers the id of the currently-running process.


# kill -9 $$

kills the current process

Showtime gotta see us guys, im sorry i dont keep if they are real(The Real Lword) or not but these guys rocks the stage in every episode!!!! And this pecafrmonre,everytime i watch i got chills!!! Jennifer Beals such a great actress,such a great person,i adore her.And i cant believe that her best works are just Flashdance and The L World. She is 47 yrs old now.dont miss this woman

> and >>

> and >> can be used to redirect the output (only stdout, not stderr, for that see below) of a command to a file. The difference between them is, that >> appends to the given files, while > will truncate it.


ls -alh > files-in-directory.txt
ls -alh /otherdir >> files-in-directory.txt  # Append list of files in other directory

2> or 2>>

stderr can be redirected by using 2> or 2>>.

But sometimes, you need stderr to be included in stdout for certain purpose, e.g. when another program is parsing/logging only stdout and you need errors to be in there, too. In these cases, you can redirect stderr to stdout with "2>&1". (Of course, it is also possible to redirect stdout to stderr this way, if you need to silence a program on stdout for some purpose).


cat notexisting 2> test  # Writes "File or directory not found." to test
cat notexisting 2>&1  # Outputs "File or directory not found." to stdout


| is used to pipe the stdout of one programm to the stdin of another one.

tail -n 100 foo | sort -r  # Reverse sort the last 100 lines of the file foo


&& evaluates the binary result of the command left of it and ands it with the binary result right of it if it is not already false, where false!=0. In other words, if you write

command1 && command2

command2 will only be executed if command1 returned success.

See also