There is a command
l available on my machine which appears to do nothing.
which l also produces no output. Is this a real command, and does it actually do anything?
ls print output in column form. When stdout is a terminal (rather than being redirected to a file or non-terminal device, or piped to another command),
-C is implied. So running
ls -C is the same as running
ls. But they are not equivalent when
ls is redirected or piped. For example:
ek@Kip:~/firefox$ ls application.ini libfreebl3.chk libxpcom.so blocklist.xml libfreebl3.so libxul.so chrome libmozalloc.so mozilla-xremote-client chrome.manifest libmozsqlite3.so omni.ja components libnspr4.so platform.ini crashreporter libnss3.so plugin-container ... ek@Kip:~/firefox$ ls | less application.ini blocklist.xml chrome chrome.manifest components crashreporter ...
ls -C (or
-l) outputs in column form regardless of what kind of device
ls -C | less looks like the top output (but paged by
less, of course).
The main visible difference between
l is due to the
-F flag, which causes
ls to append symbolic suffixes (called indicators) to the entries it displays. These indicators identify what kind of file or directory they are.
Compare this to the output of the first
ls command above:
ek@Kip:~/firefox$ ls -F application.ini libfreebl3.chk libxpcom.so* blocklist.xml libfreebl3.so* libxul.so* chrome/ libmozalloc.so* mozilla-xremote-client* chrome.manifest libmozsqlite3.so* omni.ja components/ libnspr4.so* platform.ini crashreporter* libnss3.so* plugin-container* ...
/means the entry is a directory.
*means the entry is a normal file and is executable (i.e., has executable permissions).
- The absence of any indicator means the entry is a normal file that is not executable.
There are several other indicators:
@means the entry is a symbolic link (see also
|means the entry is a FIFO device (remember, many resources in Unix-like OSes are represented by filesystem entries).
=means the entry is a socket.
>means the entry is a door.
--classify flag and
--indicator-style=classify are equivalent to
ls -CF) is similar to but not the same as
It's also good to keep in mind:
The same text can be both a regular command and an alias.
This is commonly used to specify options that are widely considered both highly useful and harmless, such as automatic colorization (where color is applied when
stdout is unredirected or is a terminal, so the escape codes specifying colors are virtually guaranteed not to be misinterpreted).
By this principle,
ls is itself an alias.
ek@Kip:/$ alias ls alias ls='ls --color=auto'
So what command really gets executed when you run
l? This one:
/bin/ls --color=auto -CF
- The shell (
bash) resolves commands that don't contain a
/to the first match appearing in
PATH, which in Ubuntu for
- Aliases can contain aliases. Alias resolution is not recursive (an alias cannot call itself, though it can call a regular command that has the same name). But it does support nesting.
l resolves to
ls -CF which resolves to
ls --color=auto -CF.
Aliases can be changed.
These aliases exist because they're set up that way by default, but every user can change their aliases. See
man alias, Chapter 25 and Appendix M in the Advanced Bash-Scripting Guide, and How save my "alias" entries forever.
l are equal
raja@badfox:~/Pictures$ l des.png Screenshot from 2012-09-22 19:37:03.png Screenshot from 2012-09-22 19:37:11.png Screenshot from 2012-09-22 19:37:12.png Untitled.png raja@badfox:~/Pictures$ ls des.png Screenshot from 2012-09-22 19:37:03.png Screenshot from 2012-09-22 19:37:11.png Screenshot from 2012-09-22 19:37:12.png Untitled.png
why means there is a in-built system alias causing for this . if you want to see , open your terminal and type
alias then you will get output like this
raja@badfox:~/Pictures$ alias alias alert='notify-send --urgency=low -i "$([ $? = 0 ] && echo terminal || echo error)" "$(history|tail -n1|sed -e '\''s/^\s*[0-9]\+\s*//;s/[;&|]\s*alert$//'\'')"' alias egrep='egrep --color=auto' alias fgrep='fgrep --color=auto' alias grep='grep --color=auto' alias l='ls -CF' alias la='ls -A' alias ll='ls -alF' alias lock='gnome-screensaver-command -l' alias ls='ls --color=auto' alias sms='php .sms.php' raja@badfox:~/Pictures$
When in doubt,
l is aliased to `ls -alF'
(see also What does the la command do)