Basic Commands in Linux

                        Few Basic linux commands manual in brief for your referance.

ls

The basic file "list" command. It is all too easy to underestimate the power of this humble command. For example, using the -R, recursive option, ls provides a tree-like listing of a directory structure. Other useful options are -S, sort listing by file size, -t, sort by file modification time, -b, show escape characters, and -i, show file inodes.

The ls command returns a non-zero exit status when attempting to list a non-existent file. bash$ ls abc ls: abc: No such file or directory bash$ echo $? 2

Example 15-1. Using ls to create a table of contents for burning a CDR disk

#!/bin/bash # ex40.sh (burn-cd.sh) # Script to automate burning a CDR. SPEED=2 # May use higher speed if your hardware supports it. IMAGEFILE=cdimage.iso CONTENTSFILE=contents DEVICE=cdrom # DEVICE="0,0" For older versions of cdrecord DEFAULTDIR=/opt # This is the directory containing the data to be burned. # Make sure it exists. # Exercise: Add a test for this. # Uses Joerg Schilling's "cdrecord" package: # http://www.fokus.fhg.de/usr/schilling/cdrecord.html # If this script invoked as an ordinary user, may need to suid cdrecord #+ chmod u+s /usr/bin/cdrecord, as root. # Of course, this creates a security hole, though a relatively minor one. if [ -z "$1" ] then IMAGE_DIRECTORY=$DEFAULTDIR # Default directory, if not specified on command line. else IMAGE_DIRECTORY=$1 fi # Create a "table of contents" file. ls -lRF $IMAGE_DIRECTORY > $IMAGE_DIRECTORY/$CONTENTSFILE # The "l" option gives a "long" file listing. # The "R" option makes the listing recursive. # The "F" option marks the file types (directories get a trailing /). echo "Creating table of contents." # Create an image file preparatory to burning it onto the CDR. mkisofs -r -o $IMAGEFILE $IMAGE_DIRECTORY echo "Creating ISO9660 file system image ($IMAGEFILE)." # Burn the CDR. echo "Burning the disk." echo "Please be patient, this will take a while." cdrecord -v -isosize speed=$SPEED dev=$DEVICE $IMAGEFILE exit $?

cat, tac

cat, an acronym for concatenate, lists a file to stdout. When combined with redirection (> or >>), it is commonly used to concatenate files.

# Uses of 'cat' cat filename # Lists the file. cat file.1 file.2 file.3 > file.123 # Combines three files into one.

The -n option to cat inserts consecutive numbers before all lines of the target file(s). The -b option numbers only the non-blank lines. The -v option echoes nonprintable characters, using ^ notation. The -s option squeezes multiple consecutive blank lines into a single blank line.

 

In a pipe, it may be more efficient to redirect the stdin to a file, rather than to cat the file. cat filename | tr a-z A-Z tr a-z A-Z < filename # Same effect, but starts one less process, #+ and also dispenses with the pipe.

tac, is the inverse of cat, listing a file backwards from its end.

rev

reverses each line of a file, and outputs to stdout. This does not have the same effect as tac, as it preserves the order of the lines, but flips each one around (mirror image).

bash$ cat file1.txt This is line 1. This is line 2. bash$ tac file1.txt This is line 2. This is line 1. bash$ rev file1.txt .1 enil si sihT .2 enil si sihT

cp

This is the file copy command. cp file1 file2 copies file1 to file2, overwriting file2 if it already exists

Particularly useful are the -a archive flag (for copying an entire directory tree), the -u update flag (which prevents overwriting identically-named newer files), and the -r and -R recursive flags. cp -u source_dir/* dest_dir # "Synchronize" dest_dir to source_dir #+ by copying over all newer and not previously existing files.

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