allows directing input into a program from a file
$cat < input.txt
Differences between using file as arg & input redir
$cat myfile.txt
--argument--
cat is sent the string myfile.txt
cat opens the file & read the content
$cat < mefile.txt
The shell opens myfile.txt & makes the content available to cat
$wc myfile.txt
> lines words letters myfile.txt
$wc < mtfile.txt
> lines words letters
# wc has no way to know the file name
Every Linux process has 3 streams allowed
standard input standard output
(stdin) --------- (stdout)
-----------------> | process | ---------------->
default:keyboard --------- default:screen
| use out redir (>)
use input redir (<) | to change the file
to change the file |
|-------> standard error (stderr)
default:screen
use 2> to change the file
example:
$mycmd arg < in.txt
$mycmd > out.txt
# overwirte existing file (creating new at first execution)
$mycmd 2> err.txt
$mycmd >> out.txt
# append to existing file (creating new at first execution)
$mycmd 2>&1 out.txt
# put stdout & stderr in same file
# &1 is where stdout is redirecter now
##Wildcard matching (Global Pattern)##
*.txt
It's the ability of shell (not command)
How it works?
- when the shell sees a global pattern, it finds all files that match the pattern & replace the pattern with file names
- when using global pattern, it passes several arguments (instead of one) to the command
example:
$rm -rf *.cc
$echo *.txt
# 1.txt 2.txt 3.txt
$echo "*.txt"
$echo '*.txt'
# *.txt
# both prevent global pattern from expanding
####connect the stdout of a program to the stdin of another program####
$head -20 sample.txt | wc -w
--> temp -->
print a duplicate-free list of word1.txt and word2.txt (one word per line)
$cat word*.txt | sort | uniq
# uniq remove duplicate adjent to each other
####use the output of a program as an argument of another program####
use $(----) to embed command
$echo "Today is $(date)"
# one argument
#$echo Today is $(date)
$echo 'Today is $(date)'
#Single quote prevents embedding command from expanding
-
search within text file
usage: $egrep pattern files (regex)
-
the output is lines that match pattern
-
By default, it is case-sensitive
-
And matches all results in prefix, suffix or in middle
i.e
$egrep "cs246|CS246" index.html
| or in regex
- single quote supports global pattern & regex from expanding
- double quote only supports global pattern from expanding
Some regex:
"cs246|CS246" == "(cs|CS)246"
"(c|C)(s|S)246" == "[cC][sS]246"
[ ] : choose one char from the set of char
[^ ] : choose any one char not in the set
| : choose one string
? : 0 or 1 of the proceeding expression
"cs ?246" == "cs246|cs 246"
"(cs)?246" == "246|cs246"
* : 0 or more of the proceeding pattern
"(cs)*246" == "246|cs246|cscs246..."
+ : 1 or more occurance of the proceeding pattern
. : any one char
.* : any number of any char
- Use
^to indicate that the pattern must match for the start of the line - Use
$to indicate that the pattern must match for the end of the line - Use
\to escape char that has special meaning
exercises
print all words that start with "e" and have 5 char
$egrep "^e....$" <filename>
print words of even length
$egrep "^(..)*$" <filename>
print words with exactly one "a"
$egrep "^[^a]*a[^a]*$" <filename>
##File permission##
$ls -l
-rw-r-xr-- 1 Carey staff 123 Sep 1st ... <filename>
| \ # of links owner group sign time last modified
file permissions the file belongs to
type
rw- r-x r--
1 2 3
- owner's permission
- all users in the file's group (exclude owner)
- all others
.
r w x
read write execute
meaning of execute
- x for non-directory file : can attempt to execute a file
- x for directory file: can navigate to that dir (cd)
Owner is the only one that has right to change permissions
chmod <mode> <file>
/ | \
ownership operator permission
class
u - owner + add r
g - group - remove w
o - others = set exacly x
a - all
$chmod g-r <file>
$chmod o=rm
$chmod a+x
x=1
# create a variable called x with the string value 1
$echo "${x}"
$echo $x
# display the value of x
${PATH}
# list of directories seperated by:
# where command are stored
# when runing a command, shell will search command in those dir
# change PATH variable
PATH=${PATH}:newdir
##Shell Scripts##
A text file containing a sequence of command executed as a program
#!/bin/bash ---shebang: "I am bash script"
To run bash script "basic" in current dir,
$./basic
# you can put "." in PATH
$./myscript arg1 arg2
$0 $1 $2 $3
(not argu) (empty string)
Given a script, $1 refers to arg1 and so on.
.......
something here
##Shell Loops##
####while loop#### Print number 1 to $1
lectures/shell/scripts/count
#!/bin/bash
usage () {
echo "Usage: $0 limit" 1>&2
echo " where limit is at least 1" 1>&2
exit 1
}
if [ $# -ne 1 ]; then
usage
fi
if [ $1 -lt 1 ]; then
usage
fi
x=1
while [ $x -le $1 ]; do
echo $x
x=$((x + 1))
done
Notes:
$((...)) treat it as math expression
x=$(x + 1)
=$x + 1
=1 + 1 (string)
Rename all files ending in .c to ending in .cc
lectures/shell/scripts/renameC
#!/bin/bash
# Renames all .C files to .cc
for name in *.C; do
mv ${name} ${name%C}cc
done
Notes:
${name%C} if name ends in C, it will be removed; otherwise, nothing happens
${z#C} if z begins with C, it will be removed
Count the number of times of word $1 appears in file $2
lectures/shell/scripts/countWords
#!/bin/bash
# countWords word file
# Prints the number of times word occurs in file
x=0
for word in $(cat "$2"); do
if [ $word == $1 ]; then
x=$((x + 1))
fi
done
echo $x
#!/bin/bash
# Returns the date of the next payday (last Friday of the month)
# Examples:
# payday (no arguments) -- gives this month's payday
# payday June 2016 -- gives payday in June 2016
if [ $# -ne 0 -o $# -ne 2 ]; then
exit 1;
fi
answer () {
if [ $2 ]; then
preamble=${2}
else
preamble="This month"
fi
if [ $1 -eq 31 ]; then
echo "${preamble}'s payday is on the 31st."
else
echo "${preamble}'s payday is on the ${1}th."
fi
}
answer $(cal $1 $2 | awk '{print $6}' | grep "[0-9]" | tail -1) $1
year
------------------$1 for anwser----------------- --$2 for anwser---
awk 'pattern {action}' input-file > output-file
If the pattern is omitted, the action is applied to all line.
awk '{ print $5 }' table1.txt > output1.txt