Bash
Basic
Save the output of a command to a variable :
a=$(echo xyz); echo $a
>>> xyz
Write the output to a file :
- overwrite the file
echo 'writing test' > file_name.txt - append to the file
echo 'writing test' >> file_name.txt
Get the last several characters of a string :
a=my.fits; echo "${a: -5}"
- Output is
.fits
Get the characters of a string before several charactors :
a=my.fits; echo "${a: :-5}".sdf
- Output is
my.sdf - works on Ubuntu not on Mac
For loop :
for i in *; do echo $i; done
Check if a file exist or not (more file test operators):
if [ -e ${file_name} ]; then echo ${file_name}; fi
Find out the number of files in a folder:
ls | wc -l
Note 1 : a folder is one file no matter how much files in it
Note 2 : ls -l | wc -l will give (the number of files + 1)
Generate a random number between 0 and N (not include), where N is an integer :
echo $(( RANDOM % N ))
Extract substring :
b=${a:2:3}
- 2 is the zero-based offset and 3 is the length
Unpack (unzip, untar) files
Unpack a .tar file :
tar -xvf filename.tar
Note: .tar is the extension for the source files downloaded from the arXiv.org
Unpack a .tar.gz file :
tar -zxvf file.tar.gz
Unpack a .gz file:
gunzip file.gz
Errors
syntax error: unexpected end of file
- might be caused by incomplete loops, e.g. if ‘done’ is missed in the following :
while : do echo 'check me' done